. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . $ gametes). From thispoint of view, therefore, gamogenesis although includ-ing sexual reproduction is not identical with it. Itmust be remarked, however, that the gradation is sofine, especially in the Chlorophycese, that the distinc-tion is scarcely worth making. Sometimes, as inmany of the Zygnemaeeae, the gametes are morpho-logically indistinguishable but physiologically differ-entiated, and in these cases there is often a morpho-logical d
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . $ gametes). From thispoint of view, therefore, gamogenesis although includ-ing sexual reproduction is not identical with it. Itmust be remarked, however, that the gradation is sofine, especially in the Chlorophycese, that the distinc-tion is scarcely worth making. Sometimes, as inmany of the Zygnemaeeae, 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 A). The zygospore is formed by the union oflike (or nearly like) gametes, such as takesplace in many of the Protococcales, Ulotri-chales, Desmidiacea3, 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-midiacese, many Pr-otococcales, etc.), and inthe multicellular forms .the ordinary vege-tative cells become the gametangia with or. Fig. 92. Various forms of little-differentiated gametes. A, Spiro-Uyrti tenuissima (Hass.) aplanogametes andslightly differentiated gametan-gia. B—H, Planogametes. B,Isogamete of Trentepohlia Blci-schii (after Karsten); C, fusionof heterogametes of PandorinaMorum (Miill.) Bory; D-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. without some external change of form (Zyg-nemaceaB, Ulotrichales, etc.). In the Conjugate; 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 gametangium. 136 Chlorophyceae The planogametes are usually very simila
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