Zygneniace' 127 



gating filaments have entirely perished. In Spirogyra the two 

 outer coats are ruptured, generally at one end of the spore, and 

 the protoplasm is protruded as a filament clothed with the inner- 

 most cellulose wall. This filament becomes divided by a transverse 

 septum, one cell becoming an organ of attachment and containing 

 little or no chlorophyll, whereas the other cell increases in size, 

 contains one or more chloroplasts, and by repeated divisions forms 

 a filament. The distinction between base and apex is soon lost 

 and the filament floats freely in the water. Organs of attachment 

 have, however, been observed to be developed subsequently from 

 older cells. In the germination of zygospores of Zygnema the 

 distinction into base and apex is scarcely evident. 



Certain of the zygospores of Spirogyra velata Nordst. have 

 been observed to germinate immediately after their formation 

 without the development of a thick cell- wall (fig. 49 E and F) 1 . 



Genus Debarya Wittr., 1872. [Mougeotiopsis Palla 1894.] 

 The thallus consists of simple, cylindrical filaments, with or without 

 a distinct constriction between the cells, and with a thin external 

 mucous covering. There is one chloroplast in each cell, disposed 

 as an axile plate similar to that of Mougeotia, and containing 

 several pyrenoids. In rare instances pyrenoids are absent. The 

 gametes are formed from the entire contents of the gametangium 

 and conjugation takes place in the conjugating- tube, the mature 

 zygospore occupying a position between the gametangia. The 

 zygospores are very variable in character and there are consider- 

 able differences in the actual processes of conjugation, but in one 

 curious fact all the species agree. A peculiar change comes over 

 the empty gametangia as the zygospore is being formed; they 

 become very clear and refractive, and sometimes a series of stria- 

 tions become visible parallel to the transverse walls. They have 

 at this stage the appearance of solidity, most likely owing to the 

 deposition of annular thickenings of cellulose inside the cell-wall 

 on the receding of the protoplasm during conjugation. This 

 feature is noticeable not only in living specimens, but also in old, 

 preserved examples. 



All the species of this genus are of rare occurrence. D. glyptosperma (De 

 Bary) Wittr. is more widely distributed than the others; the cells are 9 15 ^ 

 in thickness and 6 15 times longer than the diameter ; the conjugating- tubes 



1 W. & G. S. West iu Ann. Bot. xii, 1898, t. v, f. 84, 85. 



