1904] 



On the Origin of Parasitism in Fungi, 



119 



In the case of obligate parasites the cell-sap of the host-plant proved 

 to be the most marked positive chemotatic agent. Malic acid is the 

 specific substance that attracts the germ-tubes of Monilia fructigena into 

 the tissues of young apples ; whereas the enzyme pectase performs the 

 same function for the germ-tubes of Cercospora cucumis, an obligate 

 parasite on the cucumber. 



Immune specimens of plants belonging to species that are attacked 

 by some obligate parasite owe their immunity to the absence of the 

 substance chemotactic to the parasite. 



Purely saprophytic fungi can be educated to become parasitic, by 

 sowing the spores on living leaves that have been injected with a 

 substance positively chemotactic to the germ-tubes of the fungus 

 experimented with. By a similar method of procedure, a parasitic 

 fungus can be induced to attack a different species of host-plant. 



These experiments prove what has previously only been assumed, 

 namely, that parasitism in fungi is an acquired habit. 



A series of experiments prove that infection of plants by fungi 

 occurs more especially during the night, or in dull, damp weather. 

 This is due to the greater turgidity of the cells, and also to the presence 

 of a larger amount of sugar and other chemotactic substances present 

 in the cell-sap under those conditions. 



VOL LXXIII. 



