VARIATION AND DISEASE. 171 



In such cases we cannot trace what alterations 

 have occurred in the cells and tissues concerned, 

 though we may be sure that some changes do 

 occur. 



No experienced cultivator doubts that some 

 varieties of Potato, Wheat, Vine, Chrysanthemum, 

 etc., suffer more from epidemic diseases than others, 

 and our yearly catalogues furnish us with plenty 

 of promises of " disease-proof" varieties. Here 

 also we may imagine several ways in which a 

 particular variety may resist or escape the 

 epidemic attacks of fungi which in the same 

 neighbourhood decimate other varieties. If we 

 could breed a variety of the Larch which opened 

 its buds later than the ordinary form in our 

 northern plains, the probability of its escaping 

 the Larch-disease would be increased in proportion 

 to the shortness of the period of tender foliation 

 described on p. 153. It has been claimed for 

 certain varieties of Wheat that increased thickness 

 of the cuticle and fewer stomata per square unit 

 of surface have diminished the risk of infection 

 by Rust fungi, and for certain varieties of Potato, 

 that the thicker periderm of the tuber protects 

 them against fungi in the soil. That certain thick- 

 skinned Apples, Tomatoes, and Plums pack and 

 store better than those with a more tender 

 epidermis seems proved that is to say, they 

 suffer less from fungi which gain access through 

 bruises and other wounds ; but it cannot be 

 said that any convincing proof is yet to hand 

 explaining in detail why some races of wheat 



