58 The Potato 



once given, he has the power, in most cases, to perpetuate 

 it and to modify its characters. There are two very 

 different factors or problems in the origination of potato 

 varieties by selection, — the production of the first de- 

 parture or variation, and the subsequent breeding of it. 



It is apparent that the very first effort on the part of 

 the potato-breeder must be to secure individual differ- 

 ences. If the plants which he grows are very much alike, 

 there w^ill be very little hope of obtaining new varieties. 

 He should, if possible, cause his plants to vary. This 

 initial variation may usually be induced by changing the 

 conditions in which the plant has habitually gro\\ai, as 

 a change of seed, change of soil, tillage, varying the food 

 supply, crossing and the like. As a matter of fact, how- 

 ever, nearly all plants which have been long cultivated, 

 like the potato, are already sufficiently variable to afford 

 a starting point for breeding. The grower should have 

 a vivid mental picture of the variety which he desires to 

 obtain; then he should choose those hills which most 

 nearly meet that ideal. 



Experimental breeding has demonstrated that the 

 different varieties of farm and garden plants, as we ordi- 

 narily know them, are not entirely homogeneous and 

 uniform, but are made up of smaller groups now known 

 as "pure lines." It is ordinarily understood that a pure 

 line is the progeny of a self-fertilized indi\idual, that is, 

 one in which all effects of crossing have been eliminated 

 by constant self-fertilization. In potatoes which are 

 ordinarily propagated asexually, a pure tuber line would 

 be the progeny of a single tuber. 



Experiments seem to demonstrate that the group of 

 individuals known as a pure line vary much less than the 

 variety as a whole, and that selection of the best indi- 



