HOW PLANTS ARE INFECTED BY FUNGUS SPORES n 



Europe, has spread from the potato to weeds belonging to the 

 potato family only. In like manner the hollyhock fungus (Puc- 

 cinia ma Iv ace arum}, which followed the hollyhock to Europe, 

 has only succeeded in infecting European weeds belonging to 

 the hollyhock family. The rust of wheat {Puccinia graminis) 

 has followed wheat throughout the world, but has not been 

 able to extend its range of host-plants beyond that of a few 

 grasses belonging to the wheat family. Now it is perfectly 

 certain that the spores of all these fungi, which are produced 

 in myriads, must necessarily have alighted times out of 

 number on living leaves of hundreds of different kinds of 

 plants, so that it cannot be for lack of opportunity that a 

 more varied assortment of plants have not been infected. 



In the instance of some highly specialised fungus parasites, 

 it has been shown that the power of infection is much more 

 restricted than in the examples given above. In some cases 

 only one kind of plant can be infected, or even only one 

 particular form or variety of a plant. 



If these well-proved facts are borne in mind, much loss of 

 time and expense might often be saved. In one instance a 

 plum-tree, growing near to a house in which cucumbers were 

 grown, was cut down because its leaves were infested with 

 plum ' leaf-rust ' (Puccinia pruni), as it was assumed that the 

 ' leaf-rust ' was also the cause of cucumber ' leaf-blotch,' due in 

 reality to a totally different fungus (Cercospora melonis\ which is 

 quite as incapable of infesting plum leaves as the Puccinia 

 is of infesting cucumber leaves. 



The reason for the apparent selective power exercised by 

 fungi in infecting plants, I have dealt with in detail elsewhere. 

 The following are the most essential points bearing on the 

 subject. When the spores of a fungus are made to germin- 

 ate in water, the presence of certain substances can be 

 shown to exert an attractive influence on the germ-tubes 

 of the fungus ; in other words, the germ-tubes formed by the 

 spores grow towards the substance in question. Other 

 substances are found to exercise an opposite effect on the 

 germ-tubes, which are repelled, or grow in a direction away 

 from the exciting substance. This directive action of 

 certain substances in solution, on the germ-tubes of fungus- 

 spores is called chemotropism ; positive when the germ-tubes 

 are attracted ; negative when they are repelled. 



Every degree of parasitism exists amongst fungi, ranging 

 from those very highly differentiated forms that are restricted 



