The Origin and Tendencies of Parasitism in Fungi. 291 



is followed by some remarkalile modifications bearing on the 

 life-histon' of the organisms concerned. The most pronounced 

 change is that of cutting down the method of reproduction by 

 means of spores, and depending entire)}^ on a vegetative mode 

 of reproduction. The disappearance of spores is in proportion 

 to the progress made by the organism in parasitism. When the 

 parasite has succeeded in establishing a perennial mycelium 

 in the root, tuber, or seed of its host-plant, then the higher form 

 of spore-formation is often arrested. As an example, Phvto- 

 phthora infestans, the cause of the potato disease, forms myce- 

 lium in the tubers, which originates the disease the following 

 season, if weather conditions are favourable for the extension of 

 the fungus up the stem and into the leaves, also down into the 

 new tubers. Now as this perrenial mycelium present in the 

 tubers serves the purpose of continuing the fungus in time, or 

 from one season to another, the oospore stage, which at one time 

 answered the same purpose, has been completely suppressed, 

 whereas the conidial stage of reproduction, which serves to 

 extend the area of distribution of the fungus, has been retained. 

 In the other species of Phytophthora which have not evolved a 

 perrenial mycelium in the host plant, the oospore condition is 

 present as a necessity, to continue the fungus in time or from 

 one season to another. 



In other still more highly evolved parasites, the fungus 

 never leaves the host-plant, and all traces of spore formation 

 of every kind has disappeared, as in Darnel and Italian rye- 

 grass, where the mycelium of the fungus is located in the seed. 

 When the latter germinates the fungus grows along with its 

 host-plant and again enters the new seed, which, w^hen sown, 

 is again accompanied by its internal fungus, and thus the cycle 

 of host and fungus is continued from year to year. In this 

 instance the seed is not in any way injured or prevented from 

 germinating, as in the case of the oat grain when attacked by 

 smut. It has been shown that over fifty per cent, of the seed of 

 Darnel and of Italian rye-grass are infected with the fungus, 

 hence we have two distinct races of these plants ; one race in- 

 fected with the fungus, and must ever remain so ; another race 

 that is free from infection and must always remain so, as there 

 are no spores produced by the fungus to set up an infection. 

 Owing to the entire absence of fruit the nature of the fungus 

 infecting darnel cannot be determined. It has been shown that 

 infected plants are more vigorous, and produce a heavier crop 



1910 Aug. I. 



