8 INTRODUCTION 



various organic acids, asparagin, peptone, gelatin, glycerine, etc., and 

 under both aerobic and anaerobic conditions. 



On the other hand many fungi are obhgatory parasites and methods 

 have not yet been devised by which they can be grown except upon 

 suitable living host tissues. This is true of the Rusts (Uredinales), the 

 White Rusts (Albuginaceae), most of the Downy Mildews (Peronospo- 

 raceae), the minute insect parasites belonging to the Laboulbeniales, etc. 

 This would seem to indicate that for such parasites the choice of food is 

 very strictly limited. Among some of the commoner parasites, e.g., stem 

 rusts of wheat, Puccinia graminis tritici Erikss. & Henn., and powdery 

 mildew of various grasses, Erysiphe graminis DC, there have been 

 found what are called biologic or physiologic forms. These are races of 

 the fungus that are indistinguishable except for the fact that one will 

 grow only on certain species or varieties while the other races grow only 

 on other varieties. 



Parasitic fungi may be destructive or balanced parasites. The former 

 may kill the host cells or tissues by means of some poisonous substances 

 which may even diffuse out in advance of the fungus hyphae with the 

 consequence that the latter actually enter dead tissues. Some species of 

 Botrytis, Sderotinia, Pythium, etc., are of this type. On the other hand a 

 balanced parasite is so well adapted in its demands upon the host to the 

 ability of the latter to supply these needs and to continue to live and 

 grow that both fungus and host develop together until the time comes 

 when the fungus is ready to produce its spores. Then it too destroys the 

 surrounding tissues. Many of the Smuts (Ustilaginales) are balanced 

 parasites. Actually all gradations between these two extremes may be 

 found. Mostly the destructive parasites are less strictly confined to 

 definite hosts than are the balanced parasites. 



Some fungi belonging to the Polyporaceae attack and destroy only 

 the dead cells (wood fibers, tracheary tissues, etc.) of the wood of living 

 trees and from that viewpoint are saprophytes, yet their growth ceases 

 when the death of the tree occurs. The conditions within the host tissues 

 that favor the growth of the fungus are evidently sufficiently changed 

 when the tree dies so that the fungus no longer finds the conditions of 

 environment that are requisite for its continued growth. Many fungi are 

 parasitic in certain stages of their growth and saprophytic later on. Thus 

 the fungus of apple scab {Venturia inaequalis (Cke.) Wint.) grows and 

 produces its conidia upon the living leaves and fruits of the apple but 

 overwinters saprophytically and produces its sexual stage of reproduction 

 within the dead leaves on the giound. This is true of very many of the 

 leaf spot fungi of economic as well as wild plants. 



Just how a parasitic fungus obtains its food from its host is not clear 

 in all cases. Apparently it may be by the action of some secretion from 



