BUD SELECTION 



395 



or 4 years and found 12 bud mutations. Changes were noted in the 

 color, shape and habit of growth of the tubers and in the depth of the eyes. 

 But as for the bearing of bud mutations on origin of new varieties East 

 reached the conclusion that, while isolated cases of improvement might 

 be due to selection of bud mutations, yet comparatively few (probably less 

 than 0.5 per cent.) of our present varieties arose in this manner. This 

 evidence on the origin of varieties has led East to adopt the view that 

 probably all bud mutations are so exceedingly rare in the potato that few, 

 if any, cases of "running-out" or "degeneration" in varieties are to be 

 explained on this basis. He believes the principal factor in such dete- 

 rioration is disease, and that in numerous experiments on potatoes, in 







FIG. 162. Variation in yield between tuber-units from the same hill. Above, the 

 progeny of two tubers from hill selection No. 35; below, that from hill selection No. 4. 

 (After Stuart.) 



which it is shown that successive selections have raised the average 

 yield over that of the unselected tubers, the results are entirely due to the 

 elimination of diseased tubers. 



While the elimination of diseased tubers or of tubers that were weak- 

 ened by disease in the leaves or stem does undoubtedly explain the success 

 of many selection experiments it may not account for all of them. Tests 

 of individual tubers of almost any commercial variety apparently reveal 

 inherent differences in the tubers. Although the plant is very susceptible 

 to environmental conditions and some tuber characters such as shape and 

 size are very unstable, yet sometimes the product of two closely similar 

 tubers which came from the same hill when grown under closely similar 

 conditions will differ widely (see Fig. 162). The most satisfactory method 

 of testing individual tubers is the tuber-unit method which was introduced 

 by Webber. Each tuber which is to be tested is cut lengthwise into 

 four equal pieces which are planted at equal distances from each other. 

 The four hills thus comprise a tuber-unit. 



