" TUBER-UNIT " METHOD OF SEED-POTATO IMPROVEMENT. 27 



VALUE OF THE "TUBER-UNIT" METHOD. 



The tuber-unit method was principally intended to serve as a 

 ready means by which high-yielding or uniformly shaped strains 

 could be isolated from the general mass. The writer conceives that 

 its most valuable feature lies not in the isolation of highly produc- 

 tive strains, but in the elimination of the unproductive and diseased 

 plants. Extensive experiments conducted by this office show con- 

 clusively that the simple elimination of the unfit tends to materially 

 increase the per-acre yields. A careful study of strong and weak 



FIG. 1.— Strong (1) and weak (2) tuber units of Rural Blush potatoes. (Photograph of plants made about 



August 20, 1911.) 



tuber units of 12 varieties during the past two seasons, 1911 and 

 1912, shows in every instance a very close relationship between the 

 vigor of the plant and the tuber yield. The average production of 

 merchantable tubers from the strong plants was over 16 times 

 greater than from the weak ones, while in the case of the culls the 

 yield was only a little over twice as great. The data further show 

 that the average weight of merchantable tubers from the strong 

 plants was 5.3 ounces, as against 3.8 ounces from the weak ones. 

 The small tubers or culls from the strong plants averaged 1.7 ounces, 



[Cir. 113] 



