Predisposition. 63 



the mildew of potato leaves. After four days, the tubers grown in the 

 soil containing copper did not show any infection, while those girown in 

 untreated soil were infected in a very decided manner from being brought 

 into contact with a mildewed leaf. 



Marchal 2 , following on the same lines, tried to secure immunity to 

 lettuce against Bremia lactucae, and he found that /the plants treated showed 

 considerable resistance, and the immunity seemed to be in proportion to 

 the strength of the solution. He also experimented with cereals for pro- 

 tection against rusts, but without success, and the probability is that he did 

 not use the proper substance which repelled the germ-tubes of the irust- 

 spores when they attempted to enter the plant. 



Massee 7 experimented with cucumber and tomato plants, watering the 

 soil on which they were grown with a solution of sulphate of copper. The 

 result was that " not a single one of the treated tomato plants showed a trace 

 of disease " after being sprayed with water containing the spores of the 

 fungus, while the untreated check plants were badly diseased. 



It is important to note that tomatoes produced from plants treated with 

 solutions of copper sulphate were found on analysis to show amounts of 

 coppesr 'not sensibly greater than that found in the fruits obtained from un- 

 treated plants. 



These experiments at least show that certain substances entering into the 

 constitution of the host-plant render it for the time being immune to the 

 attacks of certain fungi, even although it was naturally predisposed to that 

 particular form of disease. 



