226 BOTANY. 



coated with a thick wall (/, Fig. 153). This zygospore is a 

 resting spore, and may retain its vitality for an indefinite 

 period. 



302. In the germination of the zygospore the first notice- 

 able change is the partial separation of the contents into two 

 portions, and the escape of the whole, surrounded by a deli- 

 cate wall, through a rent in the exospore (g, h, Fig. 153) ; 

 the separation of the protoplasm now becomes complete 

 (i, Fig. 153), and each portion becomes again partly divided 

 by lateral constrictions, which, however, do not quite reach 

 the centre ; in this way, within the mass which escaped from 

 the zygospore there are formed two constricted cells, which 



Fig. 153. Conjugation of Cosmarium Mtnenghinil. a, front ; b, end ; c, side 

 view of the adult plants ; d, two cells conjugating ; e, young zygospore formed ; /, 

 ripe zygospore, with spiny wall the four halves of the parent cells are empty : ff. 

 the zygospore germinating after a period of rest ; k, the young cell escaped from 

 zygospore ; i, young cell dividing, showing two new plants similar to a, placed 

 crosswise in the interior of the cell, x 475. After CErsted. 



are, in fact, new individuals resembling the original ones 

 which conjugated (a, Z>, c, Fig. 153). 



The descriptions above given are of the processes as they 

 take place in the bilobed Desmids ; in those which are not 

 lobed it takes place in essentially the same way, with differ- 

 ences only in the minor details. 



303. Desmids have the power of slow locomotion, and 

 they may often be seen moving across the field of the micro- 

 scope, or in a jar or bottle they may frequently be seen to 

 congregate in particular places. The mechanism of the 

 movement is unknown, but it appears to be certain that it is 

 not ciliary. 



Desmids are exclusively inhabitants of fresh water (not 

 salt), and in almost all cases they appear to prefer pure and 



