DIRECT PENETRATION 



241 



penetration to be accomplished within approximately 5 minutes. 

 They also found the hvphae to possess an osmotic pressure suffi- 

 cient to penetrate turgid potato cells. Few measurements of 

 osmotic pressure in fungi have been made; thev might be found 

 valuable in an interpretation of factors concerned in penetration. 

 Studies of this type were conducted bv Thatcher (1939, 1942). 

 He used the plasmolvtic method in osmotic pressure and permea- 

 bility determinations and was able to show that certain parasitic 

 funs^i increase the permeability of the plasma membrane of the 

 host cells. His measurements of the osmotic pressure of parasite 

 and host are shown in Table 24. In each fungus the osmotic 



pressure of the parasite is greater than that of its host. Moreover, 

 Thatcher was able to demonstrate an increased permeability in 

 diseased tissues over healthy tissues, indicating that the parasite 

 causes certain substances to leach from the host cells and thus to 

 lower their osmotic pressure. 



Although these data as a whole have a bearing on the problem 

 of penetration and might be taken to prove that penetration is 

 the result of mechanical pressure in certain species of fungi, it 

 does not necessarily follow that all species which effect their own 



