542 TRICHOMYCETES, ACTINOMYCETES, HYPHOMYCETES 



which is usually coarsely or finely granular. In the lower forms, 

 Phycomycetes, each hypha is a unicellular multinuclear cell, which 

 may be branched; in the higher forms, My corny cetes, the filaments 

 are multicellular, each cell being separated from its fellows by distinct 

 septa. A nucleus is demonstrable in a majority of the molds and it 

 is probable that it is present in all. 



Reproduction. The reproductive cells of the lowest and simplest 

 forms are scarcely differentiated morphologically from the vegetative 

 cells, indeed in some instances the distinction has never been made. 

 The hyphse break up and the fragments give rise to new colonies. 

 Reproduction in the Phycomycetes, of which the widely distributed 

 genus Mucor is a familiar type, occurs in the following manner a 

 constriction occurs near the tip of an aerial hypha and the extremity 



FIG. 85. Aspergillus sporangia. 



then increases in size until a spherical mass, the sporangium, is formed, 

 which divides into a number of spores. These escape with the rupture 

 of the sporangium and, if they reach a favorable medium, form the 

 starting points of new colonies. This is asexual reproduction. Sexual 

 reproduction takes place somewhat differently: lateral branches 

 from two adjacent hyphse meet and fuse. These branches or gameto- 

 phores are morphologically indistinguishable but differ in sex. The 

 fused cell enlarges to form a zygospore, separated from the hyphse 

 by septa, and eventually grows into a sporangium, from which asexual 

 spores escape and start new colonies. 



Among the Mycomycetes or higher molds, asexual reproduction alone 

 occurs. The simplest type begins as a thickening of the end of a hypha, 

 which soon constricts at regular intervals to form small spherical or 



