VARIETIES 8 1 



quiring more solution, and it is always lying over the 

 ground when the last spraying ought to be given, and 

 is in the way at lifting-time, whether the potatoes are 

 raised by hand or digger. Modern breeders aim to 

 produce a short haulmed, upright, heavily leaved top. 

 because the upright habit of growth (Fig. 22) is more 

 likely to keep clear of disease than a spreading habit, 

 ow'ing to water being shed from the former more read- 

 ily than from the latter, and not offering a foothold to 

 the disease spores (Fig. 37). Plants whose branches 

 lie on the ground are more liable to disease because 

 they cover a greater area, their leaves, touching the 

 ground, are almost always damp from conta(5l with it, 

 and sun and wdnd cannot so readily reach them. Very 

 tall haulmed varieties are readily beaten down by 

 storm and wind, and in this state they cannot dry so 

 readily; hence, they fall in a clammy mass, very favor- 

 able for the growth of disease spores. 



Varieties with strong, hardy haulm suffer less from 

 spring frosts. L,ate varieties usually have taller haulm 

 than first early varieties. Some varieties make their 

 heaviest growth of foliage late in the season, and in 

 this way are not so subje(5l to attacks of early blight. 



10. The leaf. The British disease-resisting varieties 

 have hard, thick leaves. Whether the thickness of the 

 leaf is an important factor in their resistance to rot {Phy- 

 tophthora infestans) is not determined. The fadt that 

 spraying the upper surface of the leaf tends to prevent 

 blight would seem to show that access to the inside of 

 the leaf is obtained by growth through the cell walls 

 as well as through the stomata, on its under surface. 

 If this be true, then the thickening and hardening 



