64 THE FUNGI WHICH CAUSE PLANT DISEASE 



since no photosynthesis occurs, this process is never masked as it 

 is in the case of the chlorophyll-bearing plants. 



In nutrition requirements there is great diversity; but in all 

 cases carbon must be taken from some organic source. Starch, 

 sugar, cellulose and kindred compounds are frequent sources of 

 the carbon food supply. Nitrogenous foods are, generally speaking, 

 not required in such abundance by the Eumycetes as by the 

 bacteria and advantage may frequently be taken of this fact in 

 isolating the fungi from bacteria by growing them on media poor 

 in nitrogen, in which case the fungi often outgrow the bacteria. 



The color of the fungi is determined largely by the constitution 

 of the media upon which they grow.^; '' ''• * 



Many fungi exhibit a peculiar heteroecism, that is, part of their 

 life cycle is passed through upon one host, part of it upon another 

 host, even of very distant botanical kinship. Thus among the 

 rusts; in one instance part of the life cycle is upon the apple, the 

 remainder upon the cedar tree. Fungi also exhibit polymor- 

 phism, i. e., in one stage they exhibit one spore form and in an- 

 other stage another spore form totally different. In this way 

 several apparently quite distinct types of spores and sporiferous 

 structures may belong to the same species. 



Classification of Fungi.*- ''•"'"•**■ " The true fungi in them- 

 selves constitute a very large group made up of diverse forms, many 

 of which are as yet little known. Any satisfactory system of classi- 

 fication is impossible until much more knowledge is gotten regard- 

 ing their morphology, cytology, life histories and especially their re- 

 lations to their hosts. According to present knowledge they com- 

 prise very numerous species distributed in three classes as follows: 



Key to Classes of Eumycetes 

 Mycelium continuous in vegetative 



stage Class 1. Phycomycetes, p. 65. 



Mycelium septate 



Spores in asci Class 2. Ascomycetes, p. 113. 



Spores on basidia* Class 3. Basidiomycetes, p. 298. 



Not as above; spores on conidio- 

 phores, naked, or in pycnidia; 



or spores quite unknown Fungi Imperfecti, p. 475. 



* In the rusts and smuts the promycelium is regarded as a basidium. 



