PHTC0PH7TA. 143 



253. 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, Pig. 73). Upon 

 meeting, the ends of the tubes flatten upon each other, 

 the walls fuse together, and soon afterward become ab- 

 sorbed, thus making a channel leading from one cell to 

 the other {B, Fig. 73). 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. 73, B and C). 

 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. 



254. 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 begin- 

 ning ; after a while a transverse partition forms in it, and 

 this is followed by another and another, until an extended 

 filament is formed. 



255. The Black Moulds (Mucoracece) are saprophytic and 

 sometimes parasitic plants; they are composed of long 

 branching filaments {hyplice), 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. Tlje protoplasmic contents of 

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

 velop chlorophyll. The cell-walls are colorless, except in 

 the fruiting hyphae, which are usually dark-colored or 

 smoky (fuliginous) ; hence the name of Black Moulds. 



