ENVIRONMENTAL FACTORS 63 



Considering the fungi as a whole, there is neeessarily no sharp 

 line of demarcation between the saprophytic and the parasitic habit. 

 Some organisms which attain their best development as parasites 

 may, if occasion demands it, sustain themselves saprophytically, 

 or they may normally undergo a portion of their existence as sapro- 

 phytes. The converse of this is also true-. With respect to this 

 habit four subdivisions may be recognized. 



Obligate parasites are, practically speaking, entirely dependent 

 upon other living things for requisite conditions of growth and 

 particularly, perhaps, for the organic nutrients. 



Obligate saprophytes grow upon nonliving substances and are 

 unable to penetrate living tissues. 



Facultative saprophytes are organisms which normally pass 

 through life as parasites, but which are capable for a time, or in 

 certain stages of development, of a true saprophytie existence. 



Facultative parasites are saprophytes which only occasionally, 

 or under very special conditions, may become parasitic. 



In making these distinctions it should not be assumed that 

 special weight is given to the form of the organic food materials 

 utilized by the fungus, since it is quite probable that a parasite on 

 the one hand and a saprophyte on the other may secure from its 

 host or from the substratum precisely the same compounds. Bio- 

 logical relations should be regarded as most important. For general 

 descriptive purposes and for biological discussion the classification 

 mentioned above is a matter of convenience, but opinions would vary 

 greatly if it were necessary to apply this specifically. 



The group in which obligate parasitism seems most clearly de- 

 fined is that of the rusts, Uredinales. It would be useless to try to 

 cultivate these fungi in nonliving substrata, that is, in artificial 

 cultures. The germination of the spores alone may proceed apart 

 from the host. In such groups as the Chytridiales and the downy 

 mildews, Peronosporaceae, the majority are obligate parasites ; yet 

 a few of the former order, and species of Phytophthora in the 

 latter family, have been grown on dead substrata. The smuts, 

 Ustilaginales, and the plum pockets and witches' brooms of stone 

 fruits, Exoascaceaej are strictly parasitic, although upon artificial 

 media the conidia of many species may sprout vigorously. The sur- 

 face mildews, Erysiphaceae, are doubtless also properly classed here. 



