. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . Sometimes, as inmany of the Zygnemaceae, the gametes are morpho-logically indistinguishable but physiologically differ-entiated, and in these cases there is often a morpho-logical differentiation of the gametangia (consult ). The zygospore is formed by the union oflike (or nearly like) gametes, such as takesplace in many of the Protococcales, Ulotri-chales, Desmidiacea?, etc., and is the resultof a gamogenesis in which there are noext
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . Sometimes, as inmany of the Zygnemaceae, the gametes are morpho-logically indistinguishable but physiologically differ-entiated, and in these cases there is often a morpho-logical differentiation of the gametangia (consult ). The zygospore is formed by the union oflike (or nearly like) gametes, such as takesplace in many of the Protococcales, Ulotri-chales, Desmidiacea?, etc., and is the resultof a gamogenesis in which there are noexternal sexual differences between thegametes themselves. The gametes arise ingametangia. In the unicellular forms thecell itself becomes the gametangium (Des-midiacea?, many Protococcales, etc.), and inthe multicellular forms the ordinary vege-tative cells become the gametangia with orwithout some external change of form (Zyg-nemaceae, Ulotrichales, etc.). In the Conjugates the gametangium givesorigin to only one large non-ciliated aplanogamete, but in other groups it ismore usual for several ciliated planogametes to be formed in one Fig. 92. Various forms of little-differentiated gametes. A, Spira-lly ra tenuissima (Hass.) Kilt/,showing aplanogarnetes andslightly differentiated gametan-gia. B—H, Planogametes. B,Isogamete of Trentepohlia Blei-schii (after Karsten); C, fusionof heterogametes of PandorinaMorum (Mull.) Bory; B -F, es-cape of isogametes from gamet-angium, and fusion (E) to formzygote (F), of Monostroma mem-branacea W. & G. S. West; Gand H, gamete (G) and gamo-genesis (H) of Chlamydomonasmedia (after Klebs) ; gt, gamet-angium. 136 ChLorophycese The planogametes are usually very similar to the zoogonidia, although theyare generally somewhat smaller and there may be slight differences in theciliation. In those Chlorophycete with dissimilar sexual cells the female organconsists of an oogonium, which is usually more or less spherical (CEdogoniaceae)
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