I N T R O D U C T I O N.
UNTIL a recent period, the study of the minute objects which form the subject of this work had been more neglected in this kingdom than almost any other branch of Natural History, and I commenced my researches with the intention of acquiring for myself some fuller and more satisfactory information in regard to disputed points in their history, and also with the hope that I might be able to present to the British Naturalist such a description of our species as seemed necessary towards making the knowledge of them at homekeep pace with its advance on the Continent. I soon discovered not only that we possessed many species hitherto undescribed, but that various points in their economy, not devoid of interest, remained still unexplained or doubtful; and, rewarded beyond my expectations, I hastened to communicate the result of my investigations in a series of papers to the Botanical Society of Edinburgh. As these memoirs have received the gratifying approbation of distinguished naturalists, both in this country and abroad, I have been induced, at the solicitations of my friends, to publish as complete a monograph of the British species as the present state of our knowledge will permit.
The Desmidieae have been for a long time a common territory, claimed both by zoologist and botanist. In consequence, a greater number of persons have devoted themselves to their study, and as they have often entered on the subject with
2
different and conflicting views, every fact relating to their history has undergone a more rigorous examination.
The Desmidieae are of an herbaceous green colour; a few only of the Closteria have the integument coloured, but in all the internal matter is green. All the family are inhabitants of fresh water. Mr. Thwaites indeed has gathered two or three species in water slightly brackish, but the same species are also found in localities remote from the sea. Certain marine objects that have been classed with the Desmidieae have the internal matter of a brown colour, but these, in my opinion, belong to the Diatomaceae.
Their most obvious peculiarities are the beauty and variety of their forms and their external markings and appendages; but their most distinctive character is the evident division into two valves or segments.
Each cell or joint in the Desmidieae consists of two symmetrical valves or segments, and the suture or line of junction is in general well-marked; in a few instances however, as in Scenedesmus, it is determined principally by analogy.
In Pediastrum its situation is shown by a more or less evident notch on the outer side, but no separation has been noticed. The structures of Scenedesmus and Pediastrum are in fact less known than those of the rest of the family; and of their modes of reproduction we are altogether ignorant. In the other genera the suture eventually opens and allows the escape of the contents, and it is indicated by either a transverse line or a pale band, and usually also by a constriction.
An uninterrupted gradation may be traced from species in which these characters are inconspicuous to those in which they are fully developed: thus in Closterium and some species of Penium there is no constriction; in Tetmemorus, in some Cosmaria, and in Hyalotheca, it is quite evident, although still but slight; in Didymoprium and Desmidium it is denoted by a notch at each angle; but in Sphaerozosma, Micrasterias and some other genera, the constriction is very deep, and the connecting portion forms a mere chord between the segments,
3
which appear like distinct cells, and are so considered by
Ehrenberg and others.
When the papers on the Desmidieae were publishing in the 'Annals of Natural History', I stood alone in regarding the frond as a single cell, differing on this point not only from Ehrenberg, but from every author whose works I had seen. Professor Kützing, however, in his ´Phycologia Germanica,' has by independent observations arrived at the same conclusion; an important corroboration of the correctness of the opinion I then advanced. That the frond in Euastrum and allied genera is really a single constricted cell, and not a binate one, will, I am persuaded, be apparent to any one who traces the gradations mentioned above; but as the. opinions of such distinguished naturalists as Ehrenberg and Meneghini are deservedly of much influence, and the subject is so important, - since upon the view which we take of it depends the explanation of the division of the frond presently to be described,-I shall proceed to notice some facts which seem to me quite decisive.
In Navicula and other genera of Diatomaceae the frustules are often truly binate, and, as each frustule is complete in itself, though they be separated from each other their respective contents will still be protected on all sides ; and even should one be broken the contents of the other will be undisturbed. In the Desmidieae, on the other hand, as there is no septum between the segments, if these separate, or an opening be made in one, the contents of both will escape;and I have more than once observed the minute granules passing from one segment through the connecting tube into the other. The conjugation of the fronds and formation of sporangia I believe to be altogether irreconcilable with the supposition of binate cells. For in the simple Algae are many examples where the contents of two cells meet and form a compact seed-like mass ; but I know of no instance in which the contents of more than two cells are thus united : nor does it appear probable that the process of reproduction in either an animal or a vegetable should require, or indeed permit,
4
the conjunction of. four individuals for that purpose; but if the fronds were binate, it must follow that the cooperation of four individuals would be necessary for the continuation of the species. In Didymoprimum Grevillii the reproductive body is contained within one of the coupling joints; so that if they were binate we should have the contents of two cells passing into two other cells, the contents of all four uniting into one body, two of the cells at all events forming a single chamber. It may indeed be suggested that the joint in Didymoprium differs from the frond in some of the other genera; but such a supposition is utterly untenable, and will never be advanced by any one acquainted with these objects.. In both the endochrome is divided into two by a pale transverse band marking the junction of the valves; and here they in both eventually open, and permit the escape of their contents. In the one case, as in the other, the coupling bodies alike communicate at this point, and the entire process is essentially the same. I will now presume that I have proved that the bipartite Desmidieae are truly cells more or less constricted, and in the following details I shall so designate them.
In the Desmidieae the multiplication of the cells by repeated transverse division is full of interest, both on account of the remarkable manner in which it takes place, and because it unfolds, as I believe, the nature of the process in other families, and furnishes a valuable addition to our knowledge of their structure and physiology.
The compressed and deeply constricted cells of Euastrum offer most favourable opportunities for ascertaining the manner of the division ; for although the frond is really a single cell, yet this cell in all its stages appears like two, the segments being always distinct, even from the commencement. As the connecting portion is so small, and necessarily produces the new segments, which cannot arise from a broader base than its opening, these are at first very minute, though they rapidly increase in size. The segments are separated by the elongation of the collecting tube, which is converted into two roundish hyaline lobules. These lobules increase
5
in size, acquire colour, and gradually put on the appearance of the old portions. Of course, as they increase the original segments are pushed farther asunder, and at length are disconnected, each taking with it a new segment to supply the place of that from which it has separated.
It is curious to trace the progressive development of the new portions. At first they are devoid of colour, and have much the appearance of condensed gelatine, but as they increase in size the internal fluid acquires a green tint, which is at first very faint, but soon becomes darker; at length it assumes a granular state. At the same time the new segments increase in size and obtain their normal figure; the covering in some species shows the presence of puncta or granules; and lastly, in Xanthidium and Staurastrum the spines and processes make their appearance, beginning as mere tubercles, and then lengthening until they attain their perfect form and size; but complete separation frequently occurs before the whole process is completed. This singular process is repeated again and again, so that the older segments are united successively, as it were, with many generations. In Sphaerozosma the same changes take place, and are just as evident, but the cells continue linked together, and a filament is formed, which elongates more and more rapidly as the joints increase in number. This continued multiplication by division has its limits; the segments gradually enlarge whilst they divide, and at length the plant ceases to grow ; the division of the cells is no longer repeated; the internal matter changes its appearance, increases in density, and contains starch-granules which soon become numerous; the reproductive granules are perfected, and the individual perishes. In a filament the two oldest segments are found at its opposite extremities; for so long as the joints divide they are necessarily separated further and further from each other. Whilst this process is in progress the filament in Sphaerozosma consists of segments of all sizes ; but after it has reached maturity there is little inequality between them, except in some of the last-formed segments,
6
which are permanently smaller. The case is the same with those genera in which the separation of the cells is complete. I admit that the division of the cells just described apparently differs greatly from that in other simple Algae; but I believe that the process in all is essentially the same, and that whatever differences exist are modifications necessarily resulting from the different forms of the cells. In the examples already given the cell itself consists of two distinct portions, having a constriction between them; hence each of the new-formed portions is similarly distinct from the older one which forms it and to which it is united.
In order fully to elucidate the subject, cells may be distributed into three principal kinds, distinguished by their form:
Bipartite cells belong only to the Desmidieae; cells globose or roundish at the ends are seen in the Nostocs and Palmelleae; attenuated cells in the Desmidieae; and cylindrical ones in the Conjugatae, Tiresias, &c.
It is obvious that the new portions must arise from the whole of the junction margin of the original valves; consequently when the junction occupies only a part of the breadth the new portion will be narrower than the old; but when the junction of the valves is as broad as the cell, the new portion will from the beginning be of the same breadth. From this important fact, we may explain the different sorts of division.
Since in the two latter kinds of cell the valves are united by their entire breadth, the new portions cannot be distinguished by their size, we must therefor have recourse to other aids to enable us to trace the changes and satisfy ourselves of their real identity with that already described; and I hope to be able to show that this identity does exist.
In Nostoc and Anabaina the cells are globular, and as there is no constriction we might remain ignorant of the real me-
1st. Bipartite cells, already described, and. more or less constricted at the middle;
2nd. Cells globose or rounded at the ends, or having the extremities attenuated;
3rd. Cylindrical cells.
thod of division; but, guided by the analogical process, in the Desmidieae, I hope to make it sufficiently plain. The hemispheres are thrust apart by the new formation; but now it is the outer rounded margin that we look to for an explanation. If a globe be cut into two equal portions, each will represent half a circle. By comparison with the neighbouring cells, we find that these two half circles remain unaltered, and are mere1y separated from each other, for if again brought together they would reconstitute the former globe. The new formations however separate them further and further, until the intervening space equals that occupied by the original globe, and then we find two globes exactly like the primary one, the internal half of each being the newly-formed one. During this time the inner portions, as they extend, develop more and more of the circle, until each becomes, as I have stated, a perfect hemisphere. The whole process cannot, of course, be seen in the same cell; but in a dividing filament some joints may be observed in one stage and some in another, which renders the evidence complete.
When the cell is oblong, or only rounded at the extremities, the process, though similar, is less evident: the cell at first seems merely to elongate until it obtains nearly twice its original length, when the division commences and the rounding of the new ends becomes apparent. The tapering cell presents but little difference, for the separation takes place before its extremities are fully developed. Sometimes these cells. separate obliquely, as in Spirotaenia and Scenedesmus.
I ought to state however that the opinions advocated above do not agree with those of M. De Brébisson, who has attained so high a reputation for his intimate acquaintance with the freshwater Algae, and to whose kindness I have been so often indebted during the progress of the present work. He considers that there is an essential distinction in the mode of division between the Desmidieae and Nostochineae (including in the latter the Palmelleae), and that from it indeed differential characters are obtained by which we can
8
distinguish these nearly-allied groups. He observes of Hormospora mutabilis, Bréb.*, " Ils sont le plus souvent géminés, se multipliant par une division spontanée (déduplication) transversale, comme cela arrive dans quelques autres Pleurococcoidées. Une division analogue a lieu dans les Desmidiées, auxquelles on serait d'abord tenté de rapporter les Hormospora; mais les demi- corpuscules (hémisomates) des Desmidiées développent à leur point de séparation une nouvelle portion semblable à la première, tandis que, dans l'accroissement des Nostocinées, les corpuscules sont divisés en deux par un étranglement transversal, sans qu'il s'ensuive une reproduction sur chacun des points de rupture. Il y a dans ce cas, comme je l'ai dit ailleurs, déduplication simple. Dans les Desmidiées, il y a deduplication et réduplication."
It is with unfeigned diffidence that I venture to dissent from the opinion of one possessing so profound a knowledge of these tribes, and I do so only from conviction, the result of close and repeated investigations.
I have stated my belief that the same changes occur in both the Desmidieae and the Nostochineae. A cell in Micrasterias has two hemispheres, just as a joint in Anabaina has;in both these separate, and in both each hemisphere becomes again a perfect sphere; and if in Micrasterias the two hemispheres. were united by their whole bases, there would not remain even an apparent difference between them.
The form of the cylindrical cells no longer helps us in tracing the method of division. In Penium as in the Conjugatae, they seem merely to elongate and then divide. As I formerly suggested, in a paper read before the Botanical Society of Edinburgh, I consider it extremely probable that in all the simple Algae the cell or joint consists of two valves, and that additions occur at their junction, the original parts remaining unaffected: but this it may never be possible to
*Annales des Sciences Naturelles, Jan. 1844.
9
demonstrate satisfactorily, unless a species of Conferva with a coloured integument should be detected, or some means can be devised for permanently colouring the filaments without impairing their growth. Then indeed the question might be determined; at present I can merely show the probability that the cell in cylindrical species of Desmidieae agrees with the joint in a Zygnema or Tyndaridea; since whenever the covering is colourless and free from markings not the slightest difference can be perceived. This is the case in a few species of Penium; and hence Penium Brebissonii is by some authors placed in the Palmelleae. In Penium margaritaceum and Penium Cylindrus the integument is coloured, and we are enabled, by means of the paler appearance of the newly-formed portions, to satisfy our-selves that in these also each half of the original cell is acquiring during the division a new partner. In Didymoprium the same fact is rendered apparent, because the suture passes between minute teeth ; these teeth recede from each other, and the new teeth which appear between them show the place where the separation of the joint has occurred.
The spontaneous division of the frond is included by some writers amongst the modes of reproduction; but this is not strictly correct, for it is rather the manner in which the individual plant grows, since all the cells arrive at maturity nearly at the same period and terminate their existence about the same time.
The Desmidieae are most probably reproduced only in two modes; one by the escape of the granular contents of the mature frond, and the other by the formation of sporangia, the result of the coupling of the cells.
When the cells approach maturity, molecular movements may be at times noticed in their contents, precisely similar to what has been described by Agardh and others as occurring in the Confervae. This movement has been aptly termed a swarming. It has been seen by numerous observers, in this country by Messrs. Dalrymple, Jenner, Thwaites, Sidebotham, Dr. Dickie, and others. The cause
10
of this sudden commotion cannot be ascertained ; but I have met with it more frequently in specimens that have been kept some days than in fresh-gathered ones. When released by the opening of the suture, the granules still move, but more rapidly and to a greater distance. With the subsequent history of these granules I am altogether unacquainted, but I conclude: that it is similar to what has been traced in other Algae.
The second mode of reproduction is by coupling, and the formation of sporangia. A communication is established between two cells and a seed-like mass is formed in the same manner as in the Conjugatae. This is green and granular at first, but soon becomes of a homogeneous appearance and of a brown or even reddish colour. There are however some variations in the process , in the two families which require notice. In the Conjugatae, the cells conjugate whilst still forming parts of a filament; but in the Desmidieae the filamentous species almost invariably separate into single joints before their conjugation, and in most of the species the valves of the cells become detached after they are emptied of their contents.
In many genera the sporangia remain smooth and unaltered; in other they become granulated, tuberculated or spinous; the spines, being either simple or forked. at the apex. In fact a sporangium may pass successively through all these stages, and hence may so change its appearance that its different states are liable to be taken for sporangia belonging to different species. In Tiresias also we sometimes meet with sporangia bearing spines, but in that genus they are arranged like the spokes of al wheel, and not scattered as in the Desmidieae. What is the nature of the sporangia, and why so complicated a process is necessary, since the species is also propagated by means of the granules or zoospores which escape from the ruptured cell, are questions to which we cannot, in the present state of science, return a satisfactory answer. The sporangia I consider capsules; and this view seems to be confirmed by the experience of
11
Mr. Jenner, who informs me that the covering of the sporangium swells, and a mucus is secreted, in which minute fronds appear and, by their increase, at length rupture the attenuated covering*. That some purpose distinct from that performed by the zoospores is served by the coupling of the cells and formation of the sporangium cannot be doubted: for, where we can trace the operations of nature, we find that nothing is useless or in vain; nor is it reasonable to suppose that this complicated process should fulfil no other purpose than one already provided for without it. The sporangia are most abundant in spring before the pools dry up; and I would suggest, as no improbable conjecture, that the zoospores may be gemmae, analogous to those present in Marchantia polymorpha and Lunularia vulgaris, and that they possess merely a limited vitality, which is destroyed unless they are at once placed in circumstances favourable to their growth, whilst on the other hand, in the conjugated cells some important change takes place during the commingling of their contents and the formation of the sporangium, like what happens in the production of seeds in general, which renders the sporangia capable of retaining the vital principle uninjured throughout long periods of drought.
That the Closteria couple and produce sporangia, in a manner similar to the Conjugatae, has been recorded by Turpin and other writers. Correct figures of some species in that state are given by Ehrenberg in his 'Infusoria,' and Meneghini mentions that Brébisson had detected it in Desmidium, but I am not aware that the conjugation of other Desmidieae was noticed by any writer before I published in the 'Annals of Natural History' full descriptions of the formation of sporangia in Tetmemorus granulatus and Staurastrum mucronatum. Subsequently examples have been detected in almost every genus, and we cannot hesitate to consider it charac-
*
12
teristic of the family. In this country the conjugation of about forty species has been noticed by different observers, and M. De Brébisson informs me that he has gathered sporangia of the following species in France: Hyalotheca dissiliens, Didymo-prium Grevilli, Staurastrum pygmaeum, S. controversum, S. muticum, Cosmarium Botrytis, Closterium Lunula, C. seta-ceum, C. acutum, and C. lineatum.
In defining the genera and species, I have made no use of the reproductive bodies, for as yet too little is known about them to render them available for that purpose, and in many cases we are still uncertain whether the mature form is yet known. They are likewise so early detached from the emptied cells, that it is often very difficult to determine to what species they belong *.
I have gathered sporangia of other species besides those mentioned in this work; but whenever they were not still adherent to the fronds I have thought it best to pass them over, lest I should be the cause of error.
That the orbicular spinous bodies so frequent in flint are fossil sporangia of Desmidieae cannot, I think, be doubtful when they are compared with figures of recent ones. Indeed one celebrated geologist, Dr. G. Mantell, who, in his 'Medals of Creation,' without any misgiving had adopted Ehrenberg's ideas concerning them, has changed his opinion, and in his last work regards them as having been reproductive bodies, although he is still uncertain whether they are of vegetable origin.
Ehrenberg and his followers describe these bodies as fossil
*
species of Xanthidium, but no doubt erroneously, since their structure is very different. For the true Xanthidium has a compressed, bipartite, and bivalved cell, whilst these fossils have a globose and entire one.
The fossil forms vary like recent sporangia in being smooth, bristly, or furnished with spines, which in some are simple, and in others branched at the extremity. Sometimes too a membrane may be traced, even more distinctly than in recent specimens, either covering the spines or entangled with them.
Some writers describe the fossil forms as having been siliceous in their living state, but Mr. Williamson informs me that he possesses specimens which exhibit bent spines and torn margins, and thus wholly contradict the idea that they were siliceous before they were imbedded in the flint. In the present state of our knowledge it would be premature to attempt identifying the fossil with recent species: it is better therefore, at least for the present, to retain the names bestowed on the former by those who have described them. A paper on fossil Xanthidia by Mr. H. H. White, containing descriptions of eleven supposed species, accompanied by characteristic figures, may be consulted with advantage*.
In all the Desmidieae, but especially in Closterium and Micrasterias, small, compact, seed-like bodies of a blackish colour are at times met with. Their situation is uncertain, and their number varies from one to four. In their immediate neighbourhood the endochrome is wanting, as if it had been required to form them, but in the rest of the frond it retains its usual colour and appearance. I cannot satisfy myself respecting the nature of these bodies, but I believe them either to arise from an unhealthy condition of the plant, or else to be parasitic.
The only account I have seen of the discovery of fossil
* Microscopic Journal, vol. ii. p. 35.
fronds of Desmidieae is by Professor Bailey, who detected various species of Closterium and Euastrum in calcareous marls, collected in New Hampshire and New York by Professors Hubbard. and Hall, and also in marl at Scotchtown, New York, by Mr. Connors *. Professor Bailey informed me, that the specimens from the last-named station, and in which he found several Closteria, Euastra, &c., were taken from below the bones of the Mastdon giganteus. As sporangia of the Desmidieae and other membranous bodies in a fossil state have lately been detected by Mr. Deane and Dr. G. Mantell in the grey chalk of Folkestone, it is probable that a careful search in that neighbourhood would also bring to light the fossil fronds of the Desmidieae.
The production of the Desmidieae in newly-formed collections of water is involved in obscurity. The late Mr. Miller of Penzance pointed out to me an instance of this kind well-worthy of notice. He found Hyalotheca dissiliens and other species of this family in an old water-butt, which stood in a yard remote from any apparent station for the Desmidieae, and derived its water from the clouds alone; and the question naturally arises, How came the Algae there ? The theory of spontaneous generation has never obtained currency in this kingdom, and for my own part I am not unwilling to acknow-ledge that there are mysteries in nature which we cannot pene-trate. I can therefore only attempt to account for the appearance of the Desmidieae under such circumstances in two ways, - by supposing either that the atmosphere contains countless myriads of the sporules of the Desmidieae and other Cryptogamia, which vegetate only when they meet a congenial situation, or that the seeds are conveyed by means of aquatic insects, many of which, it is well known, roam during the night by means of their wings from one piece of water to another. The latter I consider the more probable conjecture.
The entire question of the vegetation of the conjugating
* 15
Algae is far from being understood. A few years back I paid considerable attention to the subject, but without arriving at any satisfactory conclusion. The Staurocarpus caerulescens is not uncommon near Penzance, is generally in large quantity where it occurs, and, from its peculiar colour, cannot escape detection; on these accounts I made it a principal subject of my observation. Although I have yearly gathered it in several pools, and the sporangia are always abundantly produced, I have particularly noticed during five or six years' observation, that it has never in a single instance reappeared in the same pool. At Dolgelley, where also in some years it is common, I met with the same result, with a single exception when I gathered it in one pool for two successive years. I have noticed the same fact with regard to Zygnema curvatum, and I believe it holds good in regard to most if not all the other Conjugatae ; but as they are more liable to be overlooked, I cannot speak of them with the same certainty as of the above-mentioned species. Algae in running water commonly recur every season. I called Mr. Jenner's attention to the subject: we were alike unsuccessful in our attempts to ascertain the cause of this singularity. His observations in general agreed with mine, that the plant will not appear in the same pool for two successive years; but he found too many exceptions to justify any certain conclusion.
All the Desmidieae are gelatinous. In some the mucus is condensed into a distinct and well-defined hyaline sheath or covering, as in Didymoprium Grevillii and Staurastrum tumidum; in others it is more attenuated, and the fact that it forms a covering, is discerned only by its preventing the contact of the coloured cells. In general its quantity is merely sufficient to hold the fronds together in a kind of filmy cloud ,which is dispersed by the slightest touch. When they are left exposed by the evaporation of the water, this mucus becomes denser, and is apparently secreted in larger quantities to protect them from the effects of drought. I have observed more especially that Tetmemorus granulatus
16
and Penium Brebissonii, under such circumstances, form a distinct mucous stratum ; and on this account some authors have placed the latter with the Palmelleae, although when it occurs in water it is less gelatinous than many other species belonging to the family of the Desmidieae.
I have never obtained a clear view of the circulation witnessed by various authors in species of Closterium and Docidium, but I have no doubt that it is correctly described in the following account by Mr. Dalrymple of his own observations: - "A circulation of the fluids within the shell was observed independent of the active molecules; this was regular, passing in two opposite currents, one along the side of the shell, and the other along the periphery of the gelatinous body*." Professor Bailey observes, that "The account by Mr. Dalrymple agrees with what I have witnessed in several species. The currents are very distinct; so much so in fact, that they attracted my attention before I was aware that they had been noticed by others Dr. Williams of Swansea has kindly translated from the German a paper on this subject by Labarzewski
I now approach a question on which I feel the greatest anxiety, lest I should not do justice to the arguments of those from whose opinion I may differ, or should fail satis-
* Annals of Natural History, vol. v. p. 416.
† American Journal of Science and Arts, vol. xli. p. 300.
‡ Linnaea, 1840, p. 278.
factorily to impress upon my reader the reasons which have appeared to my own mind incontrovertible.
The question is, - are the Desmidieae animals or vegetables?
The arguments I have seen advanced in support of their animal nature appear to me so inconclusive, when contrasted with those adduced in favour of their being vegetables, that, although in the course of a long scientific correspondence I have sought to become thoroughly acquainted with the facts relied upon by the advocates of the former opinion, I have at times almost doubted whether my distance from the metropolis may not have precluded me from the opportunity of hearing others of a more convincing description.
I will however claim the merit of being at least desirous of stating the case impartially, and I have in vain consulted some distinguished naturalists who differ from me, that I might learn whether their opinions were supported by other reasons than those which are so generally known, and to which I shall presently refer. I will also add, what may be a fact of some weight, that I formerly considered the Desmidieae animals, and the Diatomaceae vegetables, and that careful observations alone have in a great measure reversed my opinions. The Desmidieae I now believe have as strong a claim as the Conjugatae or Palmellae can have to rank with the Algae. On the other hand I consider the proper station of the Diatomaceae very doubtful. They have at least as much right to a place in the animal as in the vegetable kingdom; and perhaps the safest course would be that adopted by several celebrated continental naturalists, who regard them as belonging to a distinct and intermediate group, and partaking almost equally of the characters of animal and of vegetable.
The chief reasons advanced by Ehrenberg and others for placing the Desmidieae in the animal kingdom are the following: - that they exert a voluntary motion; that they increase by transverse self-division; and that the Closteria have at their extremities apertures and protruding organs
18
continually in motion. Although two of these reasons apply only to the genus Closterium, I freely admit that if the Closteria can be proved animals, the question as to the other genera will be decided.
Few indeed hint at such a motive for adopting the opinion, nevertheless I feel persuaded that the animal-like forms of the Desmidieae have had a great, though not avowed, influence on the determination of this question. Did we trust solely to the eye. we should conclude that objects so different in form and variable in appearance, were far more like animals than vegetables. Their symmetrical division into two segments; the beautiful disciform, finely-cut and toothed Micrasterias, the lobed Euastrum, the Cosmarium glittering as it were with gems, the Xanthidium armed with spines, the scimitar-shaped Closterium embellished with striae, the Desmidium resembling a tape-worm, and the strangely insect-like Staurastrum sometimes furnished with arms, as if for the purpose of seizing its prey; - all these characters seem indeed to pertain more to the lower animals than to vegetables. We are thus induced, however unconsciously, to judge before examination, and we naturally search for arguments in support of our preconceived opinion instead of those which may elicit the truth. But experience has proved that form alone is a most deceptive guide, the implicit dependence on which has, in many similar instances, been the cause of error ; and I believe that if a person unacquainted with what has been written respecting the two groups should look at the representations of the Desmidieae, and examine the graceful and arborescent Zoophyte, having its branches to all appearance loaded with fruit that is periodically produced, matured, and shed, he would without hesitation place the Desmidieae in the animal, and the Zoophyte in the vegetable kingdom. Nor could we wonder at such a decision, since in former times, even observers of high scientific attainments, judging by external appearances, did in fact class Zoophytes with the Algae.
Again, Ellis, who first established the animal nature of
19
the Zoophytes, and thus dispelled one error, presently committed a similar fault by transferring along with the Zoophytes (simply on account of their agreement in certain external characters), Corallina and other genera which, it is almost unnecessary to add, have not until a recent period been restored to their proper place amongst the Algae*. The above is a sufficient illustration of the danger of trusting solely to the evidence of sight : I shall endeavour to show that it would in the present case also lead to an incorrect conclusion.
That the Desmidieae have been so long associated in the same family with the Diatomaceae, whose proper position is so doubtful, presents another obstacle to the recognition of the claims of the former to rank with the Algae. For when so many eminent observers, -botanists and zoologists, notwithstanding they differ widely in respect of the department of Natural History to which these forms should be assigned, - concur in classing them together, we naturally suppose that there must be valid reasons for such a course. I shall therefore point out the distinctions between these groups, and show that, at all events, they can no longer be united in one family, but must be separated, as they have been by Kützing in his more recent work, as well as by others.
I have shown that the cell in the Desmidieae consists of two valves united by a central suture, and that during its division the new-formed portions are interposed between these valves. The Desmidieae are membranous, or should a few species contain silica, it is not present in sufficient quantity to interfere with their flexibility. They rarely have acute angles, and are seldom (if ever) rectangular. They are often deeply incised or lobed, warted or spinous. The internal matter is of a herbaceous-green colour, and starch vesicles abound in the mature cell. They couple and form either orbicular or quadrate seed-like bodies, and are remarkable for the resistance which they oppose to decomposition.
* See the introductions to Johnston's admirable works on the British Zoophytes and Corallines.
20
In all these respects they differ from the Diatomaceae. In the latter each frustule consists of three pieces, one central and ring-like or continuous all round, and the others lateral. The division is completed by the formation of new portions within the enlarged central piece, which then falls off, or else by a new septum arising at the centre; but I believe that, in every case, the separation commences internally before it extends to the covering *. Their coverings, with very few exceptions. are siliceous, withstand the actions of fire and acids, and may be broken but not bent ; the frustules are often rectangular in form, are never warted, and scarcely ever spinous. Their internal matter is usually brown when recent; and, although some species are greenish, or become green after they have been gathered, none are of a truly herbaceous colour. Their vesicles bear some resemblance to those in the Desmidieae, but, they are of a yellower colour, and no starch has been detected in them. The Diatomaceae do not conjugate†, and in general they very soon give out an offensive odour.
I have preferred treating of these topics first and at some length, because I wish the reader to enter upon the subject unprejudiced, so that after a fair examination of the conflicting arguments he may be ready to surrender his judgement to that which is supported by facts of greater weight.
The first reason advanced by Ehrenberg in support of the animal nature of the Desmidieae is, that they possess a "voluntary motion ;" but I must protest against his use of the term voluntary as prejudging the matter in dispute and assuming more than in the present state of knowledge can be ascertained; more indeed than he has attempted to prove.
That the Desmidieae move must be admitted; for this fact has been noticed by too many accurate observers to permit
*
† Since the above was in type Mr. Thwaites has detected four species of Diatomaceae in a conjugated state; the sporangia are elongated and in pairs, and the internal matter is similar to that of the frustules.
21
any doubt of its truth, and although I have myself failed to perceive their actual movement, I have sufficient evidence of its occurrence. But, whilst making this admission, I must still maintain that in the lower tribes of organic life motion is not an indubitable sign of an animal nature *, and that the movements of the Desmidieae most be very sluggish, or exercised only under peculiar circumstances, since I have never witnessed it, notwithstanding I have almost daily living specimens under my inspection. Mr. Jenner has been equally unsuccessful, and several friends, experienced in the use of the microscope, either have not seen it or speak of it in uncertain terms†.
Professor Bailey states that "their power of locomotion
* "Motum non determinare limites regni animalis exinde patet, quod sunt animalia, quae non moventur, vegetabilia in quibus motum vividum videmus." Agardh, Conspectus Diatomacearum, p. 4.
"The active motions in plants and their parts, especially in Algea, ought not to give rise to the supposition of an animal nature, even when they are called infusorial or animal motions." - Ehrenberg, Taylor's Scient. Mem. vol. i. p. 566.
† I subjoin the opinions of various observers : -
" Actual motion, arising from internal causes, I saw only in Sphaerastrum ; and the slight movement, supposed to have been observed in some of the genera, is certainly of the same description as that of some Confervae."- Meyen, I839, as quoted in Pritchard's Infusoria, p. 180.
"These are animals instead of plants, if the faculty of locomotion will entitle them to that rank."- Carmichael, in Hook. Br. Fl. vol. ii. p. 398.
"It was impossible to determine whether the vague motions of Closterium were voluntary or not."--Dalrymple, see Annals of Nat. Hist. vol. v. p. 417.
"Motions apparently voluntary are easily seen . . . . I have seen Euastrum margaritiferum move quite distinctly." - Bailey, American Bacillaria.
"Elles n'ont pas un mouvement sensible sur le porte objet du microscope. Cependant il est facile de remarquer dans les localités où elles vivent où dans les vases où on les conserve, qu'elles se dirigent vers la lumière et se rapprochent en pellicules où en sortes de pinceaux d'un beau vert, réunies entre elles au moyen d'un mucus qui les entoure ordinairement." -- Brébisson, in Chevalier's ‘Des microscopes et leur usage.'
"The various species of Closterium, as well as the closely allied Euastra have a distinct motion which cannot be referred to any extrication of gas. I have had species of Closterium and Euastrum confined in a compressor, in water perfectly free from other bodies, and they moved so fast that I found it impossible to sketch their forms by the camera lucida until they were killed." - Professor Bailey in lit.
22
may be rendered apparent by taking a portion of mud covered with Closteria, placing it in water exposed to light, and then if the Closteria are buried in the mud they will soon work their way to the surface, covering it again with a green stratum *." I have myself frequently observed this fact, especially in Tetmemorus granulatus and Penium Brebissonii; but I presume that it is owing to the stimulus of light rather than to any voluntary effort; at any rate the same result will follow if an 0scillatoria be substituted for the Closterium. In like manner, in some species of Nostoc, Anabaina, and Palmella, the filaments or cells throng towards the light; and should the specimen he turned over, they will in a few hours appear on the new surface, whilst they have become less numerous on the previous upper one. Of course the gelatine must be in a sufficiently lax state to permit such a movement. Another proof of their power of locomotion is afforded by their retiring, in some instances, beneath the surface when the pools dry up. I have taken advantage of this circumstance in order to obtain specimens less mingled with foreign matters than they would otherwise have been. If a species be much mixed with mud, I take a saucer, fill it with earth made into a paste with water, and cover it with a piece of linen; over this I spread a thick layer, containing the Desmidieae, and allow it to become nearly dry; within a few days the specimens will form a stratum on the linen, and may be scraped off with a knife. This plan however proves successful only with the smaller species, and minute Algae accompany the Desmidieae which are so obtained.
It is thus evident that whatever be the motive power of the Desmidieae they possess it only in common with acknowledged Algae, and in a less degree than either the Diatomaceae, the Oscillatorieae, the sporules of various Algae, or indeed their own sporules.
Ehrenberg considers that increase by voluntary division is the character which separates animals from vegetables †, and,
*American Bacillaria, in American Journal of Science and Arts, vol. xli. p. 300.
†
23
in fact, he produces no other reason for denying the vegetable nature of some genera. But it will probably be thought a sufficient answer to mention that Meyen and others have proved the growth by elongation and bisection of the cells to be very frequent, if not universal, in the more simple Algae * .
Meyen remarks that Ehrenberg has interpreted " all the facts known as if these creations were undoubtedly animals, whilst the same facts would indicate quite a different signification if we proceeded upon the supposition that they were nothing but plants†," In the present instance we have a striking example of the truth of this observation, since Ehrenberg, on observing that division occurs in the lower animals, too hastily draws the inference that only animals can be the subjects of it.
0n the other hand, Mr. Shuttleworth, in a letter to Mrs. Griffiths, written in the spring of 1842, described the growth of the simple Algae by repeated spontaneous division of their cells, and illustrated it by sketches of Zygnema nitidum, &c. He states that the process is most evident in the 0scillatoriae, in Conferva fugacissima, in Lyngbya muralis, and in the Conjugatae; and, after noticing the similar process in the Diatomaceae, he adds, " I trust that now their vegetable nature is beyond all doubt; " thus showing that from the same premises two able observers came to exactly opposite conclusions. It is right to mention that Mr. Shuttleworth did not claim the credit of being the original discoverer, but merely said that he had convinced himself of the correctness of the facts published.
To Mr. Hassall is justly due the credit of first directing the attention of British naturalists to this mode of growth;
*
† See Pritchard's Infusoria, p. 178.
24
and, as it appears he was not aware that it had previously been noticed by continental botanists, another proof is thus afforded that it has attracted the attention of independent observers.
The fact of the transverse bisection of the Algae is now so well established that this character is employed by J. G. Agardh, in his generic definitions *.
In a preceding part of this introduction, I have endeavoured to show that in the Desmidieae the process is the same as in all the simple Algae.
If some should think that I have dwelt too long upon this topic, I trust I shall stand excused when it is considered that the celebrated Ehrenberg places much reliance upon this character, and that great weight is justly allowed to the authority of one who has devoted so much time and skill to the examination of the Desmidieae; moreover, that his arguments are familiar to the British naturalist, and his opinions extensively embraced, whilst the opinions of those who differ from him on this subject are comparatively unknown.
I am not in a position either to deny or to affirm with confidence the presence of openings in the extremities of the Closteria, for in objects so minute it is very difficult, frequently perhaps impossible, to distinguish between a depression and an opening †. It appears indeed to me that in Closterium there is a slight notch, or more usually the rudiment of one, at the apex of the segments, a mere indication in short of what is fully developed in Tetmemorus and Euastrum; for in C. Dianae, and some other species, there is an evident though minute notch, and in Penium not even a trace of it can be detected. In no instance can any portion of the contents of the cell be forced out from the extremities.
*
† " No one accustomed to the use of the microscope can be otherwise than aware how much very minute objects seen under a high power are apt to assume a character in accordance with preconceived notions."--J. T. Smith, in Annals of Natural History, vol. xix. p. 2.
25
Mr. Dalrymple believes that orifices exist which are closed by internal membranes.
Ehrenberg, on the contrary, describes not merely open orifices, but protruding organs or feet immediately behind them, and thus affords an example of his want of caution when he gives the rein to his poetic fancy; for in this instance at least he is clearly in error. Mr. Dalrymple, whose remarks on the Closteria are the undoubted result of careful examination, carried on with the desire of recording actual facts rather than of supporting a theory, denies the existence of any papillae or proboscides at this part, and also the supposition of Ehrenberg, that the moving molecules seen near the extremities constitute the basis of such papillae. This admission is the more valuable as coming from a person who is not only an accurate observer but a believer in their animal nature. Professor Bailey also informs us that he has failed to detect them.
In most of the Closteria there is at each extremity of the endochrome a distinct globule containing moving granules. Ehrenberg seems to have mistaken these for organs of motion. In Closterium rostratum and C. setaceum these granules are situated at a distance from the extremities, and, as they are apparently not contained within a globule, move more freely and afford better opportunities for observing them than those in the other species do. In one instance I saw them continue to move after escaping from the frond. Did the Closteria possess mouths we should expect to find them in every species; but Penium interruptum, in which there are conspicuous terminal globules and moving granules, has decidedly nothing of the kind; and as its end view is circular and rather turgid no more desirable plant could be selected for examination. If mouths are present they ought to be visible in this and the allied species, but not a single punctum can be detected in the end view of the empty frond.
I have now passed in review the four points advanced by Ehrenberg in support of the animal nature of the Desmidieae, and I submit that I have shown that from them no evidence
26
approaching to a satisfactory proof of his position can be deduced. In offering these remarks I hope I shall not be suspected of the least wish to undervalue Ehrenberg's discoveries. I believe that he is sometimes too hasty in arriving at his conclusions, and that his knowledge of the Desmidieae is less accurate than of the Diatomaceae and other tribes which have come under his observation. When we consider the extent of his researches, it is not surprising, eminent as he is, that be should fall into some errors, which a person inferior in ability, but confining his whole attention to a comparatively smaller range, is likely to detect. Nor even in respect of this family, although I must consider it the most defective portion of his great work, are the obligations slight which we owe to Ehrenberg : for not only has he enlarged our knowledge of the Desmidieae by the discovery of many new facts, by his discrimination of the species, and by producing better representations than those which previously existed, but be has given an impulse to the study, and rendered it popular.
The fifth volume of the 'Annals of Natural History *' contains an abstract of a paper by Mr. Dalrymple on the Closteria, read before the Microscopical Society of London, from which I quote the following reasons for placing them in the animal kingdom:--
" 1st. That while Closterium has a circulation of molecules greatly resembling that of plants, it has also a definite organ, unknown in the vegetable world, in which the active molecules appear to enjoy an independent motion, and the parietes of which appear capable of contracting upon its contents.
" 2nd. That the green gelatinous body is contained in a membranous envelope, which, while it is elastic, contracts also upon the action of certain reagents whose effects cannot be considered purely chemical.
" 3rd. The comparison of the supposed ova with cytoblasts
*P.415.
27
and cells of plant precludes the possibility of our considering them as the latter, while the appearance of a vitelline nucleus, transparent but molecular fluid, a chorion or shell, determines them as animal ova. It was shown to be impossible that these eggs had been deposited in the empty shell by other infusoria, or that they were the produce of some entozoon.
" 4th. That while it was impossible to determine whether the vague motions of Closterium were voluntary or not, yet the idea the author had formed of a suctorial apparatus, forbad his classing them with plants."
I confess I am unable to refer to any example in other Algae of terminal globules like those present in the Closteria, bot neither can one be found amongst animals; and if in some respects they have an analogy with organs belonging to the latter, in others they agree better with vegetable life. The contained granules seem to me to differ in no respect, except in position and uninterrupted motion, from other granules in the same frond, and, as I have already stated, I once saw the motion continue after their escape from the cell precisely as in other zoospores. Meyen observes that "the functions of these bodies is very difficult to determine, but they are to be found in very many Confervae, and are perhaps to be likened to the spermatic animalcules of plants."
The contraction of the internal membrane of the Closteria, or the expulsion of their contents on the application of iodine or other reagents, cannot be relied upon as a satisfactory test for determining their nature, for the blandest fluids will, in some cases, occasion violent action. If fresh water touches Griffithsia setacea, the joints burst and spirt out their contents; and if it be applied to a species of Elachistea, the granular contents are instantly thrown into commotion. In certain conditions of the Closteria themselves, water will produce effects like those attributed to reagents. I have frequently witnessed that by the addition of water to specimens of Closteria, which had for some time been kept merely in a damp state, the frond has been ruptured and the endo-
28
chrome has escaped with considerable force. This is especially the case with Closterium Lunula; and the same circumstance occurs in most of the Desmidieae, although in none of the others so remarkably as in that genus. Mr. P. Grant has favoured me with an account of numerous experiments which he instituted to determine the effects of different substances on the Desmidieae, Conjugatae, &c. From these it seems that the action of a reagent cannot be predicated with any certainty, and that the molecular motion is not affected by several strong poisons, whilst it yields to other substances less generally deleterious.
With regard to the " supposed ova," I fully agree with Meyen that " they are similar to the green corpuscles found in the cells of the Confervae," and, when both are removed " from their cells, I am persuaded that it would be impossible to distinguish between them. That they contain amylum has been, with good reason, pointed out by Meyen as decisive against the notion that they are eggs.
The latest advocate of the animal nature of the Desmidieae is C. Eckhard, in his memoir ‘on the Organization of the Polygastric Infusoria,' published in Weigmann's Archiv, part 3, 1846. With this memoir I am acquainted only through the medium of a translation by Dr. J. W. Griffiths in the ‘Annals of Natural History*.' It is apparently written for the purpose of confirming Ehrenberg's views, and accordingly defends his accuracy in every respect. So far as regards the Desmidieae, Eckhard notices only the Closteria, and he advances nothing additional except an opinion that the transverse suture is a fissure or mouth. In order that both sides of the question may be compared, I extract his remarks, and merely point out that he has left unnoticed the facts on which botanists place their chief reliance : --
"The grounds for their being of animal nature are derived partly from their motion, partly from their organization. On the leaves of Ceratophyllum, I observed the manner in
*
29
which several Closteria adhered elegantly by one extremity; in about a quarter or half an hour many of them were situated in the same manner upon a higher part of the leaf: not a single animalcule was found on the side of the leaf, nor adherent longitudinally to it. They had evidently moved during the above time from the lower to the upper part of the leaf. If we observe their motions under the microscope they are not so rapid as those of many other polygastric infusoria, but the motion is always evidently animal. They swim, especially in summer, in the most varied directions, and I have frequently seen Cl. acerosum and Lunula swim against the current when the water on the object-holder was flowing towards one side, whilst fragments of plants, various kinds of Spirogyra and Oscillatoria were carried away. It is difficult here to discover anything but animal motion; to explain this however by electricity, as Turpin attempted, is unnatural, and not less absurd than that of the muscular fibre by the same natural agent by Strauss. But the relations of the organization of the Closterina are likewise in favour of their animal nature. In illustration of this I shall confine myself to Cl. acerosum. We see that the animal,which is expanded in the middle, is elongated symmetrically on each side. In the middle there is a transverse fissure, which probably serves for the admission of nourishment; since, when this animal is kept for some time in coloured water, we perceive little accumulations of the colouring matters. At the extremities we see on each side a vesicle, in which minute granules (?) incessantly move. In other species there is moreover a small aperture; it is situated more posteriorly, and is perhaps connected with the cell. Ehrenberg twice saw in this animalcule filaments (feet ?) project from it. Internally there are on each side two or four cords, and a row (in other kinds several) of granular bodies. In the species figured, I have so often seen the above change in relative position, that I have been compelled to wait until they again appeared in their original position in order to delineate them. All this is not plant-like ; and if the carapace
30
of the Closterina should prove to be of a horny nature, as would appear to be the case from their becoming wrinkled when heated, they would be removed from the vegetable kingdom with still greater certainty."
Although at first sight it seems to indicate the contrary, the swarming of the zoospores or granules really affords a strong confirmation of the vegetable nature of the Desmidieae. This swarming has been noticed in the Algae by many eminent observers, and notwithstanding the extraordinary phaenomena which it presents, no fact in their history is more firmly established. Hence J. G. Agardh, who so worthily emulates his father's fame, has been induced to apply the name of "Zoospermeae" to one of his three primary divisions of the Algae. In this country the swarming has been witnessed by Mr. Borrer, the Rev. M. J. Berkeley, Mr. Hassall, and others. I have frequently seen it in Sphaeroplea crispa and Draparnaldia tenuis. A commotion arises within the cell as if all its contents were suddenly endued with life; the disturbance increasing, the cell, opens, when the zoospores hasten from their prison, and, apparently enjoying their newly-acquired liberty, dart about in every direction, until, tired of their sport, they at length resume a quiescent state *.
This description has such a marvellous character that the reader may suppose it is somewhat indebted to the imagination, and I believe no one can witness the occurrence for the first time without being startled and almost led to doubt the evidence of sight. Such movements are so contrary to our ordinary experience of vegetable life, that we involuntarily hesitate to admit their compatibility with it; and on the continent many eminent naturalists, unable to find a satisfactory explanation, consider that in this stage the zoospores are really animals, and do not acquire their vegetable nature until a subsequent period. This opinion never obtained countenance in this country: yet a Berkeley did not
* The fullest details accessible to the English reader will be found in Mr. Harvey's excellent introduction to his ´ Manual of British Algae,' and in the introduction to Hassall's ´British Freshwater Algae.'
31
esteem its refutation unworthy of his pen, and a Harvey thought it necessary to record his dissent from it and his belief that the phenomenon must be regarded as a "strictly vegetable peculiarity." All that I have alluded to as happening in the Algae occurs also in the Desmidieae; but no similar motion has ever been observed in the contents of an animal after their escape from the individual, and I therefore claim its presence in the Desmidieae as a strong presumptive evidence in support of their vegetable nature.
Starch has been observed in the Desmidieae by many persons, and I am not aware that its occurrence is now denied by any one. I should therefore have supposed that it hardly required further proof, but as the subject is altogether unnoticed by most writers on the opposite side, it becomes necessary to bring it fully before the reader.
Meyen first directed attention to the presence of starch as a conclusive proof that the Desmidieae are Algae. He states that in several genera he has "distinctly seen that the large and small granules contained amylum, and were sometimes even entirely composed of it *," and that " in the month of May he had observed many specimens of Closterium in which the whole interior substance was granulated, and all the grains gave with iodine a beautiful blue colour, as is the case with starch, which is not an animal product." I know not that Ehrenberg or his followers have ever repeated Meyen's experiments or taken the slightest notice of them, important as they are to the solution of the disputed question. In this country, Mr. Dalrymple at first failed to detect starch in the Closteriae, but afterwards observed it in specimens of Closterium (Penium) Digitus, which I sent him, and acknowledged its presence in the following terms: - "I have examined the specimens sent up, and in several I can detect the blue colour of the iodide of starch; this is by no means however universal, some being merely stained yellowish-brown; but in those instances there appears to be an absence
* See American Journal of Science and Arts, vol. xli. p. 298.
32
of granular matter, the fact of blue granules in some is how-ever decisive of the presence of starch." Mr. Dalrymple is so careful and accurate an observer, that I doubt not his failure was caused by the immature state of the specimens on which be experimented, as he indeed subsequently allowed might have been the case.
Professor Bailey of the U.S. Military Academy, in an article on the American 'Desmidiaceae' *, gives copious extracts from Mr. Dalrymple's paper on the Closteria, accompanied by his own remarks. He bears testimony to the general correctness of Mr. Dalrymple's observations, but with regard to those on the action of iodine, he says, "I cannot otherwise account for Mr. Dalrymple's statement, that iodine , 'in no instance produced in the Closteria the violet or blue colour indicating starch,' than by supposing that the specimens he examined were not in the proper state to exhibit it. Meyen expressly states that it is , 'at certain times, particularly. in spring ,' that the starch may be detected. I am able by conclusive experiments to confirm Meyen's statement as to the presence of starch in these bodies. In specimens gathered in November I find no difficulty in producing the blue colour with tincture of iodine. Sometimes however the specimen becomes so opake by the action of this reagent, that the purple colour of the granules can only be detected after crushing the specimen by means of the compressor. The characteristic colour of iodide of starch is then shown most distinctly. I have repeatedly treated in this way Closterium Trabecula, as well as others, and have uniformly found that a portion of the interior takes the purplish colour †."
I have myself repeatedly noted the effects of iodine on many of the Desmidieae. In a young state the cells are filled with a green homogeneous fluid, which as the plant approaches to maturity becomes granular. Scattered amongst
* †
33
this minutely granular matter, larger granules, or rather vesicles, make their appearance. These Ehrenberg calls ova; but I cannot perceive the slightest difference between them and the granules present in the higher Algae.
Professor Bailey's testimony is the more satisfactory because it is not the evidence of a partisan, but the admission of one inclined to regard the Desmidieae as pertaining to the animal kingdom. Although he does not " consider the presence of starch in these bodies as conclusive evidence that they are plants," yet as a professor of chemistry he is aware that starch is not an animal product, and he suggests therefore, with some ingenuity, " Is it not possible that they are animals which feed, wholly or in part, on amylaceous matter extracted from the aquatic plants among which they live ? if so, the detection of starch in their stomachs is not surprising."
In the young cell there is no starch, but after its first appearance it continues to increase and is most plentiful in the sporangium; facts quite incompatible with Professor Bailey's
34
suggestion, but strictly analogous with its presence in the Conjugatae, and indeed in plants in general, for it is well known that starch is often most abundantly produced in the seed.
Of all the facts which indicate the vegetable nature of the Desmidieae, this is undoubtedly the mast important, since it is the most easily subjected to the test of experiment. The swarming of the zoospores is seen only now and then, and it is not always easy to trace the process of coupling; but every person can apply the test for starch, and needs only to bear in mind that unless granular matter be present there is no starch *.
The conjugation of the fronds in this family supplies an equally striking fact in proof that it belongs to the vegetable kingdom. I have already mentioned that we have amongst the Algae many examples of the junction of two individuals†, and the commingling and union of their entire contents into a seed-like body for the purpose of reproduction. The case of the Conjugatae is well known, and Mr. Hassall has proved that the same phenomenon takes place in other genera.
*
†
35
In the Desmidieae, in like manner, a bag or cell forms between two individuals, and the entire contents of both these (or indeed of four, if we regard the fronds as binate) pass out and unite together to form one reproductive body, which becoming detached, leaves the parent corpuscles altogether empty. Such an occurrence is, I believe, not only unknown amongst animals, but is contrary to all our notions of animal propagation.
Among the Conjugatae, Tyndaridea has an orbicular spo-rangium, Staurocarpus a cruciform or quadrate one; in Zygnema it is formed within one of the coupling cells, and in Mougeotia it is situated in the connecting tube. The Desmidieae present us with corresponding variations. Their sporangia are generally orbicular, but Staurastrum, Tetmemorus, Closterium and Penium afford examples of cruciform and quadrate ones: and although the reproductive body is usually contained in the connecting tube, yet in Didymoprium Grevillii it is placed in one of the conjugating cells. Lastly, we have the Didyoprium Borreri, in which the conjugated filaments form a kind of network in the same manner as in Mougeotia.
That the Desmidiae resist decomposition, exhale oxygen on exposure to the sun, preserve the purity of the water containing them, and when burnt do not emit the peculiar odour usually so characteristic of animal combustion, are other facts respecting this family, which taken singly might have less value, but in their combination furnish a most important support to the arguments already adduced.
On the foregoing statements I rest the claim of the Des-midieae to be considered Algae ; and I confidently appeal to any impartial person whether they do not at least require to be answered before the conclusion which I have drawn can be rejected. But on consulting the works of those who profess to prove the contrary, we find that the important facts which I have here investigated are either altogether overlooked, or passed by without an attempt to controvert them. For my own part I believe them to be unanswerable.
I have pointed out that the swarming of the granules
36
occurs in acknowledged Algae as well as in the Desmidieae; and that nothing which can fairly be compared with it occurs in the animal kingdom. Can either of these assertions be disproved? Again, it has been seen that starch is abundantly produced in this family. Can a single example be referred to where it is an animal product? I have shown that the reproductive body is formed in a manner well known amongst the Algae, but never detected in animals. Until these facts have been denied, or the arguments deducted from them refuted, I shall presume that the claim of the Desmidieae to be considered vegetables is firmly established*.
The Desmidieae I regard then as Algae, allied on the one side to the Conjugatae by similarity of reproduction, and on the other to the Palmelleae, by the usually complete transverse division and by the presence of gelatine. Indeed the relation to the latter is so intimate, that it is difficult to say to which
*
"The Closteria and the Desmidieae are universally plants and not animals. In the actual state of science we are obliged to admit this proposition. The organic structure, the physiological phaenomena, the history of their development, their chemical composition, manifest in these beings a perfect correspondence with others, which, under all their aspects, are comprised in the abstract idea of a plant. On the other hand, what they present in common with those beings evidently animal is but an appearance, or at most a similarity of external form. Ehrenberg was deceived by this appearance, and, guided by this fallacious resemblance, thought he discovered even in the Desmidieae the same organic peculiarities which prove the animality of other beings. What must we infer from this ? That even the most accurate observer and man of genius may err. This can never diminish his merit or render less important the services which he has rendered to Science. The loss will fall only on those who, averse to the fatigue of observing, content themselves with the authority of the master, and embrace without distinction his real discoveries and his errors. Thank Heaven, the epoch of authority is passed, and whoever submits to its yoke may be allowed to err, because science will not advance the less for him, and may even derive advantage from those very errors. From the study of the Desmidieae, and from their being brought into comparison with animals, valuable notions upon the in-timate structure of vegetables have already been derived." - Sulla Animalita della Diatomee, p. l72.
37
family some genera belong. Thus Merismopaedia, placed by Meneghini and Kützing in the Desmidieae, I believe, with Meyen, to belong properly to the Palmelleae. On the other hand, Cylindrocystis, placed in the latter by Meneghini and Brébisson, has been rightly removed to the Desmidieae by Kützing and Hassall. Brébisson's discovery of conjugated specimens of Coccochloris protuberans and Cocc. rubescens brings to our notice another link connecting these families, whilst some species of Scenedesmus may be allowed to have an almost equal claim to rank with either.
Respecting the uses of the Desmidieae little is known. Doubtless, in common with other aquatic vegetables, they tend to preserve the purity of the water in which they live, and Mr. Williamson has ascertained that to a great extent they furnish food for the bivalved mollusks which inhabit fresh waters.
As the Desmidieae are unattached and very minute, they are rarely gathered in streams : nevertheless interesting species may occasionally be obtained where the current is so sluggish as to permit the thin retaining mucus to elude its force. In small shallow pools that do not dry up in summer they are most abundant; hence pools in boggy places are generally productive.
The Desmidieae prefer an open country. They abound on moors and in exposed places, but are rarely found in shady woods or in deep ditches. To search for them in turbid waters is useless: such situations are the haunts of animals, not the habitats of the Desmidieae, and the waters in which the latter are present are always clear to the very bottom.
M. de Brébisson informs me that in France, calcareous districts, which are so favourable to the Diatomaceae, are very unproductive of Desmidieae. I have myself had no opportunity of ascertaining whether the same fact obtains in England. Mr. Thwaites and Mr. Jenner have indeed furnished me with fewer habitats from calcareous soils, but the less frequent occurrence of small pools and bogs in such districts may perhaps partly explain the deficiency.
38
In the water the filamentous species resemble the Zygnemata, but their green colour is generally paler and more opake. They often occur in considerable quantity, and, notwithstanding their fragility, can generally be removed by the hand in the usual manner. When they are much diffused in the water, I take a piece of linen about the size of a pocket-handkerchief, lay it on the ground in the form of a bag, and then, by the aid of a tin box, scoop up the water and strain it through the bag, repeating the process as often as may be required. The larger species of Euastrum, Micrasterias, Closterium, &c., are generally situated at the bottom of the pool, either spread out as a thin gelatinous stratum, or collected into finger-like tufts. If the finger be gently passed beneath them they will rise to the surface in little masses, and with care may be removed and strained through the linen as above described. At first nothing appears on the linen except a mere stain or a little dirt ; but by repeated fillings-up and strainings a considerable quantity will be obtained. If not very gelatinous, the water passes freely through the linen, from which the specimen can be scraped with a knife and transferred to a smaller piece ; but in many species the fluid at length does not admit of being strained off without the employment of such force as would cause the fronds also to pass through, and in this case it should be poured into bottles until they are quite full. But many species of Staurastrum, Pediastrum, &c. usually form a greenish or dirty cloud upon the stems and leaves of the filiform aquatic plants, and to collect them requires more care than is necessary in the former instances. In this state the slightest touch will break up the whole mass and disperse it through the water. I would recommend the following method as the best-adapted for securing them. Let the hand be passed very gently into the water and beneath the cloud, the palm upwards and the fingers apart, so that the leaves or stem of the invested plant may lie between them and as near the palm as possible; then close the fingers, and keeping the hand in the same position, but concave, draw it
39
cautiously towards the surface, when, if the plant has been allowed to slip easily and with an equable movement through the fingers, the Desmidieae, in this way brushed off, will be found lying in the palm. The greatest difficulty is in with-drawing the hand from the surface of the water, and probably but little will be retained at first ; practice, however, will soon render the operation easy and successful. The contents of the hand should be transferred at once either to a bottle or, in case much water has been taken up, into the box, which must be close at hand, and when this is full it can be emptied on the linen as before. But in this case the linen should be pressed gently and a portion only of the water expelled, the remainder being poured into the bottle, and the process repeated as often as necessary.
Sporangia are collected more frequently by the last than the preceding methods. When carried home, the bottles will apparently contain only foul water, but if it remain undisturbed for a few hours, the Desmidieae will sink to the bottom, and most of the water may then be poured off. If a little fresh water be added occasionally to replace what has been drawn off and the bottle be exposed to the light of the sun, the Desmidieae will remain unaltered for a long time. I have now before me some specimens of Euastrum insigne, the fronds of which are in as good condition as when I gathered them at Dolgelly five months ago.
Mr. Thwaites's kindness has enabled me to render this introduction more complete by the following account of two methods adopted by him in mounting minute Algae for the microscope, which he has drawn up at my request. The remarks which I have appended have been derived from other sources, as well as from my own experience: -
"In making preparations of the Algae for the microscope, there are two things which principally require to be attended to: first, to obtain a fluid which shall preserve the plant as little altered as possible from its appearance when living; and secondly, to adopt the best means for preventing the escape of this fluid after the object has been mounted in it.
40
With respect to the first point, the fluid which I have found to answer best is made in the following way: - To sixteen parts of distilled water add one part of rectified spirits of wine and a few drops of creosote sufficient to saturate it; stir in a small quantity of prepared chalk and then filter : with this fluid mix an equal measure of camphor-water (water saturated with camphor), and before using strain off through a piece of fine linen.
"This fluid I do not find to alter the appearance of the endochrome of Algae more than distilled water alone does after some time, and there is certainly less probability of confervoid filaments making their appearance in the preparations; and there would seem to be nothing to prevent such a growth from taking place when the object is mounted in water only, provided a germ of one of these minute plants happen to be present, as well as a small quantity of free carbonic acid.
"Fluids containing a larger quantity of spirits of wine, and consequently of creosote also, than the one of which I have given the formula, produce a greater change in the appearance of the endochrome. I at one time thought that by increasing the density of the mounting fluid, the endochrome of the plant might be less disturbed, and I dissolved a small quantity of sugar in the fluid; but this made the cell-membrane too transparent, and rendered completely invisible the gelatinous sheath with which many species of Algae are furnished.
" I now proceed to describe my method of making cells in which to mount preparations of Algae. Some objects require very shallow and others somewhat deeper cells. The former may be made with a mixture of japanners' gold-size and litharge, to which (if a dark colour is preferred) a small quantity of lamp-black can be added. These materials should be rubbed op together with a painter's muller, and the mixture laid on the slips of glass with a camel-hair pencil as expeditiously as possible, since it quickly becomes hard; so that it is expedient to make but a small quantity at
41
a time. For the deeper cells marine-glue answers extremely well, provided it is not too soft. It must be melted and dropped upon the slip of glass, then flattened, whilst warm, with a piece of wet glass, and what is superfluous cut away with a knife, so as to leave only the walls of the cell; these, if they have become loosened, may be made firm again by warming the under surface of the slip of glass. The surface of the cells must be made quite flat, which can be easily done by rubbing them upon a wet piece of smooth marble covered with the finest emery powder.
"When about to mount a preparation, a very thin layer of gold-size must be put upon the wall of the cell as well as on the edge of the piece of thin glass which is to cover it; before this is quite dry, the fluid with the object is to be put into the cell, and the cover of thin glass slowly laid upon it, beginning at one end: gentle pressure must then be used to squeeze out the superfluous fluid, and, after carefully wiping the slide dry, a thin coat of gold-size should be applied round the edge of the cell, and a second coat so soon as the first is dry: a thin coat or two of black sealing-wax varnish may then be put on with advantage, in order to prevent effectually the admission of air into the cell or the escape of fluid out
of it.
"I at first mounted objects for the microscope without enclosing them in a cell previously prepared for their reception, but merely by laying them on the slip of glass with a drop of the fluid, and then covering them up with a piece of thin glass or talc, and afterwards surrounding the latter with a border of thick gold-size, in order to prevent the evaporation of the enclosed fluid. Preparations so made will frequently last some considerable time; but eventually the contraction, as it becomes dry, of the outer surface of goldsize forces the remainder, which still continues soft, between the two glasses, and the mounted object is thus injured. I found the same thing frequently to occur when the cells were made of gold-size only without the litharge; but this
42
inconvenience seems to be completely obviated by the plan I have recommended above.
"I would remark that the gold-size employed should be of the consistence of treacle ; when purchased it is usually too fluid, and should be exposed for some time in an open vessel, a process which renders it fit for use."
Mr. Sidebotham also has favoured me with directions for mounting the Desmidieae. His method is nearly similar to that employed by Mr. Thwaites, but when the last coat of varnish is nearly dry, he applies a fine bronze with a camelhair pencil.This not only has a neat effect, but prevents the risk of adhesion consequent on the softening of the varnish in warm weather. Mr. Sidebotham uses distilled water as the mounting fluid, but although his specimens retain the green colour of the endochrome better than any other I have seen, yet, for the reason assigned by Mr. Thwaites, I doubt whether such a mode is suitable for their permanent preservation.
Mr. Thwaites' fluid is superior to camphorated-water and various other liquids which I have tried; but as it requires more time for its preparation than may be at all times convenient, I find the following the best substitute: -
Bay-salt and alum, one grain each, dissolved in an ounce of distilled water.
Goadby´s solution acts too powerfully on freshwater Algae, and corrosive sublimate injures the specimens.
In mounting the Desmidieae great attention is necessary to exclude air-bubbles, which cannot be avoided unless the fluid completely fills the cell; and also not to use too much fluid, as in this case the smaller species will often be washed away on the escape of the superfluous portion.
As the cells cannot be sealed whilst any moisture remains on their edge, it should be removed by blotting-paper, which is preferable to any other mode.
The thin glass manufactured for the purpose is preferable to talc for covering the specimen, and should always be used
43
by those who possess an achromatic microscope; but with a simple instrument the triplet can be used only when talc is the covering.
Specimens are frequently spoiled by the intrusion of the gold-size into the cell. Experience will best teach how this accident is to be avoided; attention however to the following particulars will assist the inexperienced. The cells should be prepared some days before they are used, in order that their walls may become firm. When the cell is closed the brush should be passed round the edge of the cover, with just sufficient size to prevent the admission of air into the cell; and upon the operator's care in this respect will depend his success. If too little size be used the air finds admission, and if much be put on, or if the cell be not completely filled with fluid, the size will enter and spoil the specimen. When the first layer of size is quite dry, he should proceed as directed by Mr. Thwaites.
Mr. Topping has kindly sent me a description of his method of preparing cells for mounting microscopic objects. He uses strips of plate-glass of an uniform size (three inches by one), and marks on them the size of the cell, by taking two thin pieces of mahogany of the size of the glass, each having a hole (circular, oval or square, as may be desired) cut in its centre, the smaller corresponding with the inner margin of the cell, and the larger with the outer. These, when laid over the glass, offer a ready means of tracing with a diamond the space around the cell, which must then be filled up with japan. This is next hardened by placing the glasses in an oven, the heat of which should be raised gradually, as otherwise the japan will blister; but if care be taken in this part of the process the cells will resist the action
of proof spirit.
The fluid which Mr. Topping has used for mounting consists of one ounce of rectified spirit to five ounces of distilled water, which he thinks superior to any other combination. To preserve delicate colours however, he prefers to use a solution of acetate of alumina - one ounce of the acetate to four
44
ounces of' distilled water. Of other solutions, he says that they "tend to destroy the colouring-matter of delicate objects, and ultimately spoil them by rendering them opake *."
To those who will read a book on this subject, it is quite unnecessary to enter upon a formal vindication of the study of the lower tribes of organized life. I shall content myself with the observation that, whilst this study is not wanting in those qualities which recommend the other branches of Natural History as a means of intellectual improvement, it has a peculiar claim on account of the light which it reflects on the ultimate organization of living bodies in general. Physiologists have of late pursued the investigation of the structure and development of cells, both animal and vegetable, with the greatest zeal; and with good reason, since it is obviously most desirable that we should thoroughly understand these elements of organization before we attempt to explain its more complicated arrangements. For the attainment of so important an object, the Desmidieae furnish the most valuable assistance. If the view which I have taken be correct, their frond, in most of the genera at least, consists of a single cell, which, although it is certainly more complex than cells in general, enables us to trace its own history with ease and certainty, and reveals to us that of forms still more simple.
It may suffice thus briefly to show the utility of such inquiries; but the improvements which the microscope has received in modern times may well suggest a wider application of the language in which the illustrious Ray vindicated his favourite pursuits† (p. 45).
*
What the Allwise did not disdain to create cannot be unworthy of our notice; and if in the minute Desmidieae, so long concealed from the unassisted eye, we have been at length enabled to recognize objects as carefully organized as the bulky elephant or the majestic oak, and as happily adapted to their position in nature, possessing too an economy whose laws are no less constant and regular, shall we not gladly examine this fresh evidence of an Almighty hand, as distinctly impressed on them as on the rest of his creation?
"To Him no high, no low, no great, no small;
He fills, He bounds, connects and equals all."
† (p. 44) "Non deerit qui me vanae curiositatis arguat, quòd res adeò viles et abjectas, nullius in vita usûs, indagaverim, iisque describendis tantum temporis et operae impenderim. Cui respondeo, quòd Dei opera sunt in quibus contemplandis memet exerceo; quòd Divinae Artis et Potentiae effecta, quibus exquirendis subsecivas horas addico; quòd Ille me in hunc mundum intro-duxerit, tam inexplicabili rerum varietate instructum et ornatum; quòd oculis, quos mihi contulit, ea videnda, animo consideranda objecerit. In Dei ergo contumeliam redundat, quòd haec, quae eum creâsse negare non audes, supervacua et inutilia esse affirmes." -Hist. Plant. v. 3. praef. p. ii.