THE THALLOPIIYTA 



289 



because of their relation to nitrogen (Fig. 18). These are the 

 7iitrogen-fixing bacteria, which are found in tubercles on the roots 

 of plants belonging to the Legume family and which are able to 

 take nitrogen directly from the air and to incorporate it into 

 their bodies; and the nitrifying bacteria which convert ammonia, 

 the end product of protein decay, into nitrite salts and these, in 

 turn, into nitrate salts, the only form in which nitrogen can 

 generally be utihzed by green plants. 



Phycomycetes or Alga-like Fungi.^These fungi, as their name 

 implies, resemble rather closely certain of the algae, particularly 



Fig. 164. — The bread-mold (Rhizopus). General habit of the plant. The 

 mycelium (A) of much-branched hyphae penetrates the substratum and sends up 

 into the air stouter hyphae, the sporangiophores (B), on which are borne the 

 spherical sporangia (C). One sporangium has burst, shedding its spores, a few of 

 which still adhere to the central axis or columella of the sporangium. The two 

 groups of sporangiophores are connected by a stolon (D) . 



in their methods of reproduction. They seem especially near 

 the Siphonales because of the fact that their filaments (hyphae) 

 are coenocytic, and it is probably from that general region of 

 the algal series that they originated. 



The Phycomycetes include a great many of the forms which we 

 know as molds and blights. Three orders are particularly notable 

 and we shall describe them briefly. These are the Mucorales, 

 the Saprolegniales, and the Peronosporales. 



