PARASITISM IN FUNGI 137 



saprophyte so far as at present known, was induced to 

 become a destructive parasite on a species of Begonia, by 

 injecting the leaves of the latter with a solution of sugar, 

 to which the germ-tubes of the Trichothecium respond 

 strongly. After twelve generations of the fungus produced 

 by the method described for Cercospora, the spores of 

 Trichothecium, when sown on a living Begonia leaf not 

 injected with a solution of sugar, entered the tissues and 

 produced fruit as a parasite. This experiment shows that 

 it is possible to change a saprophyte into a parasite, and 

 also proves that parasitism is an acquired habit on the 

 part of fungi. 



In the most severe cases of an epidemic caused by a 

 fungus parasite it is not unusual to observe certain indi- 

 viduals of the same kind as the plants attacked, remaining 

 perfectly healthy, in fact immune against the disease; 

 such immune plants in the case of the cucumber I was 

 enabled to prove to be due to the absence in these plants 

 of the special chemotactic substance that rendered possible 

 the entry of the parasite into the plant. This suggests the 

 line to be taken for the purpose of producing a strain of 

 plants immune against a given parasite. 



Sugar in some form is the positive chemotactic substance 

 to which the germ-tubes of nearly all fungi, parasitic and 

 saprophytic, respond, and as this substance is present in 

 the cell-sap of almost every plant in one form or another, 

 the reason why it does not enable any parasite to enter the 

 tissues of any kind of plant is because the positive chemo- 

 tactic power of sugar is neutralised by the presence of a 

 more powerful repellent substance in the cells. 



The results of my observations on the origin of para- 

 sitism were summarised as follows : 



