. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . .; H—J, aplanospores («); A, zygospore, x , Micrasterias denticidatu Br£b., x 100. M, Euastrum oblongum (Grev.) Balfs, x , Cosmarium bioculatum Breb., x425. 0, Penium suboctangulart. W. West, x , Arthrodesmus octocornis Ehrenb., 425. Q, Staurastrum granulosum (Ehrenb.) Balfs, x 468. Sexual reproduction of a low, and probably degenerate, type occurs by theconjugation of two individual cells and the fusion of two large equal game
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . .; H—J, aplanospores («); A, zygospore, x , Micrasterias denticidatu Br£b., x 100. M, Euastrum oblongum (Grev.) Balfs, x , Cosmarium bioculatum Breb., x425. 0, Penium suboctangulart. W. West, x , Arthrodesmus octocornis Ehrenb., 425. Q, Staurastrum granulosum (Ehrenb.) Balfs, x 468. Sexual reproduction of a low, and probably degenerate, type occurs by theconjugation of two individual cells and the fusion of two large equal gametesto form a zygospore. The conjugation is similar to that in the Zygnemacea? 24—2 372 Conjugate and with the solitary exception of Desmidium cylindricum, there is no definiteconjugation-tube. Except in Desmidium Baileyi, and to a much less-markeddegree in Desmidium Swartzii and D. aptogonum, all the filamentous Desmidsdissociate into individual cells prior to normal conjugation (consult fig. 233 C,D and K; and fig. 235 J). Also, the zygospores are formed between thegametangia (or conjugating cells) in all Desmids with the solitary exception. Fig. 234. A—C, three stages in the conjugation of Staurastrum Dickiei Ralfs, x 356. D and E,Penium didymocarpum Luiid.; D, conjugation of four individuals just produced by division ;E, completed conjugation showing the double zygospore, x 464. F, Closteriiirn lineatumEhrenb., showing the double zygospore, x 100. z, zygospore. of Desmidium cylindricum Grev., in which the zygospore is formed within thefemale gametangium and a distinct conj ugation-tube is developed (fig. 235 J).The two conjugating cells, which are sexually indistinguishable, approximateand usually become enveloped in mucus. In the Cosmariese the longitudinalaxis of one conjugating cell is very often at right-angles to that of the other(fig. 234 A and B; fig. 235 J) and at the commencement of conjugation thesemicells of each individual come apart at the isthmus.
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