THE FUNGI 



265 



O 



favorable medium and are then placed in a suitable temperature 

 (15-30 C.) for twenty-four hours, all stages in the germina- 

 tion process, can be observed with a sufficient magnification. 

 The elongation of the inner coat to form a hypha is accom- 

 panied by nuclear and cell division, so that the resulting hyphse 

 and mycelium are multicellular. The individual hyphse have a 

 delicate cell wall with highly granular vacuolate cytoplasm, and 

 repeatedly give off branches, ultimately 

 forming the patches of mold already de- 

 scribed. In nature and in the artificial 

 sowings of spores more than one spore 

 usually starts a mycelium at a given point. 

 The centrifugal method of growth which 

 follows is advantageous in bringing the my- 

 celium in contact with more of the nutrient 

 material and in forming a larger surface 

 for the formation of spores. As growth 

 proceeds submerged hyphse grow down 

 into the fruit or other nutritive medium 

 and perform the functions of digesting 

 and absorbing food for the entire plant. 



Asexual reproduction. If patches of 

 Penicillium are examined, it will be seen 

 that they gradually change their color from 

 pure white to sage green. This change in 

 color usually begins in the older central portion of the cir- 

 cular patches of mycelium and spreads from the center toward 

 the circumference of any given mold colony. It is due to the 

 formation of innumerable spores, which, taken in the mass, 

 are green in color. The spores are formed upon erect aerial 

 hyphse, which branch repeatedly at the end, forming a brush- 

 like or treelike growth of short hyphal branches (Fig. 144). 

 Each branch then begins to constrict below the apex just as 

 though a thread had been tied around the hypha at this point 

 and then gradually tightened. This process is repeated until 

 each hyphal branch of the main spore-bearing hypha is con- 

 verted into a row of spores. The older spores at the ends of 



FIG. 144. The method 



of producing spores in 



Penicillium 



Consult the text for a 

 discussion of the spore- 

 forming process. At the 

 right are germinating 

 spores. After Thorn 



