BLACK MOLDS 189 



and thereupon the egg secretes a thick double wall, and 

 becomes a resting spore. 



280. The resting spores remain in the tissues of the 

 host until the latter decay, which is generally in the 

 spring. Germination then takes place, in some species 

 by the production of a tube (either germ-tube, or co- 

 nidiophore), in others by the division of the protoplasm 

 into zoospores whose subsequent development is like 

 that described above in case of the conidia. 



281. The Black Molds (Mucoraceae) are saprophytic 

 and sometimes parasitic plants; they are composed of 

 long branching non-septate filaments (hyphae), which 

 always form a more or less felted mass, the mycelium. 

 The protoplasmic contents of the filaments are more or 

 less granular, but they never develop chlorophyll. The 

 cell walls are colorless, except in the fruiting filaments, 

 which are often dark-colored or smoky (fuliginous); 

 hence the name of Black Molds. 



282. The mycelium sometimes develops exclusively in 

 the interior of the nutrient medium; in 



other cases it develops partly in the me- 

 dium and partly in the air. In some 

 species the mycelium may attack the fila- 

 ments of other plants of the same order, 

 and even exhibit a weak parasitism upon 

 higher plants. 



283. The reproduction of black molds is asexual and 

 sexual. In the asexual reproduction (propagation) the 

 mycelium sends up erect filaments, which produce few or 

 many separable reproductive cells the spores. The 

 method of formation of the spores in a common black 

 mold (Mucor) is as follows: The vertical filaments, 

 which are filled with protoplasm, become enlarged at the 

 top, and in each an arched partition forms, constitut- 



