Frond plane, circular, composed of several cells, which form by their union a flattened star, and are generally arranged either in a single circle or in two or more concentric ones; marginal cells bipartite.
Frond minute, composed of four or more cells united together in the figure of a flattened star; when these are only four in number they are not arranged in a circle and the star is somewhat angular; in most species, however, they form either a single circle or two or more concentric ones, and one or two cells usually occupy the centre. The cells are combined into a frond by a mucous matrix, which is generally colourless and constitutes hyaline interstices. Occasionally some are ruptured, but in this case their endochrome alone escapes,
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and the others are not affected. In all the species the free margin of the outer cells is bipartite, a character which I consider important, and in fact a modification of the form observed in Cosmarium and other genera with constricted fronds. The cell in Pediastrum, however, is different from theirs, since its division into two segments exists only on one side, and in regard to the inner cells is at most a slight concavity of the external margin.
The flat, star-like fronds of Pediastrum are so characteristic, that there is no risk of mistaking it for any other genus, Crucigenia perhaps excepted; that however differs by having entire, quadrangular cells.
It is far more difficult to distinguish its species. Ehrenberg relies chiefly on the number of the circles, but this character, as Meneghini and Professor Bailey have observed, cannot always be depended on: the latter says, " There appears to me to be much confusion in the specific characters, arising from the circumstance that the number of corpuscles in the different rows has been made a character of specific importance. From what I have seen of the species, I am satisfied that the number of corpuscles in a star is liable to great variation in the same species."
I have myself noticed that, in one species at least, the number of circles varies from one to three; and the same observation has been made by Mr. Jenner. He has also remarked that the number of cells in the inner circle, on which I was inclined to place greater reliance, is subject to variation. Some species have one cell in the centre and others two; but the central cells are said to be sometimes deficient, a variation which I have not observed.
All the above characters, it must be allowed, are more or less uncertain; still upon them we must for the most part depend in discriminating between nearly allied species.
Meneghini adopts as a specific character the number and position of hyaline vesicles in each cell; but I am afraid that both of these are also variable.
The synonyms in this genus are so confused that I quote them with much hesitation. Ehrenberg, having relied almost entirely on the number of the circles and the cells in each, has neglected the form of the cells, and consequently his species are intermixed, states of some being referred to others. Mr. Hassall indeed has constituted several new species; but as he has attempted this from an examination merely
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of Ehrenberg's figures, and not of the plants themselves, his opinions are entitled to less weight than they would have been if founded on his own observation.
Professor Bailey has suggested, in his paper on the American Desmidieae, that the form of the cells might afford the most certain character. Kützing, who has described nine species in his ' Phycologia Germanica,' seems to have arrived at the same conclusion. I have ventured in some instances to differ from Kützing's views; yet I feel persuaded that his definitions of the species in Pediastrum are truer to nature than those of Ehrenberg.
I have not seen the cells during the process of division; but I am informed by M. de Brébisson that it takes place at the notch, in the same manner as in other Desmidieae: hence the cells in each circle are connected at their ends, like those of the filamentous genera. I do not however understand in what manner the additional circles are formed, nor why the numbers in each circle are so constant.
The segments or lobes are in some species more or less emarginate, and in others entire; as this character seems to be constant, I have taken advantage of it to divide the genus into two sections.
1. P. Tetras (Ehr.); cells four, separated by colourless interstices which form a cross; lobes truncato-emarginate.
Micrasterias Tetras, Ehr. Infus. p. 155. t. 11. f. 1 (1838). Bailey, Amer. Journ. of Science and Arts, v. 41. p. 293. t. 1. f. 19.
Pediastrum Tetras, Ralfs, Annals of Nat. Hist. v. 14. p. 469. t. 12. f . 4 (1844); Trans. of Bot. Soc. of Edinburgh, v. 2. p. 157. t. 17. Hassall, Brit. Freshwater Alg. p. 388.
Barmouth; Dolgelley and Penzance, J. R. Beckley Furnace near Battle, Sussex, Mr. Jenner. Ambleside, Mr. Sidebotham. Near Aberdeen, Mr. P. Grant.
Germany, Ehrenberg. Maine to Virginia, Bailey.
Frond extremely minute, composed of four cells, which constitute a star-like figure; the cells are somewhat triangular, and connected by a hyaline matrix, which forms colourless interstices in the figure of a cross; the free margin is bilobed; the lobes terminate in a broad shallow notch, which has acute corners. The colouring matter is pale green.
Meneghini makes Pediastrum Tetras a synonym of P. Heptactis; but I have not seen any intermediate forms, and in fact the frond of this species is sometimes as large as that of P. Heptactis, although the latter contains twice as many cells.
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Length of cell 1/2941 of an inch; breadth 1/2272.
Tab. XXXI. fig. 1. a, b. fronds with endochrome; c. distorted frond; d. empty frond.
2. P. Heptactis (Ehr.); frond constituted of seven cells disposed in a circle round another in its centre; cells bipartite with emarginate lobes.
Barmouth and Penzance, J. R. Beckley Furnace, Sussex, and near Southampton, Mr. Jenner. Rochdale, Mr. Coates. Near Aberdeen, Mr. P. Grant.
Germany, Ehrenberg and Corda. Mexico, Bailey.
Frond very minute, composed of eight cells, one in the centre and seven in a circle round it; the cells connected by a gelatinous matrix, which forms colourless interstices. The seven external cells are bipartite, their lobes ending in a broad shallow notch with acute corners; in other respects the cells are variable; the figure of the central one especially differs in almost every specimen, being sometimes angular and entire, and sometimes deeply divided with rounded segments. The colouring matter is very pale.
Length of cell 1/2906 of an inch; breadth 1/2500.
Tab. XXXI. fig. 2. a, b. fronds with endochrome; c. distorted frond; d. empty frond.
3. P. biradiatum (Meyen); outer circle generally of eleven bipartite cells, with bifid or emarginate lobes.
Pediastrum biradiatum, Meyen, Nov. Act. Acad. Nat. Cur. v. 14. f. 21, 22 (1828). Meneghini, Synop. Desmid. in Linnaea 1840, p. 211.
Micrasterias Rotula, Ehr. Infus. p. 158. t. 11. f. 7 (1838).
Pediastrum Rotula, Kützing, Phycologia Germanica, p. 143 (1845).
alpha. Penzance, J. R. Ambleside, Mr. Sidebotham. Rochdale, Mr. Coates.
beta. Beckley Furnace, Sussex, and near Southampton, Mr. Jenner.
Germany, Meyen, Ehrenberg, Kützing. New Jersey, Bailey.
Frond minute, generally composed of 16 or 17 cells arranged in two concentric circles, with a solitary cell in the centre; this central cell is said to be often wanting. I have never found it in British specimens, nor indeed even a vacancy in its place. The five angular cells forming the inner circle are often quadrilateral, and the exterior of each has a linear notch. The outer circle consists of eleven cells, which are either quadrilateral or somewhat cuneate;
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the free margin is deeply bipartite, and the lobes are again divided or truncato-emargiate. The endochrome is bluish-green, with minute scattered granules.
Interstices between the cells hyaline.
Kützing unites this and the two preceding species under Ehrenberg's name, Rotula; Meneghini keeps it distinct. Typal specimens of this species differ from P. Tetras and P. Heptactis, not only in the number of the cells, but also in the deeply divided and tapering lobes of the outer cells. Specimens however of the variety beta, gathered in Sussex by Mr. Jenner, render it doubtful whether Kützing's view is not the correct one; for whilst these specimens have the same number and disposition of cells as P. biradiatum, the lobes of their marginal cells are merely slightly emarginate like those of P. Heptactis.
Length of cell in the outer circle 1/2000 of an inch; breadth 1/1754. Length of beta. 1/2550; breadth 1/2040.
Tab. XXXI. fig. 3. a. frond with endochrome; b. empty frond: fig. 4. a, b. fronds of beta. with endochrome; c. empty frond.
4. P. Selenaea (Kütz.); cells crescent-shaped, arranged in one or more circles around one or two central ones; connecting medium coloured.
Barmouth, North Wales, J. R.
Germany, Kützing, Ehrenberg.
Pediastrum Selenaea, like the other species of this genus, varies in the number of cells. Kützing's figures represent three states, having respectively one, two, and three circles, round a central cell, all the cells being lunate; the number of cells consisting of 5 in the first circle, 10 in the second, and 15 in the third.
The following description applies to British specimens only.
Frond minute, with two angular cells in the centre, and six crescent-shaped ones arranged in a circle round them. The cells are not approximate, or separated by hyaline interstices, as in other species; but are connected by a coloured portion, which indeed may be said to constitute the frond, the cells being imbedded in it. It will be seen that the British specimens differ from those figured by Kützing by having two cells in the centre instead of one, and six in the circle instead of five. I have never seen specimens with more than
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one circle. Notwithstanding these differences, I believe that our plant is correctly referred to this species. It agrees both in the crescent-shaped cells and in the coloured connecting medium; characters which distinguish it from all others.
Tab. XXXI. fig. 5. frond with endochrome.
5. P. simplex (Meyen); frond composed of four cells with or without one or two central ones; lobes of marginal cells ovate, tapering to a point.
beta. cruciatum; cells angular at the base, connected in a cruciform manner without a central space.
Near Aberdeen, Mr. P. Grant.
Germany, Kützing, Ehrenberg.
In all the specimens that I have seen of this species, the number of cells was invariably four; but, according to Ehrenberg and other authors, it occasionally has one or two other cells in the centre. I have observed two forms: in one of them the cells were truncated at the base and the frond had a central square vacancy; in the other the bases of the cells extended into angles, and, as these exactly fitted each other, the interstices or joinings formed a cross. The latter state I believe to be the Pediastrum cruciatum of Kützing, although the figures which he sent me represent smaller fronds.
At the base of each cell in beta . there is generally a large vesicle, which however is sometimes absent. At first sight this variety appears to be distinct, but I have seen intermediate specimens, and, in all, the lobes of the cells are ovate at the base and taper into a cuspidate point.
Length of cell including lobes 1/1020 of an inch; breadth 1/1632; length of beta . 1/1666; breadth from 1/3448 to 1/2040.
Tab. XXXIV. fig. 15. a. frond with endochrome; b. empty frond; c. frond of beta; d. P. cruciatum from a drawing by Kützing.
6. P. pertusum (Kütz.); inner cells leaving hyaline intervals resembling foramina; outer cells rectangular, notch triangular, as broad as the cell.
Micrasterias Boryana, Ehr. Infusor. t. 11. f. 5. b, d, i (1838).
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Micrasterias tricyclia, Ehr. Infusor. t. 11. f. 8. a, b (1838).
Pediastrum pertusum, Kützing, Phycologia Germanica, p. 143 (1845).
Pediastrum tricyclicum, Hassall, Brit. Freshwater Algae, t. 92. f. 1 (1845).
Aberdeen, Mr. P. Grant. Storrington, Sussex, Mr. Jenner.
Germany, Ehrenberg.
Frond consisting of from one to three circles of cells arranged round one or two central ones. The number of cells, as in Pediastrum Selenaea and P. Boryanum, generally consists of 5 in the first circle, 10 in the second, and 15 in the third; but I have seen fronds with four cells forming a circle about two central ones. Ehrenberg's figures represent all the cells as alike in form, and this was the case in specimens sent me by Mr. P. Grant; but Mr. Jenner finds the inner cells quadrate, and only the marginal ones notched. In all the varieties, howeverm, the inner cells are so placed as to leave hyaline spaces which have the appearance of perforations. The marginal cells have, on their free side, a wide triangular notch, about equal to the fourth part of a square; were this filled it would make the cell quadrate, consequently the sides and base of the cell form rectangular triangles, and the terminal lobes taper to an acute point.
Length of cell in outer circle 1/2266 of an inch; breadth 1/3268.
Tab. XXXI. fig. 6. a, b. fronds with endochrome.
7. P. granulatum (Kütz.); cells granulated; lobes of marginal cells tapering.
Pediastrum granulatum, Kützing, Phycologia Germanica, p. 143 (1845); in lit. cum icone.
Weston Bogs near Southampton, Mr. Jenner.
Germany, Kützing.
Frond composed of six cells circularly arranged around two subquadrate central ones; the marginal cells have lobes gradually tapering into short points, and all are rough with minute granules.
I have seen no specimens of this species, but Mr. Jenner's drawings exactly agree with one sent me by Professor Kützing.
Length of cell in outer circle 1/2000 of an inch; breadth 1/1851.
Tab. XXXI. fig. 8. a. frond with endochrome; b. empty frond.
8. P. Napoleonis (Turpin); six angular cells forming a circle round two central ones; lobes of marginal cells cuspidate; notch wide.
Ashdown Forest, Sussex, Mr. Jenner. Swansea, J. R. Near Aberdeen, Mr. P. Grant.
Germany, Kützing, Ehrenberg.
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I greatly doubt whether this species is distinct from Pediastrum Boryanum. Its claim to rank as a species depends on the number and arrangement of its cells. The cells themselves are variable in form. The notch of the marginal ones is rounded, and usually wider than in the allied species.
Length of cell in outer circle from 1/1569 to 1/1483 of an inch; breadth from 1/1813 to 1/1088.
Tab. XXXI. fig. 7. a, b, c. fronds with endochrome; d, e. empty fronds.
9. P. angulosum (Ehr.); marginal cells with angular lobes which are not extended into rays; interstices hyaline.
Dolgelley, J. R. Ambleside, Mr. Sidebotham. Aberdeen, Mr. P. Grant. Beckley Furnace near Battle, Sussex, Mr. Jenner.
Germany, Ehrenberg.
Frond minute, consisting of one or more circles of cells, with a single cell in the centre; the number of cells are generally five in the inner circle, ten in the second, and fifteen in the third; the inner cells are angular, and similar to those of Pediastrum Boryanum and P. vagum. The marginal cells are less deeply notched than in those species, and the lobes are merely angular, not in the least extended into processes or rays. The angle is produced by the sloping of the outer margin, which frequently might be termed obliquely truncate, or almost emarginate.
Length of cell in outer circle 1/2732 of an inch; breadth 1/1942.
Tab. XXXI. fig. 11. a. frond with endochrome; b, empty frond.
10. P. Boryanum (Turpin); cells arranged in one or more circles round one or two central ones; marginal cells gradually tapering into two long subulate points; notch narrow.
Pediastrum angulosum, Meneghini, Synopsis Desmid. in Linnaea 1840, p. 211. Hassall, British Freshwater Algae, p. 391.
Pediastrum Boryanum, Ralfs, Annals of Nat. Hist v. 14. t. 12. f. 7 (upper figure) (1844); Trans. of Bot. Soc. of Edinburgh, v. 2. t. 17.
Helierella Boryana, Turpin, Dict. des Sc. Nat. par Levr. Atl. Veg. ves. f. 22 (1820); Mém. du Mus. v. 16. t. 13. f. 22.
Micrasterias Boryi, Kützing, Synop. Diatom. in Linnaea 1833, p. 603.
Micrasterias Boryana, Ehrenberg, Abh. der Berlin. Ak. p. 300 (1833); Infus. p. 157. t. 11. f. 5. a, c, g, h.
Pediastrum Boryanum, Meneghini, Synop. Desmid. in Linnaea 1840, p. 210. Ralfs, Annals of Nat. Hist. v. 14. p. 470. t. 12. f. 7 (lower figure); Trans. of Bot. Soc. of Edinburgh, v. 2. p. 158. t. 17. Kützing, Phyc. Germ. p. 143. Jenner, Fl. of Tunbridge Wells, p. 198. Hassall, Brit. Freshwater Algae, p. 389.
Dolgelley, Barmouth, and Penzance, J. R. Cheshunt, Mr. Hassall. Beckley Furnace near Battle, Storrington, Tunbridge Wells, &c., Sussex; Dorking, Surrey; and near Southampton, Mr. Jenner. Manchester, Mr.Gray and Mr. Williamson.
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Germany, Kützing, Ehrenberg. Falaise, Brébisson. Maine to Mexico, Bailey.
Cells generally arranged in one to three circles round a central cell. The number of cells consists of five in the first circle, ten in the second, and fifteen in the third; but these numbers are not invariable; sometimes there are two central cells, and still more frequently the numbers in the circles differ from those just stated. The inner cells are five- or six-angled, broader than long, with a slight concavity on the outer margin; the marginal cells are biradiate, the rays subulate, the notch narrow.
Pediastrum Boryanum differs from P. vagum in the gradually tapering acute lobes of the outer cells; from P. angulosum it may be known by the lobes, which are elongated into rays, and not merely angular, as in that species.
Length of cell in outer circle from 1/2083 to 1/1633 of an inch; breadth from 1/2732 to 1/2222.
Tab. XXXI. fig. 9. a. frond with endochrome; b. empty frond.
11. P. ellipticum (Ehr.); cells variable in number and arrangement; lobes of marginal ones suddenly contracted into short, cylindrical, obtuse processes.
beta. processes of the lobes truncato-emarginate.
Micrasterias elliptica, Ehrenberg, Abh. der Berlin. Ak. p. 302 (1833); Infus. p. 158. t. 11. f. 9.
Dolgelley, J. R. Aberdeenshire and Banffshire, Mr. P. Grant. Near Bristol, Mr. Thwaites. Beckley Furnace near Battle, Sussex; Dorking, Surrey; and near Southampton, Mr. Jenner.
Germany, Ehrenberg, Kützing. Maine to Virginia, Bailey.
Frond larger than that of any other species, often very irregular in form; cells numerous, but varying much in number; the arrangement of the inner ones, especially in the larger specimens, is more or less irregular, and not in distinct circles, like those in the preceding species. The notch of the marginal cells is narrow, the lobes terminated by a cylindrical process, which appears to me rounded at its apex; but Mr. Jenner usually finds two minute teeth at the end.
Length of cell in outer circle from 1/1754 to 1/906 of an inch; breadth from 1/1515 to 1/1020.
Tab. XXXI. fig. 10. a, b, c. fronds with endochrome; d. empty frond.
Micrasterias, No. 3, Bailey, American Journal of Science and Arts, v. 41. p. 293. t. 1. f. 21 (1841).
Pediastrum Boryanum, Ralfs, Annals of Nat. Hist. v. 14. t. 12. f. 8 (1844); Trans. of Bot. Soc. of Edinburgh, v. 2. t. 17.
Pediastrum vagum, Kützing, Phycologia Germanica, p. 143 (1845); in lit. cum icone.
Pediastrum constrictum, Hassall, Brit. Freshwater Algae, p. 391 (1845).
Pediastrum ellipticum, Hassall, Brit. Freshwater Algae, t. 92. f. 2 (1845).
