DIRECT PENETRATION 



241 



host and parasite nuclei and protoplasm freely intermingle with 

 each other. 



The stimulus causing penetration. The observations of 

 Pfeffer that chemotropic stimuli are responsible for the migra- 

 tion of antherozoids of mosses and ferns to the archegones fur- 

 nished the stimulus for similar studies involving the proximate 

 cause of penetration by fungi. Miyoshi, working under PfefTer's 

 direction, published two reports (1894, 1895), in which he con- 



Fig. 42. Stages in penetration by the hyperparasite Parasitella (Mucor) para- 

 siticus into the hypha of Mucor. The buffer cell is cut off in B. In C the 

 wall between buffer cell and host has been dissolved, permitting direct 

 protoolasmic contact. In D branches are developing from the buffer cell. 



(After Burgeff.) 



eludes that membranes are penetrated by germ tubes or hyphae 

 only when a certain concentration of an attractive substance 

 (chemical attractant) is present on the opposite side. According 

 to this theory, concentration of the chemical must exceed a spe- 

 cific threshold value if it is to attract; but, if a certain maximum 

 concentration is employed, the result is repellant action. He 

 dealt with penetration of collodion films, parchment, gold leaf, 

 cellulose films, and the epidermis of onion scales. The spores of 

 the various species used were placed in films of agar that were 

 separated from the chemical to be tested by perforated sheets of 

 mica. He also injected with water leaves of Begonia and Trades- 

 cantia to which spores were applied, but no penetration resulted, 

 whereas active penetration followed injection with cane sugar. 

 Admittedly his conclusion that chemotropic factors are funda- 

 mental in determining whether germ tubes oenetrate is supported 

 by quite convincing evidence. 



