220 ALG^E 



zoids of the Muscineae and Vascular Cryptogams. The vegetative 

 thallus is here either composed of branching filaments, or is reduced to 

 a flat plate of cells. In the two lower orders, the CEDOGONIACE^E and 

 the SPH^ROPLEACE^E, it consists of an unbranched filament of uni- 

 nucleated or multinucleated cells ; and in them the impregnation of the 

 oosphere by the motile antherozoids is brought about directly, without 

 the intervention of a trichogyne. Non-sexual propagation by means of 

 swarming biciliated zoospores, formed within zoosporanges, and bearing a 

 close resemblance to the antherozoids, occurs in the Coleochaetaceae and 

 Sphaeropleacae ; in the QEdogoniaceae the zoospores are much larger 

 bodies, bearing a tuft of cilia. The class displays a rudimentary alter- 

 nation of generations. 



ORDER i. COLEOCH^TACE^:. 



The typical genus of this small order, Coleochczte (Breb.), comprises 

 several species of small fresh- water Algae forming minute discs or cushions 

 attached to submerged plants, from -^ to inch in diameter, consisting, 

 in the simpler forms, of a single layer of cells, often arranged in rays 

 proceeding from a common centre. Some of these cells are furnished 

 with colourless bristle-like protuberances fixed into narrow sheaths. All 

 the cells of which the thallus is composed usually lie in one plane, but 

 their degree of union with one another varies. In C. scutata (Breb.) 

 they are closely united into a compact disc, which continues to grow by 

 peripheral increase, the marginal cells dividing by radiating and tangential 

 walls. In C. soluta (Pringsh.) the thallus consists of a number of branches, 

 which ramify dichotomously, and lie side by side, more or less closely 

 crowded, in one plane. In other species, as C. pulvinata (A. Br.) and 

 divergens (Pringsh.), the branches do not ramify in one plane only, but 

 develop also segmented ascending branches, which form, in the latter 

 species, together with the original disc, a nearly hemispherical cushion. 

 The entire thallus is always enveloped in mucilage. 



Non-sexual propagation takes place in Coleochaete by means of 

 biciliated zoospores, produced either in all the cells of the thallus or only 

 in the terminal cells of the branches. The entire protoplasmic contents 

 of the mother-cell or zoosporange are used up in the production of the 

 zoospores, which escape from them either at the side or at the back. 

 'Sexual reproduction is effected by the impregnation of an oosphere formed 

 within an oogone by motile antherozoids, through the agency of a tricho- 

 gyne. The oogone is, in C. pulvinata, always constituted out of the 

 terminal cell of a branch, which swells up and at the same time 

 elongates at its extremity into a narrow hair-like trichogyne, which then 



