. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . points along its length. 16—2 24:4 Siplionales Considering the diameter of the filaments the cell-wall is as a rulerelatively thin. This fact, combined with an absence of elasticity of the wall,causes the filaments to collapse very readily unless carefully handled. The cytoplasm forms a thick lining layer in which numerous minutenuclei are embedded. The chloroplasts are very small and discoidal, with anoval or subcircular outline, and occur i
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . points along its length. 16—2 24:4 Siplionales Considering the diameter of the filaments the cell-wall is as a rulerelatively thin. This fact, combined with an absence of elasticity of the wall,causes the filaments to collapse very readily unless carefully handled. The cytoplasm forms a thick lining layer in which numerous minutenuclei are embedded. The chloroplasts are very small and discoidal, with anoval or subcircular outline, and occur in very large numbers in the lininglayer. They are without pyrenoids. The stored reserve material consists ofglobules of oil in Vaucheria and grains of starch in Dichotomosiphon. As inother cases in the Chlorophycea3, and as pointed out by Fleissig (00), thesereserves are physiologically analogous. Septa normally appear in the tubular thallus only in connection with thereproductive organs. Injury, however, results in the appearance of septacutting off the injured parts, the uninjured portions developing into newplants (consult fig. 157 A and B).. Fig. 157. A and B, portions of thallus of Vaucheria showing formation of septa on injury ;A, Vaucheria gcminata (Vauch.) DC.; B, V. ornithocephala Ag. C, apex of filament ofV. sessilis (Vauch.) DC. showing the zoogonidangium. D, germination of the zoogonidium ofV. ornithocephala. E, V. sessilis showing developing oogoniuin (on right) and antheridium(on left). F—H, V. geminata (Vauch.) DC. showing development of sexual x 75. a, antheridium ; oo, oogoninm ; zg, zoogonidangium. Asexual reproduction usually takes place by large solitary zoogonidia of aunique character, which have received the name of synzoospores owing totheir obviously compound nature. During the development of the zoogoni-dium the growing apex of a filament assumes a club-shaped form andbecomes of an intense green colour, after which a transverse sept
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