Filament elongated, triangular or plane; joints bicrenate at the margins; an oval foramen between the joints.
The filaments are elongated jointed, fragile, and either triangular or plane; the joints at the angles are bicrenate, and at their junction are excavated at the centre, and thus form a series of oval foramina. The endochrome is bipartite.
By Brébisson, Meneghini and Kützing this plant is included in Desmidium, and certainly the state with triangular filaments does at first sight appear closely allied to Desmidium Swartzii; but the large oval foramen between the joints is so remarkable a character, that I must concur with Ehrenberg in placing it in a separate genus. As however Ehrenberg's name, Odontella, had previously been given by Agardh to some Diatomaceae, it became necessary to find another name.
Ehrenberg included Sphaerozosma in his Odontella, but the present genus is essentially distinct from Sphaerozosma ; in the latter the joints have their margins incised or sinuated, and gland-like processes at their junctions, and it is merely by the interposition of these processes that the joints are now and then slightly separated. In this genus,
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on the contrary, the joints have two prominent teeth at the margins or angles, and the foramen results from the excavation or concavity of the joint itself, and not from the presence of glands.
1. A. Desmidium (Ehr.); joints in the front view quadrangular, broader than long; crenatures distinct.
alpha. Filaments triangular, regularly twisted; crenatures rounded.
Desmidium aptogonum, Bréb. Alg. Fal. p. 65. t. 2. (1835). Menegh. Syn. Desm. in Linn. 1840, p. 203. Kütz. Phy. Germ. p. 141.
beta. Filaments plane; crenatures shallower and slightly angular.
Odontella Desmidium, Ehr. Infus. p. 153. t. 16. f. 4. (1838).
alpha. Ambleside, Westmoreland, Mr. Sidebotham. Dolgelley, J. R.
beta. Ambleside, Mr. Sidebotham.
North Germany, Ehrenberg. Carlsbad, Corda. Falaise, Brébisson.
The filaments, which are slenderer than those of Desmidium Swartzii, are slightly mucous, but have no distinct sheath. The joints are nearly as long as broad, and being excavated at their junction and connected only at the margins, they leave oval foramina between them.
The two states of this plant are so different that they seem at first sight distinct species. In alpha . the filaments are triangular, and being regularly twisted as in Desmidium, they in like manner exhibit a dark waved longitudinal line, marking the third angle; hence the foramina are far less conspicuous in this than in the plane variety, as they are obscured by the junctions of the third angle. When the dark line which denotes that angle approaches the margin of the filament they are more evident, and their size and figure may be ascertained. The marginal crenatures are deeper and more rounded, which fact, as Mr. Jenner has justly remarked to me, makes it doubtful whether a similar character can be relied on for distinguishing Desmidium quadrangulatum from Desmidium Swartzii. The end view is triangular; the sides are slightly concave and the angles rounded and somewhat thickened. The endochrome in this aspect has the rays equal in number to the angles, each ray being usually bipartite.
The variety beta . is an elegant microscopic object. Its foramina are large and conspicuous. The only specimens I have seen were gathered by Mr. Sidebotham. Its characters are so distinct that it is unnecessary to compare this variety with any other plant.
A careless observer might mistake the triangular form for Desmidium Swartzii. It differs however not only in the presence of foramina, but its filaments are more slender and its joints longer in comparison with their breadth.
Several filamentous Desmidieae have no slight resemblance to Taeniae, and none more than this species.
Length of joint of alpha . (Dolgelley specimen), including connecting processes, 1/1495 of an inch; length between foramina 1/1818; breadth of joint 1/1000.
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beta. (Ambleside specimen.) Length of joint, including the connecting pro-cesses, 1/1295 ; length between foramina 1/1938 ; breadth of joint 1/925.
Table XXXII f. I. a. portion of mature filament; b, c, d. transverse views; e. portion of a filament of beta. (Ambleside specimen); f . portion of a filament with dividing joints; g. two joints in an oblique position; h. lateral view.
