124 BOTANY. 



their corners; finally only the centres of the rounded ends 

 are left in slight contact, which soon hreaks. 



261. The sexual reproduction is well illustrated in Spi- 

 rogyra, one of the principal genera. At the close of 

 their growth in the spring, the cells push out short tubes 

 from their sides, which extend until they come in contact 

 with similar tubes from parallel filaments {A, Fig, 56), 

 Upon meeting, the ends of the tubes flatten upon each 

 other, the walls fuse together and soon afterwards become 

 absorbed, thus making a channel leading from one cell to 

 the other {B, Fig. 56). Through this channel the proto- 

 plasm of one cell passes into the other, and the two unite 

 into one mass, which becomes rounded and in a short time 

 secretes a wall of cellulose around itself (Fig. 56, £ and G). 

 The resting spore thus formed is set free by the decay of 

 the dead cell- walls of the old filament surrounding it; it 

 then falls to the bottom of the water, and remains there 

 until the proper conditions for its growth appear. 



262. The germination of the resting spore is a simple 

 process. The inner mass enlarges and bursts the outer hard 

 coat; it then extends into a columnar or club-shaped mass, 

 gradually enlarging upward from its point of beginning; 

 after awhile a transverse partition forms in it, and this is fol- 

 lowed by another and another, until an extended filament 

 is formed. 



263. The Black Moulds (Mucorini) are saprophytic and 

 some times parasitic plants; they are composed of long 

 branching filaments {hyphae), which always form a more 

 or less felted mass, the mycelium ; when first formed the 

 hyphse are continuous, but afterwards septa are formed in 

 them at irregular intervals. The protoplasmic contents of 

 the hyphse are more or less granular, but they never de- 



