Frond simple, elongated, cylindrical or fusiform, straight, entire, not constricted at the middle; ends rounded; endochrome spiral.
The fronds are simple, straight, cylindrical, slightly attenuated at the extremities; they are quite entire and not constricted at the middle. The endochrome is spiral, as in Zygnema, and, except in fronds about to divide, there is no interruption at the centre.
The spiral arrangement of the endochrome will distinguish Spirotaenia from every other genus in the family. It also differs from Tetmemorus in the entire extremities, and from that genus as well as from Docidium in the absence of a constriction at the centre. It differs likewise from Closterium by its straight fronds and the absence of a globule near the extremities.
Spirotaenia affords another example of resemblance to the Conjugatae. The endochrome is so exactly like that of Zygnema, that, until we saw M. de Brébisson's drawing, both Mr. Jenner and myself had regarded the S. condensata as the young state of a Zygnema. It however does not form a filament, the cells dividing in the manner of the Desmidieae; but the division is oblique, as in Scenedesmus; thus affording a character by which it may be distinguished from Penium, even should the spiral form of the endochrome be absent.
In Spirotaenia the complete division of the cell is prior to the division of the gelatinous covering, which thus retains the two newly parted cells together for some time longer; a fact which convincingly proves that this genus belongs to the Desmidieae.
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1. S. condensata (Bréb.); endochrome a single, broad, closely spiral band.
Spirotaenia condensata, Brébisson, in lit. cum icone (1846).
Common. Dolgelley and Penzance, J. R. Henfield, Sussex, and bogs at Fisher's Castle, Tunbridge Wells, Mr. Jenner. Ayrshire, Rev. D. Landsborough. Aberdeenshire, Dr. Dickie and Mr. P. Grant. Ambleside, Mr. Sidebotham. Near Bristol, Mr. Thwaites.
Falaise, Brébisson.
Frond bright yellow-green, cylindrical with rounded ends, or fusiform, five to ten times longer than broad, enclosed in an evident mucous covering. The endochrome consists of a single broad spiral band. The revolutions are close, and from seven to twelve in number.
This is a beautiful plant; and in a recent state the spiral arrangement of the endochrome is very conspicuous, but it collapses shortly after being gathered.
Length of frond 1/208 of an inch; breadth 1/1048.
Tab. XXXIV. fig. 1. a, b. mature frond; c. fronds after division.
2. S. obscura --- ; endochrome at first in several slender, spiral threads, afterwards uniform.
Dolgelley and Penzance, J. R. Bogs at Fisher's Castle, Tunbridge Wells,Mr. Jenner.
Frond dark green, cylindrical or fusiform, extremities attenuated, five to eight times longer than broad. When young the endochrome is distinctly spiral, and the frond resembles a joint of a many-spired species of Zygnema. The threads or fillets are several in number, slender, and each of them makes but one or two revolutions in the length of the frond. Generally however the endochrome forms a uniform dark green mass without any appearance of spires. In this state, a small hyaline space left at each extremity will frequently be found to contain a small granule.
I long regarded this species as probably a variety of Spirotaenia condensata, nor was it until I met with a young specimen that I was fully satisfied of its distinctness. Even specimens which do not exhibit the spires may be recognized by the darker colour of their endochrome and more tapering extremities.
Length of frond from 1/247 to 1/226 of an inch; greatest breadth from 1/1020 to 1/907.
Tab. XXXIV. fig. 2. a, b, c. single fronds; d, e. f'ronds after division.
