82 THE POTATO 



of the cuticle and the palisade cells (Fig. 34), or 

 thick cells on the upper surface of the leaf, will no 

 doubt prevent many spores from reaching the inside 

 cells of the leaf. They may germinate on the surface, 

 but not enter, unless they find some place where the 

 leaf has been injured. The punctures of the flea- 

 beetles are, on this account, of great importance, as 

 they furnish an entrance to the inner cells (Fig. 40). 



N. A. Cobb, of Australia, has shown that in the 

 case of wheat the varieties most resistant to rust (Puc- 

 cinia graminis and P. rubigo vcra), none being abso- 

 lutely resistant, have narrow, stiff, upright foliage, 

 while those most liable to attacks have broad, flabby, 

 and pendant foliage. In the plants resistant to rust 

 the cuticle of the leaf is much thicker than in the 

 others, and is so thick that the rust spores, when they 

 germinate on the outside of the leaf, cannot penetrate 

 it, or if they do succeed in entering the leaf through 

 stomata, the threadlike growths of the parasite cannot 

 rupture the cuticle wall to fructify ; and, further, some 

 wheats have stomata so narrow and are so well cov- 

 ered with wax that the germinating threads of the 

 rust spore fail to enter every time. These circum- 

 stances seem to support the claim that the tough, thick- 

 walled, hard, dry leaf is the one to select for disease- 

 resistant powers. It has been observed that plants of 

 the potato family having this type of leaf are fairly free 

 from fungus leaf diseases. It is essential that the 

 leaves of the potato be abundant to insure a good yield. 



1 1 . The vigor of the variety. Vigor is the power 

 stored in a plant which enables it to overcome difficul- 

 ties at different periods of growth. A variety must 



