SPIROGYRA 



37 



blends with it, forming a zygote (see p. 34). The 

 zygote secretes a membrane around itself, and passes 

 through a resting-stage before germination. The cells 

 which unite sexually are called " gametes " (Gr. gamos, 

 marriage). The second mode of conjugation is the more 

 frequent. Two filaments draw towards each other and 

 lie side by side ; very soon processes 

 are formed at the sides of the cells 

 of each thread, and unite as shown in 

 Fig. 11, and Plate II., Figs. 1 and 2. 

 In the meantime the cell-contents 

 round off, as previously described. 

 Ultimately the joined processes form 

 a conducting-tube along which the 

 gametes of one filament pass to those 

 in the other. The gametes fuse and 

 form zygotes. This process is often 

 called the " ladder " type of con- 

 jugation, because the joined pro- 

 cesses resemble the rungs of a ladder. 

 The zygotes are freed from the cell- 

 enclosures by the latter' s decay, and 

 become dispersed. They do not 

 ordinarily germinate at once, but 

 rest through the inclemency of 

 winter ; they can resist both cold 

 and drought. Each zygote, on germination, gives rise 

 to a new plant (Plate II., Fig. 3), which at first remains 

 attached to a substratum by a colourless and attenu- 

 ated base ; but after a short period of growth the fila- 

 ment detaches itself from its anchorage, and floats freely, 

 without displaying any definite base or apex. 



Fig. 11. — Spikog ytu 

 longata. x 350. 



Two filaments in conju- 

 gation, a, Early stage 

 of formation of con- 

 jugating processes ; 

 b, processes in con- 

 tact; ch, spiral chloro- 

 plast ; n, nucleus. 



