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 



Fig. SIX— Ulothrix zonata. 



I Two filaments of this plant. « Escape of gametes in packets. » Spherical packet of gametes free from the filament. * Separa- 

 tion of the gametes. « Gametes swimming about and pairing. « Products of pairing of gametes (zygotes) attached to 

 substratum. >-s Zygote giving rise to zoospores. i» Two zoospores. » x 250; ^-i" 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 usually smaller and weaker plants. 

 The gametes which conjugate (see figs. 371 ^ and 371 ^) produce zygotes which sink 

 to the bottom of the water, and after a period of rest grow into unicellular plants, 

 each of which forms 2-14 zoospores (figs. 371 ^- ''• ^' ^). These probably give rise tc 

 ordinary plants. 



