A GROWTH-INHIBITING SUBSTANCE 243 



of the tuber first grow into sprouts and that they inhibit the 

 growth of the more basal buds. If the tuber is cut into trans- 

 verse slices, the inhibitory influence of the apical buds does not 

 reach the others and there is a general development of buds 

 over the entire tuber, depending upon the thickness of the slices. 



If we consider the behavior of the pear branches previously 

 described, we shall see that their behavior can be admirably ex- 

 plained upon the assumption that the growth of new shoots is 

 regulated by the action of a growth-inhibiting substance pro- 

 duced in the apical portion of the branches. 



In the light of this hypothesis, it is rational that the new 

 shoots appear as they do. It is reasonable to assume that all 

 lateral buds on shoots such as those shown in figure 1 are capa,b^^ ! 

 of growth, indeed we have found that if the inhibiting factcMl^^^^*' 

 removed each and every bud will develop into a shoot/^Ve 

 seem warranted in assuming, therefore, that growth will take 

 place at each meristematic point unless restrained by some in- 

 hibiting factor. 



In the case of pear shoots like those shown in figures 1 and 2 

 it would appear that the buds immediately below the point o 

 amputation grow first because they are the first to be freed from 

 the inhibiting substance. As soon as the new shoots began to 

 grow, they began to produce more of the inhibiting substance 

 which in turn kept the buds below them dormant. So long as 

 the apical buds of the new shoots were uninterrupted in their 

 development all buds beneath them remained dormant. The 

 experiment on the branch shown in figure 2 is conclusive evidence 

 that the non-development of the lateral buds was not due to 

 immaturity, since when the three upper shoots were amputated, 

 their stumps soon produced new shoots, while the two uncut 

 shoots produced no corresponding growth. (The small spurs 

 near their apices developed late in the season as the apical buds 

 ceased their activity.) 



In all the cases here presented the shoot from the uppermost 

 bud (from the bud nearest the point of amputation) was the 

 most vigorous produced from that particular stump and the 

 size of shoots was regularly less as the distance from the point 



