I40 



MORPHOLOGY, 



tinues until the content of the suppl}-ing cell has passed over into that of the 

 rsceptive cell. The protoplasm of this one is now slipping away from the 

 cell wall, until finally the two masses round up into the one zygospore. 



292. Th.e zj'gospore. — This z\'gospore now acquires a thick wall which 

 eventually LecoRK's brown iu color. The chle'n-phyll color fades out. and a 

 large part of the protoplasm pas^us into an oily substance which makes it 

 more resistant to conditions which would be fatal to the vegetative threads. 

 The zygospores are capiible therefore of enduring extremes of cold and dry- 

 ness which would destroy the threads. They pass through a "resting" 

 perie)d, in which the waler in the piMid may be fntzen, or tlrieil. and with the 

 oncoming of fav<irable conditions for growth in the spring or in the autumn 

 they germinate and ]iroduce the green thread again. 



293. Life cycle.- — The growth i^f the spirogyra thread, the conjugation of 

 the gametes and fumation id" ih*.- zygospore, and the growth of the thread 

 from the zygospore again, makes what is called a complete life cycle. 



294. Fertilization. — While conjugation results in the fusion of the two 

 masses of protoplasm, fertilization is accomplish<.*d when the nuclei of the 

 two cells come togctlier in the zNgospore lukJ fuse into a single nucleus. The 



Fig. 13 !• 

 Fertilization in spirogyra ; sliows different stages of fusion uf tlit two nuclei, with mature 

 zygospore at right. (After Overton.) 



ilifferent stages in the iusii m ( 'f the two nuclei of a recentl>" formed z}'gi>spore 

 arc shown in iiginvi3i. 



In the conjugation of the twoCells. the chloreiphyll band of the supplying 

 cell is said to degenerate, so thai in the new plant the number of chlon)[)hyll 

 band:^ in a cell is not inci'eased b\' the union of the tw(.> cells. 



295. Simplicity of tlie process. — Iu spirogyra any cell of the thread 

 may f>rm a gamete (excepting the holdfasts of some sjtecies). Since all of 

 the cells e>f a threail are }>raelica ll\' alike, there is no structural difference 

 between avegetative cell andacell about to Ct.mjugate. The ditlerence is a 

 [)liysiole)gical one. All the celK are c.ipable of conjugalion if the physiolog- 

 ical conditions are present. All the cells thereforL' are potential gametes. 

 (Strictly speaking the wall of the cell is the ,j,^(7W(-/'(7;/i,'//('///, while the content 

 forms (he gamete. } 



While there is sometimes a slight dilterence in size between the conjugal- 



