. The algae. Algae. 40 THE ALGAE Under dry conditions aplanospores are formed: when this oc- curs the parent cell gelatinizes and a *palmella' stage results, the cells of which subsequently give rise to 2 or 4 biflagellate gametes. In culture solutions of low nutrient concentration reproduction takes place by means of zoospores, whilst in highly concentrated solutions they are replaced by aplanospores. The environment can therefore affect the type of reproductive body. The production of aplanospores suggests how the genus Chlorella may have Fig. 15 Chlorococcum humicolum. A-C, vegetat
. The algae. Algae. 40 THE ALGAE Under dry conditions aplanospores are formed: when this oc- curs the parent cell gelatinizes and a *palmella' stage results, the cells of which subsequently give rise to 2 or 4 biflagellate gametes. In culture solutions of low nutrient concentration reproduction takes place by means of zoospores, whilst in highly concentrated solutions they are replaced by aplanospores. The environment can therefore affect the type of reproductive body. The production of aplanospores suggests how the genus Chlorella may have Fig. 15 Chlorococcum humicolum. A-C, vegetative cells. D-Fj zoospore formation ( x 800). (After Smith.) whilst the multinucleate state indicates a transition to a coenocytic phase that is further developed in other genera {Protosiphon, Hydrodictyon), Asexual reproduction is by zoospores or aplanospores and sexual reproduction by biflagellate iso- or anisogametes. There is no evid- ence of even a primitive oogamy such as is found in Chlamydo- monas. The allied genus Trebouxia (Cystococcus) forms the algal com- ponent in many hchens (see p. 427). The cells differ from those of Chlorococcum in lacking an irregularly thickened wall and the plastid is axile and not parietal. Characiaceae : Characium (a slip or cutting). Fig. 16 Each plant is a soHtary imicell with a single parietal plastid, and motile only in the reproductive phase. It may be supposed that it has been derived from an ancestral motile organism that ceased to. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Chapman, V. J. (Valentine Jackson), 1910-. London, Macmillan; New York, St. Martin's Press
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Keywords: ., bo, bookcentury1900, bookcollectionbiodiversity, booksubjectalgae
