. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . is family is here limited tofilamentous unbranched types of the Heterokontse. Tribonema is one of themost abundant of the genera of Green Algse, T. bombycina having a world-wide distribution. In T. aflinis and other species the filaments are exactlycylindrical, but in T. bombycina the cells are normally a little barrel-shapedand not infrequently a trifle irregular in their growth (fig. 263 A). The 412 Heterotrichales apical cell may have a co
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . is family is here limited tofilamentous unbranched types of the Heterokontse. Tribonema is one of themost abundant of the genera of Green Algse, T. bombycina having a world-wide distribution. In T. aflinis and other species the filaments are exactlycylindrical, but in T. bombycina the cells are normally a little barrel-shapedand not infrequently a trifle irregular in their growth (fig. 263 A). The 412 Heterotrichales apical cell may have a conical extremity or, as in T. affinis, it may beapiculate. The cell-wall is very firm and- has a definite structure, readilybreaking up into H-pieces in the genus Tribonema, but to a much lessmarked degree in Bumilleria. Each H-piece consists of a transverse wallwith a cylindrical piece on either side, and the whole is composed of a numberof apposed layers of pectic compounds (fig. 263 G). Each cell is thusbounded by the halves of two H-pieces. The cells are uninucleate or veryrarely binucleate. The chromatophores are parietal discs in T. bombycina. Fig. 263. A—G, Tribonema bombycina (Ag.) Derb. & Sol.; A, part of vegetative filament;L, showing aplanospores (np); C, zoogonidium; D—F, germination of zoogonidium andformation of young plants; G, showing structure of cell-wall after treatment with potassiumhydrate. H and I, T. bombycina forma minor (Wille) G. S. West. G, x 570 (after Bohlin);A—F, H and I, x 450. and most other species (consult fig. 263 A and B). It is this discoidalcharacter of the parietal chromatophores which at once distinguishes thecommon species of Tribonema from all other Green A Igte. In T. affinis thechromatophores, although parietal, are few in number and very irregularin shape. The stored reserve is oil, which is scattered in small globulesthrough the cell. Asexual reproduction takes place by the formation of globular or ellipsoidaplanospores,
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