15 



or perhaps absolutely unattacked. Since in such cases the parasite is t[uickly 

 and extensively distributed on one variety and not on the other, although the 

 atmospheric conditions and other factors of vegetation are equally favorable, 

 the specific constitution of the host plant, in these two cases must have deter- 

 mined whether it would become diseased. Thus we arrive at the conclusion 

 that for the production of a parasitic disease the presence of the parasite 

 alone is not determinative but the constitution of the host organism is also 

 a determining factor. 



The many infection experiments have led to a classification of the living 

 creatures infesting other organisms and capable of attacking the tissue, in 

 which one group is described as obligate parasites when able tc attack the 

 host plant in all stages of its normal development. Of this group there have 

 been separated as zuound parasites all such organisms as cannot attack the 

 organism possessing .normal protective devices but need the changes in tissue 

 ofifefed by the surface of a wound. In a great many instances, however, we 

 have recognized the fact that the parasite only finds the environment re- 

 quired for its development when the host has been affected and its functions 

 weakened. Such conditions will appear here as were also decisive in the 

 experiments carried on by Miyoshi (see preceding section). This group 

 bears the name "parasites of weakness." 



To this last group especially belong the numerous species which during 

 many generations live on dead organic substances. They therefore must be 

 spoken of as saprophytes which occasionally become parasitic, — facultative 

 parasites. Therefore the boundary between parasitism and saprophytism is 

 lost here and even in those species which are always parasites (obligates), 

 such as the varieties of smut, we find developmental phases with a sapro- 

 phytic mode of nutrition. 



If we now, however, study more closely the families of our closest para- 

 sites among the fungi, namely, the smuts and rusts, we will find one fact 

 brought into prominence by the most recent investigations and repeatedly 

 substantiated; namely, that the energy of grozvth of the parasite depends on 

 the host plant. We have examples proving that the same fungus occurs in 

 different species of the same host genus in the same habitat, sometimes grow- 

 ing luxuriantly in many large centres, sometimes sparsely in small forms, 

 according to whether the one species has fleshy leaves and the other thin 

 ones. Indeed, the rusts are so dependent upon their host plants that biologic 

 races are formed which, agreeing formally, nevertheless show differences in 

 adjusting themselves to definite host plants and either cannot develop at all, 

 even when carefully injected upon a related host plant, or develop only 

 slightly. Thus we have a special form of the common black rust of grains 

 on rye, another on wheat, another on oats etc. Mycologists cherish the con- 

 viction that this development into individual races through the accommoda- 

 tion to a special host plant is a widespread phenomenon constantly increas- 

 ing. What else can such a race formation indicate than that parasites in 

 their demands have been and still will be most closely connected ivith the 



