508 H. MARSHALL WA[;D. 



course the seedling — having been killed by immersion in boiling 

 water — need not be supposed to contain sources of error. After 

 lying in contact with the prepared infective material until the 

 desired zoospores had been observed to escape into the sur- 

 rounding water, the infective mass was removed immediately. 

 The new material now lay in a drop of water in which were 

 the desired zoospores. After a time sufficient for contact and 

 entry of the germinal tubes, it was quickly removed with per- 

 fectly clean instruments and allowed to remain for several hours 

 in pure water, in the hope that it was infected by the desired 

 zoospores and no others. In all cases — and many trials were 

 made — the new material developed numerous zoosporangia 

 of P. gracile before a single specimen of the desired form could 

 be detected. Not only so: the P. gracile got the upper hand 

 very quickly, as it had in the original infections. Nay, in many 

 cases the desired form did not appear at all. The reason was 

 clear: the superabundant P. gracile not only formed its 

 zoospores more rapidly and in larger quantities, but they made 

 better progress in the matrix, and killed off the other form in 

 the mutual struggle for existence. In every experiment an 

 odd zoospore of P. gracile got the start, and beat its compe- 

 titor in the race, and the only result of all the care bestowed 

 appeared to be the establishment of a purer growth of the 

 mycelium bearing the P. gracile form of zoosporangia, 

 prodigious quantities of which became formed in the course of 

 a few hours. 



During these cultivations I obtained enormous extramatrical 

 developments of the mycelium of P. gracile, and, both on 

 cress and on the young buds of carrot, was enabled to watch 

 the development of the sexual organs with great success. 



At fig. 29 are drawn specimens of the extramatrical myce- 

 lium of well-developed cultures. As shown here, the other- 

 wise very slender hyphaj became swollen up here and there 

 into knob-like groups of oval or rounded protuberances, into 

 which the fine grained protoplasm became collected at length. 

 In fig. 29 A, for example, a firm septum marks off an empty 

 distal moiety of a hypha, from one full of protoplasm, and with 



