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 
Fic. 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). Themostsatisfactory 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. 
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