400 



BOTANY OF THE LIVING PLANT 



in the peripheral cytoplasm, coiled corkscrew-fashion. As in the 

 Conjugatae generally, pyrenoids occur in the chromatophores : they 

 are highly refractive, and form centres for the formation of starch, 

 while the threads that suspend the nucleus usually run out to them. 



Vegetative propagation is simply by division of the cells, which 

 occurs during the night, and may be continued indefinitely. As the 

 season progresses the filaments conjugate. Adjacent filaments 

 put out processes from cells opposite one another, which meet, 

 flatten^ and fuse at their tips, the intervening wall being absorbed. 



■M'^^-" 



T^^ 



^\ 



Fig. 338. 

 Germination of a Desmid, Closteriiim, after Klebahn. i, zygote before nuclear 

 fusion; 2, first nuclear division; 3, binuclear stage; 4, second nuclear division; 5, two 

 cells, each with a small and a large nucleus. 6, formation of two new Closterium cells 

 each with one large nucleus ; the other is disorganised. (From Oltmanns.) 



Their protoplasts contract, one earlier than the other ; it then 

 passes bodily through the now open tube, and its cytoplasm coalesces 

 with that of the other cell, while its chloroplast disorganises 

 (Fig. 337). The nuclei remain for a time distinct. The zygote 

 changes to a reddish colour, fats are stored in it, and a thickened wall 

 is formed. Freed from the parent filaments it remains dormant. 

 On germinating the outer* wall ruptures, while the inner covers the 

 enlarging protoplast. The fused nuclei divide into four, but of these 

 only one survives as the nucleus of the first cell, from which by division 

 a new filament arises. 



In Mesocavpus the conjugation is similar, except that the zygote 

 is formed actually in the tube connecting the conjugating cells. 



