Frond simple, elongated, straight, cylindrical or fusiform, slightly constricted at the middle; segments emarginate at the end, but otherwise quite entire.
The frond is elongated, as in Penium, from which however this genus may be distinguished by the emarginate ends; the same character and the elongated frond will separate it from Cosmarium.
From Euastrum, with which it agrees in the emarginate extremities, it differs in being cylindrical or nearly so, and in the segments being neither lobed nor sinuated; the fronds are also free from inflated protuberances. Sporangia have been gathered of two species.
1. T. Brebissonii (Menegh.); frond in the front view with parallel sides, but in the lateral view fusiform; the ends without any projecting processes; puncta in longitudinal lines.
beta. turgidus; larger, constriction greater; segments somewhat inflated.
Closterium Brébissonii, Menegh. Syn. Desmid. in Linnaea 1840, p. 236.
Closterium (sp. 9), Bailey, Amer. Bacil. in Amer. Journal of Science and Arts, vol. 41. no. 2. t. 1. f. 38 (1841).
Tetmemorus Brebissonii, Ralfs, Ann. of Nat. Hist. v. 14. t. 8. f. 1 (1844); Trans. of Bot. Soc. of Edinburgh, v. 2. p. 133. t. 12. Jenner, Fl. of Tunbridge Wells, p. 198. Hassall, Br. Freshwater Alg. p. 377.
Closterium monile, Kützing, Phycol. Germ. p. 132 (1845).
Dolgelley; Carmarthen; Penzance, J. R. Sussex; Kent; Surrey and Hants, Mr. Jenner. Many stations in Aberdeenshire (alt. 50-3700 feet), Mr. P. Grant and Dr. Dickie. Banffshire, Mr. P. Grant. Ambleside, Mr. Sidebotham.
beta. Ashdown Forest, Sussex, Mr. Jenner.
Falaise, Brébisson. Germany, Kützing. Maine to Virginia, Bailey.
Frond four to six times longer than broad, with a slight constriction at the middle. The front view shows the lateral margins of the segments nearly straight, and their ends rounded and emarginate. The lateral view is more constricted at the middle and the segments are attenuated towards the end.
The endochrome is dark green, and there is a series of large vesicles down the middle in a single row, either straight or with some irregularity. The frond, when empty, is found to be minutely punctate, the puncta being arranged in longitudinal rows.
The var. beta. somewhat approaches in form to Tetmemorus granulatus, but is more constricted at the middle, and the puncta form longitudinal lines.
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Tetmemorus Brebissonii differs from T. granulatus in having its front and lateral views unlike each other, and its puncta arranged in longitudinal lines; it is also generally smaller.
Length of immature frond 1/384 of an inch; breadth 1/1385; breadth at constriction 1/1666; length of mature frond 1/142; breadth 1/704; breadth at constriction 1/839.
Tab. XXIV. fig. 1. a. front view; b. side view; c. empty frond; d, e. front and side views of var. beta;. f. young frond.
2. T. laevis (Kützing); frond in the front view somewhat tapering, with truncate ends; lateral view fusiform; puncta none or very indistinct.
Tetmemors granulatus (in part), Ralfs, Annals of Natural Hist. v.14. t. 8. f 2. d, e, f, g (1844); Trans. of Bot. Soc. of Edinburgh, v. 2. t. 12.
Dolgelley, J. R. Aberdeen, Dr. Dickie. Ashdown Forest and near Tunbridge Wells, Sussex, Mr. Jenner.
Germany, Kützing.
Frond four to six times longer than broad, constricted at the middle, tapering with straight sides; ends truncate, with a hyaline lip, which is inconspicuous, and sometimes absent.
The puncta are so faint, that even when using the higher powers of Mr. Ross's achromatic microscope I have doubted their existence; but Mr. Jenner and Mr. Ross assure me that they are scattered, as in Tetmemorus granulatus.
I have gathered the sporangia of this species for three successive years near Dolgelley, forming a mucous stratum on the moist soil; I have also seen them mixed with Desmidieae sent from Aberdeen by Dr. Dickie.
After coupling, the segments of the fronds are separated by the formation of a large, quadrate, central cell, in which all the contents of both fronds are collected, the empty segments being loosely attached to its corners. The endochrome at first fills the cell, large starch globules being scattered throughout the minutely granular substance; but at length it becomes a dense, round, homogeneous body of a dark green colour, which finally changes to an olivebrown. In this stage the segments of the original fronds fall off, and leave the quadrate cell inclosing the sporangium. In the front view, as stated above, the cell is nearly square; the sides are concave, and the angles rounded and slightly produced. A lateral view shows that the cell and sporangium are both compressed.
In the present plant the process of forming the sporangium is interesting, as it exhibits a striking similarity to the change during the formation of similar bodies in Staurocarpus among the Conjugatae. In Staurocarpus, after conjugation, a subquadrate cell is formed, within which the endochrome is collected. The latter is at first of the same figure as the cell, but in one species at least is at length condensed into a compact globular body, and in every species the cell with the contained sporangium finally separates from the fila-
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ments with which it is connected. In this separate state I can discover no character by which to distinguish the sporangium of Tetmemorus from one belonging to a species of Staurocarpus.
This plant is in size intermediate between Tetmemorus Brebissonii and T. granulatus. In figure it agrees with the former; like the latter it has a hyaline lip, but one far less conspicuous. This character induced me, in my paper read before the Botanical Society of Edinburgh, to refer its sporangia to that species, and I did not discover my error until I had seen the true sporangia of T. granulatus.
Tetmemorus laevis is distinguished from T, Brébissonii either by the absence of puncta or by their scattered state when visible; its fronds also taper more in the front view. From T. granulatus it differs in being more constricted at the middle, and in having the front and lateral views dissimilar, irrespective of the terminal notch.
Length of frond from 1/374 to 1/336 of an inch; breadth from 1/1244 to 1/1073; breadth at constriction from 1/1322 to 1/1147; length of side of quadrate sporangium 1/447.
Tab. XXIV. fig. 3. a. front view; b. side view; c. empty frond; d, e, f. different stages of sporangium; g. side view of sporangium.
3. T. granulatus (Bréb.); frond fusiform both in the front and lateral views, and ending in a colourless projecting lip-like process.
Carnarvon. Dolgelley. Tal Sarn near Lampeter, and Penzance, J. R. Sussex, Surrey, Kent and Hampshire, Mr. Jenner. Kerry, Mr. Andrews. Hanham near Bristol, Mr. Thwaites. Charlton Fields, Manchester; and Ambleside, Westmoreland, Mr. Sidebotham. Aberdeenshire (alt. 1600 to 2455 feet), Dr. Dickie. Aberdeenshire and Banffshire (alt. 50 to 1600 feet), Mr. P. Grant.
Falaise, Brébisson. Germany, Kützing.
Frond fusiform, about six times as long as broad, and very slightly constricted at the middle. The extremities always have a colourless projecting lip-like process, which extends beyond the notch. The colouring is dark green, and a few large vesicles are arranged in a longitudinal row down the middle.
The empty frond is minutely punctated; the puncta generally form one or two transverse lines in each segment near the central constriction; in the other parts of the frond they are not in rows, but scattered.
Under a low power of the microscope this species much resembles the preceding; but it may always be distinguished by the front and lateral views being similar, and only differing in the terminal notch, which is not seen in the lateral view. There is also beyond the notch a remarkable lip-like pro-
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jection, which is wanting in Tetmemorus Brebissonii, and the puncta seen in its empty frond are not arranged in longitudinal rows.
Tetmemorus granulatus differs from T. laevis in its less constricted middle and its larger size. Its puncta too are usually very distinct, and the discovery of their respective sporangia leaves no doubt that these plants are distinct.
Its sporangia have been gathered by Mr. Jenner in Sussex, and by myself near Dolgelley. They differ considerably from those of T. laevis; they are orbicular, and are not inclosed in a quadrate cell, but have the empty segments of the conjugating fronds loosely attached by an imperceptible membrane; the margin of the sporangium is finely striated, a character which I have not noticed in Tetmemorus laevis.
Length of frond from 1/133 to 1/130 of an inch; breadth 1/649; breadth at constriction from 1/736 to 1/685; diameter of sporangium from 1/365 to 1/349.
Tab. XXIV. fig. 2. a. front view; b. side view; c. empty frond.
Closterium laeve, Kützing, Phycologia Germanica, p. 132 (1845).
Tetmemorus granulatus, Ralfs, Annals of Nat. Hist. v. 14. t. 8. f. 2 (1844); Trans. of Bot. Soc. of Edinburgh, p. 134. t. 12. Jenner, Fl. of Tunbridge Wells, p. 198. Hassall, Br. Freshwater Algae, p. 378.
