228 AGRICULTURE 



or of apples and pears by grafting. Such a 

 method of increase is called vegetative, as 

 opposed to propagation by means of seed, 

 which is defined as sexual reproduction. In 

 the case of the potato, what appears to 

 determine the health of the crop, and a 

 large yield, is strong development of the 

 corky layer which envelops the tuber, and 

 of the epidermis and cuticle of the leaf and 

 stem. If these protective coverings are 

 thick, the plant is able to offer successful 

 resistance, more or less complete, to the 

 attack of the many parasites that prey upon 

 it. In inclement districts there is a tendency 

 on the part of all plants, including potatoes, 

 to develop a thick skin as a protection 

 against the severe conditions of the environ- 

 ment ; and when such thick-skinned tubers 

 are subsequently planted in a district with 

 a better climate, they still, for some years, 

 retain their acquired characters, with con- 

 sequent comparative immunity from disease. 

 It is possible, also, that tubers being less 

 fully matured in a late district, sprout more 

 vigorously when used as " seed." But, 

 whatever the reason, it is at all events the 

 practice for growers, in the best potato 

 districts, such as East Lothian, Lincolnshire, 

 Bedfordshire, and Cambridgeshire, to import 



