THE CAUSE OF INFECTIOUS DISEASE. 245 



that are young and rich in water and therefore 

 less resistant, but do not attack the older plants 

 which are less rich in water. Cystopus candi- 

 dus, according to De Bary, causes the white 

 rust of the garden-cress (Lepidium sativum) ; 

 all plants, in fact, are susceptible to the attack of 

 this fungus, but only when they are in the 

 cotyledon stage ; when the cotyledon falls off 

 the leaf becomes resistant and the spores and 

 germ-tubes of Cystopus penetrate only locally 

 in every case, without being able to push far 

 into the interior. The blister-rust of spruce 

 needles occurs according to Cramer in its other 

 parasitic forms only upon the leaves of the 

 Alpine-rose and wild rosemary, Chrysomyxa 

 rhododendn 'and ledi. In localities where these 

 two plants do not occur no blister-rust is found 

 upon the spruce. Similar relations exist ac- 

 cording to De Bary, between the rust of wheat 

 and the aecidia of the barberry. The " local " 

 disposition is, therefore, in reality the local 

 presence of the host. According to De Bary, 

 if the " disease predisposition " due to the pre- 

 sence of the host be removed e. g., if the 

 barberry bush be banished to a distance the 

 disease disappears, only to arise again when 

 susceptible host-plants are introduced. 



Perhaps the most remarkable fact in this 

 connection is that definite predispositions may 



