BRITISH DESMIDIEAE.
1. HYALOTHECA, Ehr.
Filament elongated, cylindrical, very gelatinous; joints having either a slight constriction which produces a crenate appearance, or a grooved rim at one end which forms a bifid projection on each side. End view circular.
The filaments are cylindrical, simple, jointed, invested with a broad gelatinous sheath, and very fragile in one species but not so in the other. Either a groove passes round each joint, giving a crenate appearance to the margins of the filament and dividing the endo-chrome into two portions, or else a grooved rim at one extremity of the joint appears on each side like a bifid process.
A transverse view shows a radiate endochrome in one species; but I have been unable to determine whether this occurs in the other on account of its imperfect fragility.
The cylindrical filament distinguishes this genus from Desmidium and Sphaerozosma. It has no angular projections, is not twisted, and has always the same apparent breadth, and consequently in all these respects differs from Didymoprium.
1. H. dissiliens (Smith); filament fragile, crenate; a shallow groove round each joint dividing the endochrome into two portions.
Conferva dissiliens, Smith, Eng. Bot. t. 2464. (1812) (not Dillw.).
Desmidium mucosum,. Bréib. Alg. Fal. p. 65. t. 2 (1835). Menegh. Synop. Desmid. in Linnaea 1840, p. 204. Ralfs in Annals of Nat. Hist. v. 11. p. 374. t. 8. f. 2.
Desmidium limbatum, Chauv. Alg. de la Norm. f. 6. (1836), according to Meneghini.
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Gloeoprium dissiliens, Berk.. in Annals of Nat. Hist. v. 16. p. 11 (1845). Jenner, Fl. of Tunbridge Wells, p. 192. Hassall, Brit. Freshwater Algae, p. 346. t. 83. f. 3.
Hyalotheca mucosa, Kütz. Phycologia Germanica, p. 140? (1845).
Hyalotheca dissiliens, Bréb. in lit. (1846).
Common. Henfield, Sussex . and Cwm Bychan, North Wales, Mr. Borrer. Swansea; Dolgelley; Bedgelert; Penzance, &c., J. R. Hampshire and many stations in Sussex, Kent and Surrey, Mr. Jenner. Essex and Herts, Mr. Hassall. Ayrshire, Rev. D. Landsborough. Aberdeen, Dr. Dickie and Mr. P. Grant. Bandon, Professor Allman. Kerry, Mr. Andrews. Ambleside, Mr. Sidebotham. Yate near Bristol, Mr. Broome. Stoke Hill near Wells, Mr. Thwaites. Rochdale, Mr. Coates.Manchester. Mr. Williamson. Germany, Kützing. Falaise, Brébisson. West Point, New York, Professor Bailey.
Filaments mucous, adhering firmly to paper. The joints are usually broader than long, and as each has a shallow groove passing round it, it resembles a small pulley-wheel, and the filament acquires a crenate appearance; the crenatures are generally very shallow, sometimes nearly obsolete, but I have occionally seen them deeper, when the plant looks more like the figure in "Eng. Bot.;" at all times they may be detected on a careful examination with the higher powers of the microscope. The endochrome is divided into two portions by this constriction, and there is scarcely any depression between the joints.
The mucous sheath is easily observed, and on each side of the filament is nearly as broad as the central coloured portion. The sheath and filament are alike cylindrical.
If kept in water for a few days the plant separates into single joints, each of which has a perfect mucous covering. In the Desmidieae in general the filaments separate into single joints with a facility proportioned to the greater development of the sheath ; but Hyalotheca mucosa in this, as in other respects, deviates from the rest of the group. The breaking up takes place more speedily in the present plant, in Didymoprium Grevillii and Sphaerozosma vertebratum, than in Didymoprium Borreri or in the genus Desmidium. This circumstance seems to depend upon the elasticity of the mucous covering of the joint. This elasticity is proved by its becoming longer than the joint immediately upon separation, and hence into whatever number of pieces a filament may be broken, the ends of each portion are as perfectly covered as the original end of the filament itself, and with regard to each particular joint the result is the same.
The transverse view is circular, and shows a mucous border of the same form. In this aspect the endochrome is generally disposed in a stellate manner, with six or seven rays, and frequently has a colourless central spot.
Foreign specimens of "Desmidium mucosum," given me by the Rev. M. J. Berkeley agree in all points with the above description.
Hyalotheca disiliens frequently occurs in considerable quantities, unmixed with other Algae. When young it agrees in appearance with many of the Conjugatae, but it may be distinguished by its fragility. It soon becomes of a pale opake green, and is then even more fragile than before.
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Although I had seen sporangia of H. dissiliens at Penzance, I was unable to determine to what plant they belonged untill Mr. Jenner sent them from Sussex and suggested that they were its conjugated state. At first the difficulty I experienced in recognizing the original cells in their altered condition induced me to hesitate in admitting the correctness of his opinion. But having subsequently gathered this species in all stages near Dolgelley, I have traced its changes, with Mr. Jenner's assistance, and satisfied my doubts.
The filaments separate into single joints, which couple by tubes ; so far the manner of conjugation is similar to the examples I had previously seen in Staurastrum and Tetmemorus; but in those genera the segments of the cell are completely divided by the growth of the newly-formed portion; in the present case the cells separate only on the sides opposed to each other. In the cleft is formed the new portion, the enlargement of which, after it has become the connecting tube, by causing the cell to gape, produces a notch-like appearance on the outer margin. The process would be more evident if the similar texture of the old and new parts did not render it difficult at first sight to detect the joint in its altered form; and the difficulty is increased, as the coupling usually takes place in a crossed or twisted position, which thus still farther disguises the joints.
The sporangium, which is circular, is situated within the connecting tube, and the empty joints are permanently attached to it.
I have been unable to determine, by my own observation, whether the mucous covering remains on the conjugated cells, but Mr. Jenner informs me that in some instances he has observed it, and indeed that its presence led him to the identification of the plant. As in most instances, however, it is either wanting or very obscure, I have thought it advisable to omit it in the figures of the sporangia, and merely to notice its occasional occurrence.
Sporangia have been gathered also by Mr. Thwaites, Dr. Dickie, Mr. Broome, and Mr. Sidebotham; and M. de Brébisson informs me that he has met with them in France.
Length of joint from 1/2105 to 1/1351 of an inch; breadth of filament from 1/1308 to 1/833; breadth of sheath from 1/647 to 1/357. Diameter of sporangium 1/826.
Tab. I. f. 1. a, b. portions of mature filaments; c. portion of a dividing filament; d. empty joint; e, f. transverse view; g. cells conjugating; h, i. sporangia.
2. H mucosa (Mert); filament scarcely fragile; joints not constricted, but having at one of the ends a minute bidentate projection on each margin, the adjoining end of the next joint being similar.
Conferva mucosa, Mert. Dillw. Brit. Conf. t. B. (1809). Hook. Br Fl.v. 2. p. 351. Harv. Br. Alg. p.127.
Hyalotheca mucosa; Ehr. Kurze Nachricht über 274 seit dem Abschluß; der Tafeln des grössern Infusorienwerkes? (1840).
Gloeoprium mucosum, Ralfs, Annals of Nat. Hist. v. 16. p. 11. t. 3. f. 6. (1845); Trans. of.Bot. Soc. of Edinburgh, v. 2. p. 165. t. 18. Jenner, Fl. of Tunbridpe Wells, p. 192, Hass. Br. Freshwater Algae, p. 346. t. 83. f 5.
Hyalotheca mertensii; Brébisson in lit. (1846).
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In shallow pools and gently-flowing streams; most plentiful in the autumn. Bantry, Miss Hutchins. Appin, Capt. Carmichael. Trentishoe, Devonshire; Penzance; and Dolgelley, J. R. Herts and Essex, Mr. Hassall. Chiltington Common, near Pulborough, Sussex; Ashdown Forest; and in the peat-bog at Fisher's Castle, near Tunbridge Wells, Mr. Jenner. Near Aberdeen, Mr. P. Grant.
Falaise, France; Brébisson.
Filaments elongated, very gelatinous, of a pale translucent green; not fragile. Under the microscope the joints are generally about equal in length and breadth, and the endochrome forms a single irregular patch. The joints are not constricted, but at one end they have on each margin a minute bidentate projection: as the similar ones of the next joint are at its adjoining extremity, these projections occur only near the alternate dissepiments. When however the joint is elongated, preparatory to the formation of two joints, the endochrome is divided into two portions, and then these projections are present at both ends, the next joint undergoing a similar change. These projections are extremely minute, and can be detected only by employing the highest powers of the microscope ; and even then are liable to be overlooked, if not carefully sought for. I examined many specimens of this plant in 1841, but did not perceive these curious protuberances until the following year. I believe they are really formed by a grooved rim round the end of the joint, because however the filaments may be moved, they are equally apparent ; whereas if they were real processes, as in Didymoprium, they would be sometimes either entirely concealed or rendered less distinct; they can also be traced like a transverse band across the empty cell.
The filaments have a very broad mucous sheath, which from its great
breadth and absence of colour is not easily discerned; it is more evident when a specimen is dried on talc or glass, as the margins are then generally perceptible. When first gathered the filaments are very distinct, frequently parallel and subdistant even to the naked eye : this depends. on the great breadth of their mucous sheaths, which prevents the coloured filaments from coming into contact. By this character H. mucosa may in general be known, even without the aid of a microscope. The same circumstance occurs in young plants of H. dissiliens, but is less remarkable, as its mucous sheath is not more than half as broad.
Under a low power of the microscope, H. mucosa has considerable resemblance to H dissiliens, with which it is probably not unfrequently confounded. But they may always be distinguished, even without the aid of a microscope. H. dissiliens is extremely fragile, and will break into pieces if a small portion be placed on the hand and the finger gently passed over it ; this plant, on the contrary, will not break if it be taken out of the water and allowed to hang down in long strings. It always has a clear translucent appearance; but H. dissiliens, except when very young, is of an opale green. H dissiliens, if kept in water for a few days, spontaneously separates into single joints; H. mucosa, although treated in the same manner for weeks, did not separate into fragments sufficiently small to enable me to obtain a satisfactory transverse view. At length, however, I was more fortunate. When kept in water the
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mucous sheath is partially dissolved, and interferes with the examination of the joints, but by repeatedly changing the water I succeeded in procuring many single joints. The transverse view was circular, and when the cell was empty the border appeared decidedly striated. The endochrome had some tendency to a radiate form, but the treatment to which the filaments had been subjected prevented me from forming any certain conclusion as to its disposition in the recent plant.
Under the microscope H. mucosa may be known from H. dissiliens by its joints never appearing crenate, and by its endochrome being almost always in a single patch, or by the greater length of the joints if the endochrome is bipartite. Its mucous sheath is with difficulty detected, and when seen will be found to extend on each side twice the breadth of the coloured filament; whereas in H. dissiliens the mucous sheath is, except in old specimens, detected without difficulty.
The joints seem to be in pairs; a single one is consequently unsymmetrical.
H. mucosa agrees with the other Desmidieae in its capability of being kept a long time without undergoing decomposition.
This plant differs in many respects from the other Desmidieae; indeed so much so, that I had some doubts whether it would be correctly placed in this family; but as the Rev. M. J. Berkeley, as well as every other algological friend whose opinion I have solicited, considers that its place must be in the same genus with the preceding species, I have described it here.
I was first indebted to Mr. Hassall for the information that the plant under consideration is the Conferva mucosa, Dillw., as also for an opportunity of examining a foreign specimen under that name from the herbarium of Dr. Greville. I have since been enabled to compare our plant with a portion of an Irish specimen of Conferva mucosa presented to me by Mr. Borrer, who received it from Sir W. J. Hooker. From the latter I learn that this was an original specimen from Miss Hutchins. Sir W. J. Hooker has also presented me with an Appin specimen, collected by Capt. Carmichael. All these are identical with the present plant.
Length of joint from 1/1666 to 1/1250 of an inch; breadth of filament 1/1250 to 1/1111; breadth of sheath 1/167.
Tab. I. f. 2. a. portion of a filament; b. empty joints; c, d. transverse views.
