138 TEXT-BOOK OF FUNGI 



1. The entrance of the germ-tubes of a parasitic fungus 

 into the tissues of a living, healthy plant, depends on the 

 presence of some substance in the cells of the host, attrac- 

 tive to the fungus. In other words, infection is due to 

 positive chemotaxis. 



2. A saprophytic fungus can be gradually educated to 

 become an active parasite to a given host-plant by means 

 of introducing a substance positively chemotactic to the 

 fungus into the tissues of the host. By similar means a 

 parasitic fungus can be induced to become parasitic on a 

 new host. 



3. An immune plant signifies an individual of the same 

 species as the one on which a given species of fungus is 

 parasitic, but which, owing to the absence of the chemo- 

 tactic substance in its tissues necessary to enable the germ- 

 tubes of the fungus to penetrate, remains unattacked. 



Two important papers recently published have, I consider, 

 very materially supported my views respecting the origin 

 of parasitism, as briefly outlined above, although I am not 

 certain that this was the primary intention of either 

 author. 



Miss Gibson, acting on a suggestion of Professor 

 Marshall Ward, conducted a series of experiments to see 

 what happened when a plant other than the known host- 

 plant of a given fungus was inoculated. Uredospores and 

 aecidiospores of various species belonging to the Uredina- 

 ceae were used for infection, and the hosts were Ranunculus 

 ficaria, Tropaeolum, Caltha, and Valeriana. The germ-tubes 

 entered the stomata as in normal infection of the proper 

 host-plant, but within two or at most four days all the 

 hyphae were dead, and in not a single instance was a 

 haustorium formed in the cells of the plant infected. It is 



