57 



THE PRESIDENT'S ADDRESS. 



SOME PLANT DISEASES CAUSED BY FUNGI. 



[Resume.) 



By George Massee, F.L.S. 



(Delivered February 19t/t, 1904.) 



Until quite recently, no logical explanation was forthcoming 

 as to why certain fungi are parasitic on other plants. That 

 such parasitism is the outcome of a definite law is obvious from 

 the fact that, although the spores of parasitic fungi will 

 germinate on the surface of any kind of plant, provided it 

 is damp, the germ -tubes only enter the tissues and infect 

 particular species of plants. 



As a rule, a given obligate parasite is confined to one, or 

 at most a few closely allied species, and in not a few instances 

 a parasitic fungus is confined in its attacks to a single variety 

 of a species. 



The reason for this apparent selection of hosts on the part 

 of parasitic fungi is now attributed to what Pfeffer, a German 

 botanist, called chemotaxis. By chemotaxis is meant the 

 influence exerted over the direction of growth of the germ- 

 tubes of fungi by various substances, which may be of the 

 nature of acids, alkalies, sugars, and various other substances 

 present in the cell-sap of different plants. Towards certain of 

 these substances the germ-tubes of some kinds of fungi are 

 attracted, and pass through the stomata, or even pierce the 

 cell-walls and enter into the tissues of plants. Such substances 

 are said to be positively chemotactic. On the other hand, 

 the germ-tubes of some kinds of fungi are repelled, or prevented 

 from entering the tissues of certain plants, owing to the presence 



