. The natural history of plants, their forms, growth, reproduction, and distribution;. Botany. THALLOPHYTA. 649 very various form. Zoospores are formed in most genera. Gametes, where known, are motile and isogamous. Aplanospores and akinetes are very commonly formed, under unfavourable conditions. Ulothrix (fig. 371), the best-known genus, possesses cells of very variable length. The chromatophore, which contains several pyrenoids, is an interrupted cylinder, and may or may not occupy the whole length of the cell. When the conditions are suddenly changed, zoospores or gametes are very readily
. The natural history of plants, their forms, growth, reproduction, and distribution;. Botany. THALLOPHYTA. 649 very various form. Zoospores are formed in most genera. Gametes, where known, are motile and isogamous. Aplanospores and akinetes are very commonly formed, under unfavourable conditions. Ulothrix (fig. 371), the best-known genus, possesses cells of very variable length. The chromatophore, which contains several pyrenoids, is an interrupted cylinder, and may or may not occupy the whole length of the cell. When the conditions are suddenly changed, zoospores or gametes are very readily formed, the former 1-4 the latter 4-32 in a cell. According to the size of the mother-cell and the number of divisions taking place, the size of the zoospores and gametes varies greatly, the '^t£30W^. Hg. S!l.—Ulothrix zonata. 1 Two filaments of this plant. ^ Escape of gametes In packets. ^ Spherical packet of gametes free from the filament. ^ Separa- tion of the gametes, b Gametes swimming about and pairing. 6 Products of pairing of gametes (zygotes) attached to substratum. 7-9 Zygote giving rise to zoospores, lo Two zoospores. ^ x 250; 2-10 x 400. (Partly after Dodel-Port.) only constant distinction between them being the number of flagella, which in the zoospores are four, in the gametes two (c/. figs. 371 ^^ and 371 ^). The zoospores or gametes escape from the mother-cell through a hole in the wall. They are surrounded by a bladder derived, probably, from ectoplasm. The swelling of this in the water helps to drag them out of the cell-cavity. The tension of the vacuole of the mother- cell, which is visible among the zoospores or gametes as a smaller bladder, also assists in pressing out the mass (figs. 371 ^' ^' *). The zoospore settles on some solid object, and, after putting out a short root-process from its colourless anterior end, produces a new cell-thread. Some of the gametes develop parthenogenetically, in which case they germinate just like zoospores, but produce
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectbotany, bookyear1895
