3. DESMIDIUM, Ag.
Filament fragile, elongated, triangular or quadrangular, regularly twisted; joints bidentate at the angles.
The filament is fragile, of a pale green colour and slightly opake. When dried, the British species usually acquire a yellowish appearance, and adhere to paper or talc less firmly than plants belonging to allied genera. They are regularly twisted, but being triangular or quadrangular, two of the bidentate angles of each joint are always visible at the margins. The endochrome is divided into linear portions by a pale transverse line between the angles.
* Phycologia Germanica, p. 140.
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Traces of a mucous sheath have been detected by the Rev. M. J. Berkeley and more recently by myself.
A transverse view shows that the cell is triangular or quadrangular, and that the endochrome has thick rays corresponding in number with the angles; these rays are frequently cloven.
Recent specimens of Desmidium are easily known from other genera by one or two dark waved lines passing down the filament, which appearance is occasioned by the twisting of the angular filament.
1. D. Swartzii (Ag.); filament triangular, equal, with a single longitudinal, waved, dark line formed by the third angle; end view triangular with the endochrome three-rayed.
Diatoma Swartzii, Ag. Disp. Alg. Svec. (1811). Lyngb. p. 177. t. 61.
Desmidium Swartzii, Ag. Syst. p. 9 (1824). Consp. Crit. Diat. p. 56. Greville, Scot. Crypt. Fl. t. 292. Kützing, Synopsis Diatom. in Linnaea 1833, p. 613; Phycologia Germ. p. 141. Greville in Hook. Brit. Fl. v. 2. p. 402. Brébisson, Alg. Fal. p. 53. t. 2. Ehrenberg, Infus. p. 140. t. 10. f. 8. Meneghini, Synop. Desmid. in Linnaea 1840, p. 203. Bailey, American Bacillaria in Amer. Journal of Science and Arts, v. 41 . p. 288. t. 1. f. 1. Harvey, Manual of Brit. Algae, p. 196. Corda, Observ. Microscop. sur les Animal. de Carlsbad, p. 17. Ralfs, Annals of Nat. Hist. v. 11. p. 375. t. 8. f. 3; Trans. of Bot. Soc. of Edinburgh, v. 2. p. 7. t. 2. Hassall, Brit. Freshwater Algae, p. 344. t. 83. f. 7.
Desmidium bispinosum, Corda, Observ. Micros. sur les Animalc. de Carlsbad, p. 20. t. 4. f. 28 (1840)
Common. Appin, Capt. Carmichael. Tunbridge Wells. Mr. Borrer. Swansea; Carnarvon; Dolgelley; Penzance, &c, J. R. Many stations in Sussex, and near Southampton, Mr. Jenner. Essex, Mr. Hassall. Caragh Lake, Kerry, Mr. Andrews. Meath, Mr. Moore. Westmoreland, Mr. Sidebotham. Rochdale, Mr. Coates. Manchester, Mr. Williamson. Aberdeenshire, Dr. Dickie and Mr. P. Grant.
Sweden, Agardh. North Germany, Martens, Kützing and Ehrenberg. Carlsbad; Prague; Reichenberg, Corda. Throughout the United States, Bailey. Falaise, Brébisson.
The filaments are elongated and very fragile, but separate spontaneously into single joints less readily than in Hyalotheca dissiliens and Didymoprium Grevillii. They adhere but slightly to paper. They are triangular, and, when viewed under the lens, the margins are always formed by two of the three angles in turn, whilst the dark line regularly passing from side to side marks the third angle and shows that the filament has a spiral twisting in about 16 joints. On examining the filament where the dark line touches the margin, if we first raise and then depress the lens, both angles at the point of apparent contact may be distinctly seen.
The joints are in the front view somewhat quadrangular, broader than long, and each angle has two minute, slightly angular teeth. The notch between the teeth is very distinct but not gaping. The joints are connected by a
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thickened margin, which partly fills the notch formed between the joints by the projection of the angles, whence the filament has a pinnatifid appearance.
The transverse view is triangular; the angles are blunt and the sides slightly concave. The endochrome is in three portions, placed in the angles and connected together at the centre; but this arrangement of the endochrome is seen only when the joints separate spontaneously. The rays are usually bipartite. When a joint is separated by force under the microscope, a cloud of minute granules is poured out and for an instant obscures the view.
Mr. Borrer has kindly presented me with a portion of a specimen of the plant here described, which was given him by Mr. Dawson Turner as the Conferva dissiliens of Dillwyn.
I have gathered at Dolgelley some fragments of this plant which had the endochrome condensed into a sporangium-like body in the centre of each joint. As in every other species of this family in which the reproductive body has been detected, it is the result of the coupling of the cells, I think it best merely to direct attention to the fact I have mentioned, leaving its nature to be determined by future observation.
Length of joint from 1/2000 to 1/1666 of an inch; breadth of filament 1/633.
Tab. IV. a. portion of a mature filament; b. portion of a filament with the joints dividing; c, d, e. transverse views ; f . sporangia?
2. D. quadrangulatum (Ralfs); filaments quadrangular, varying in breadth from the twisting of the filament and having two longitudinal waved lines; the end view quadrangular with the endochrome four-rayed.
Desmidium quadrangulatum, Ralfs, in Annals of Nat. Hist. v. 15. p. 405. t. 12. f. 9. (1845); Trans. of Bot. Soc. of Edinburgh, v. 2. p. 163.
Desmidium quadrangulare, Kütz. Phy. Germ. p. 141 (1845).
Hab. In a boggy pool at Bologas near Penzance; Dolgelley, J. R. Ambleside, Mr. Sidebotham. Aberdeen, Dr. Dickie and Mr. P. Grant.
The Harz, Germany, Kützing.
Mr. Berkeley and Mr. Borrer regard this plant as a variety of D. Swartzii, and certainly all the more obvious distinctive marks depend upon the filament being quadrangular in one case and triangular in the other *. As I have gathered it for two succeeding years quite unmixed with Desmidium Swartzii, and as Mr. Berkeley well observes (in a letter), that "whether considered as a species or variety, it is a remarkable plant and well-deserving of notice," I have preferred to describe it as distinct, although I do not consider the point free from doubt. As there are three sides in one plant and four in the other, whilst the sides in both are equal, the filament of D. quadrangulatum is stouter; for the same reason, instead of one dark longitudinal line it has two
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lines running from side to side and crossing each other : the additional line of course depends on the additional angle. When these lines approach the opposite margins of the filament only one side is presented to the eye, and the filament is then often same breadth as in D. Swartzii, but as it is regularly twisted its apparent breadth varies, being greatest where both dark lines cross each other in the middle. The end view also has one angle more, and therefore the endochrome exhibits four instead of three rays. I may however observe, that the teeth, as Mr. Jenner has pointed out, are rounded in my specimens of D. quadrangulatum and angular in D. Swartzii; but further experience must determine whether this character is constant.
D. quadrangulatum requires about 40 joints to complete a turn, or before the same angle again appears at the same margin.
It affords some sanction to this arrangement, that shortly after my notice of this plant appeared in the 'Annals of Natural History,' it was described by Kützing in his 'Phycologia Germanica' as a distinct species.
Length of joint 1/1244 of an inch; least breadth or side in transverse view 1/603; greatest breadth, or diagonal, in transverse view 1/455.
Tab. V. a. portion of a filament; b. filament less magnified; c. empty joint; d, e. transverse views.
