348 PHYSIOLOGICAL BASIS OF POLARITY. I 



This slight inhibitory influence of a leaf upon the more apical buds 

 shown in Fig. 8 can also be demonstrated in the growth of the second 

 node above a leaf (Fig. 10). In this case the influence is noticeable 

 only in the second pair of new leaves of a bud; Leaf 1 which has the 

 same orientation as the old leaf remains smaller than Leaf 2. 



While in the basal region of a leaf the inhibitory effect is com- 

 plete, it is comparatively slight in the apical part. 



Influence of Gravity upon the Inhibitory Action of the Leaf. 



All the experiments on potted plants described in the preceding 

 pages can be repeated with the same result in stems cut out from a 

 plant. We may omit a description of such experiments since they 

 would constitute only a repetition of what has already been stated. 

 But certain of these experiments yield some additional results which 

 are of theoretical importance. 



The assumption that the inhibitory effect of the leaf upon the growth 

 of dormant shoot buds is due to chemical substances sent out by the 

 leaf is supported by the striking influence of gravity on regeneration 

 in stems suspended horizontally. Long straight stems were cut out 

 from a plant and suspended horizontally in an aquarium nearly 

 saturated with water vapor. When two leaves are left at the most 

 apical node of such pieces, none of the buds situated more basally 

 will grow out. If, however, one leaf with its axillary bud is removed 

 and the other leaf left, regeneration will occur, but the buds which 

 will grow out will show a characteristic difference according to whether 

 the leaf is on the upper or the lower side of the horizontally suspended 

 stem. 



We suspend such stems so that the axis of the two most apical buds 

 (one of which is removed with its leaf) is vertical (Fig. 11). In five 

 stems the leaf is on the upper side and in five stems on the lower side 

 of the stem (Fig. 11). All the stems were originally horizontal but 

 underwent the geotropic bending described in previous papers, 

 whereby the upper side became concave. 



When the leaf is below (right half of Fig. 11), shoots may be formed 

 in the first node basally from the leaf. This occurred in three out of 

 five stems drawn here. The other two formed shoots from the upper 

 bud of the second node. 



