Frond composed of two to ten fusiform or oblong cells, arranged side by side in a single row, but after division in two alternating rows; division oblique.
Cells always entire, and mostly fusiform or oblong, but in some species the outer ones are lunulate. There is no constriction nor suture at the middle, and the endochrome is not divided into two portions by a transverse band. The cells are few in number, varying from two to ten, and united into a frond by a hyaline matrix. Their division is oblique, and not transverse, as in most genera of the Desmidieae. As all the cells divide simultaneously, the frond when dividing consists of two rows, which are retained in connection by the matrix for some time after the complete division of the cells. From the oblique manner of division, the cells of one row alternate with those of the other.
The dividing frond is so unlike one having only a single row of cells, that, in common with Meneghini and other naturalists, I formerly considered these states as different species; and it was only on seeing a frond of Scenedesmus obtusus, in which the division of the cells was still incomplete, that I discovered my error. The fact however was previously known to Brébisson, and he has since sent me drawings illustrating the process.
The endochrome is in general very pale, and the starch granules are inconspicuous.
Scenedesmus differs from the preceding genera in the very different form of its cells, but Pediastrum supplies a connecting link between them. As in that genus, the frond in Scenedesmus is composed of several cells, but these are differently arranged; and the division into two segments, which, although modified, is still met with in the outer cells of Pediastrum, is entirely absent in Scenedesmus. In the oblique manner in which its cells divide, it agrees with Spirotaenia, which however has a different arrangement of the endochrome, and a frond consisting merely of a single cell.
190
1. S. quadricauda (Turp.); cells generally four, oblong, rounded at their ends, disposed in a single row; each extremity of the two external ones usually terminated by a bristle.
Achnanthes quadricauda, Turpin, Dict. des Sc. Nat. par Levr. Atl. Veg. ves. f. 8 (1820); Mem. du Mus. f. 6.
beta. External cells with three bristles.
gamma. ecornis (Ehr.); all the cells similar, and without bristles.
Common. Wales and Cornwall, J. R. Sussex; Kent; and near Southampton, Mr. Jenner. Bristol, Mr. Thwaites and Mr. Broome. Herts, Mr. Hassall. Ayrshire, Rev. D. Landsborough. Aberdeenshire and Banffshire, Mr. P. Grant. Northamptonshire, Rev. M. J. Berkeley. Kerry, Mr. Andrews. Manchester, Mr. Williamson. Ambleside, Mr. Sidebotham. Rochdale, Mr. Coates.
Germany, Kützing, Ehrenberg, &c. Falaise, Brébisson. Maine to Virginia, and in Mississippi River, Bailey.
Frond composed of from four to eight oblong cells, which are generally larger than those of any other species in the genus, about three times as long as broad, and rounded at their ends; the external cells are usually the most turgid, and their bristles are directed outwards.
The colouring matter is pale, with minute scattered granules.
In beta. the cells are smaller, and the external ones, besides the usual terminal bristles, have another from the centre of the outer margin.
The variety gamma. is described by some authors as a distinct species; but I agree with Ehrenberg in considering it a state of S. quadricauda, from which it differs only in having no bristles.
Length of cell 1/1121 of an inch; breadth 1/2631.
Tab XXXI. fig. 12. a, b, c, d. fronds with endochrome; e, f. empty fronds; g. variety beta; h. variety gamma; i. dividing frond of gamma.
191
2. S. dimorphus (Turp.); cells acute, four to eight, placed evenly in a single row; the inner cells fusiform, the outer externally lunate.
Dolgelley and Penzance, J. R. Bristol, Mr. Thwaites. Manchester, Mr. Sidebotham and Mr. Williamson. Aberdeen, Mr. P. Grant. Weston Bogs near Southampton, Mr. Jenner.
Germany, Kützing, Ehrenberg, Corda, &c. Falaise, Brébisson.
Frond very minute, consisting of four to eight cells placed evenly side by side in a single row; the inner cells straight, fusiform, attenuated, and acute at each end; the outer ones externally lunate. The endochrome is pale bluishgreen.
Length of cell from 1/1020 to 1/906 of an inch; breadth 1/8160.
Tab. XXXI. fig. 13. a. frond with endochrome; b. empty frond.
3. S. acutus (Meyen); cells two to eight, fusiform, acuminate, arranged in a single, irregularly alternating series.
King's Cliffe, Northamptonshire, Rev. M. J. Berkeley. Bristol, Mr. Thwaites. Dolgelley, J. R. Weston Bogs near Southampton, Mr. Jenner.
Germany, Ehrenberg, Kützing. Falaise, Brébisson.
Cells fusiform, acute at each end, frequently more or less ventricose; after division forming a single series, but projecting alternately more or less on each margin. The two outer cells are frequently crescent-shaped.
When the cells are nearly uniform this species has some resemblance to Scenedesmus dimorphus; but in the latter the cells are more slender, never ventricose, and are arranged quite evenly side by side. It is more difficult to distinguish S. acutus from S. obliquus, and I am far from certain that Ehrenberg erred in uniting them. The principal distinction is that in S. acutus
the cells form only a single series, which is nevertheless irregular on account of the alternate projection of the cells in opposite directions. In S. obliquus, on the other hand, the cells by division form two distinct rows, which, after separation, become two fronds.
Length of cell from 1/1063 to 1/1020 of an inch; breadth from 1/6181 to 1/5555.
Tab. XXXI. fig. 14. perfect frond. Tab. XXXIV. fig. 16. distorted variety.
4. S. obliquus (Turpin); cells elliptico-fusiform, after division arranged in two distinct, generally oblique series, the outermost cell of each not in contact with any of those in the other series.
Achnanthes obliqua, Turpin, Dict. des Sc. Nat. par Levr. Atl. Veg. ves. f. 9 (1820); Mem. du Mus. v. 16. t. 13. f. 9.
King's Cliffe, Rev. M. J. Berkeley. Near Bristol, Mr. Thwaites. Ayrshire, Rev. D. Landsborough. Near Manchester, Mr. Sidebotham. Dolgelley and Penzance, J. R. Near Aberdeen, Mr. P. Grant and Dr. Dickie. Storrington, Sussex; and Weston Bogs near Southampton, Mr. Jenner.
Germany, Kützing, Ehrenberg. Italy, Meneghini. Falaise, Brébisson.
Scenedesmus obliquus approaches very closely to Scenedesmus acutus, and the principal distinction depends on the different appearances presented by the dividing frond; and as this plant is most commonly seen in that state, I shall so describe it, merely premising that when separated, the frond consists of only one row. Mr. Berkeley has united S. triseriatus to S. obliquus, and I readily follow his example.
The cells are eight in number, arranged obliquely into distinct series. Each series has three cells, which are fusiform, equal, somewhat ventricose in the middle, acute at the free extremity, and subacute or rounded at the inner one. As the cells are not placed evenly, but each projects beyond its neighbour, the rows are oblique; their position with respect to each other is such that the inner ends of two cells of the lower lie between those of the upper row, whilst that of the third is outside the end of the highest in the upper. Of the two remaining cells, which are lunate, one is placed beyond the outer cell in each series, and is therefore not in contact with any cell in the other. They however take the same direction as the other cells in the series to which they respectively belong.
The different species of Scenedesmus frequently make their appearance in clear water that is kept in glasses or bottles and exposed to the light. I have received specimens of this species collected in this manner from Mr. Landsborough and Mr. Berkeley, and have myself repeatedly noticed its appearance in bottles containing other Desmidieae, and sometimes its rapid increase so as
to outnumber its companions. I must however observe, that specimens thus obtained are more variable in their characters than such as are gathered in their native abodes, and that their cells are frequently more or less distorted.
Length of cell 1/1666 of an inch; breadth 1/6250.
Tab. XXXI. fig. 15. a, b, c. different states of frond.
5. S. obtusus (Meyen); cells three to eight, ovate or oblong, and arranged in one row, or, after division, alternately in two rows.
Achnanthes quadralterna et octalterna, Turp. Dict. des Sc. Nat. par Levr. Atl. Veg. ves. f. 7, 8 (1820).
Common. Dolgelley and Penzance, J. R. Storrington and Beckley Furnace near Battle, Sussex; Reigate, Surrey; and near Southampton, Mr. Jenner. Bristol, Mr. Thwaites. Manchester, Mr. Williamson. Ambleside, Mr. Sidebotham. Aberdeenshire, Mr. P. Grant.
Germany, Kützing. Falaise, Brébisson. Maine to Virginia, and in the Mississippi River, Bailey.
Frond minute, composed of from three to eight ovate or ovato-oblong cells with rounded ends. The endochrome is very pale green.
This species is rarely met with in a simple state, but as the cells after division are still retained in connection by the mucous matrix, two rows are usually present, the broader ends of one row lying between the cells of the other.
The hyaline matrix is frequently their only bond of union, and in this state they seem to connect the Desmidieae with the Ulvaceae through Merismopedia.
Length of cell from 1/2331 to 1/1961 of an inch; greatest breadth from 1/4096 to 1/3623.
Tab. XXXI. fig. 16. a, b. ordinary state of frond; c. frond just divided.
6. S. duplex (Kütz.); cells slender fusiform, sigmoid, tapering at each end into a fine point; after division closely connected for about half their length.
Penzance, J. R.
Germany, Kützing.
Cells linear-lanceolate; extremities tapering to a fine point and curved in opposite directions. The cells after division remain closely united; frequently
the frond consists of only a single pair of cells so connected, but sometimes of two or even three of these pairs, which however are remote from each other: in this case, as the connecting mucus is colourless, they look like distinct plants, and their relation can be detected only by moving the frond. If kept in water for a few days, the cells separate from each other.
Professor Kützing formed his genus Rhaphidium for the reception of the plant above described and a species of Ankistrodesmus; but I believe the proper position of the former is in this genus.
Tab. XXXIV. fig. 17. a. frond with dividing cells; b. cell with the division more advanced.
Scenedesmus magnus et S. longus, Meyen, Nov. Act. Acad. Nat. Cur. v. 14. t. 43. f. 26-29 (1828). Kützing, Synop. Diatom. in Linnaea 1833, p. 606.
Scenedesmus quadricaudatus alpha
. cornutus, Ehr. Abh. d. Berl. Ak. 1833, p. 309. Ralfs, Annals of Nat. Hist. v. 15. p. 402. t. 12. f. 4; Trans. of. Bot Soc. of Edinburgh, v. 2. p. 159. t. 15. Hassall, Brit. Freshwater Algae, p. 393.
Scenedesmus quadricauda, Brébisson, Alg. Fal. p. 66 (1835). Menegh. Synop. Desmid. in Linnaea 1840, p. 206.
Scenedesmus caudatus, Corda, Alm. de Carlsb. 1835, p. 123. t. 4. f. 50. Kützing, Phy. Germ. p. 139.
Arthrodesmus quadricaudatus, Ehr. Infus. p. 150. t. 10. f. 16 (1838). Bailey, Amer. Journ. of Science and Arts, v. 41. p. 292. t. 1. f. 17.
Achnanthes quadrijuga, Turpin, Dict. des Sc. Nat. f. 5 (1820).
Scenedesmus Leibleinii, Kütz. Leib. Bot. Zeit. 1830, f. 3; Synop. Diatom. p. 607. Menegh. Synop. Desmid. p. 207.
Scenedesmus quadricaudatus beta. ecornis, Ehr. Abh. d. Berl. Ak. 1833, p. 309.
Scenedesmus pectinatus, Meyen, Nov. Act. Acad. Nat. Cur. v. 14. p. 2. f. 34, 35 (1828). Kützing, Synop. Diatom. p. 608; Phy. Germ. p. 140. Brébisson in lit.
Scenedesmus dimorphus, Kützing, Synop. Diatom. in Linnaea 1833, p. 608. Meneghini, Synop. Desmid. in Linnaea 1840, p. 208. Ralfs, Annals of Nat. Hist. v. 15. p. 403. t. 12. f. 5; Trans. of Bot. Soc. of Edinburgh, v. 2. p. 160. t. 15. Hassall, Brit. Freshwater Algae, p. 393.
Arthrodesmus pectinatus, Ehr. Infus. p. 151. t. 10. f. 17 (1838).
Scenedesmus fusiformis, Meneghini, Consp. Alg. Eugan. p. 18 (1837); Synop. Desmid. in Linnaea 1840, p. 208.
Arthrodesmus acutus, Ehrenberg, Infusor. p. 150. t. 10. f. 19 (1838).
Scenedesmus obliquus, Kützing, Synop. Diatom. in Linnaea 1833, p. 609 Meneghini, Synop. Desmid. in Linnaea 1840, p. 208. Berkeley, English Botany, t. 2933.
Scenedesmus triseriatus, Meneghini, Conspect. Alg. Eugan. p. 18 (1837); Synop. Desmid. in Linnaea 1840, p. 208. Ralfs, Annals of Nat. Hist. v. 15. p. 403. t. 12. f. 7. Trans. of Bot. Soc. of Edinburgh, v. 2. p. 161. t. 15.
Arthrodesmus acutus, Ehrenberg, Infusor. t. 10. f. 19. b (1838).
Scenedesmus obtusus, Meyen, Nov. Act. Acad. Nat. Cur. v. 14. f. 31 (1828). Meneghini, Synop. Desmid. in Linnaea 1840, p. 208. Ralfs, Annals of Nat. Hist. v. 15. p. 404. t. 12. f. 8; Trans. of Bot. Soc. of Edinburgh, v. 2. p. 162. t. 15. Kützing, Phy. Germ. p. 139. Hassall, Brit. Freshwater Algae, p. 394.
Scenedesmus quadralternus, Kütz. Synop. Diatom. in Linnaea 1833, p. 608. f. 94.
Scenedesmus octoalternus, Kützing, l. c. p. 609. f. 95.
Arthrodesmus acutus ?, Bailey, Amer. Journ. of Science and Arts, v. 41. p. 292. t. 1. f. 18 (1841).
