THE INHIBITORY EFFECT OF AN APICAL LEAF 



127 



This explanation is supported by the experiments represented 

 in Fig. 100 where the apical leaf of the stems placed horizontally 

 was above. In this case we have inhibition above and below. 

 On the upper (concave) side of the stem shoot formation is 

 inhibited becavise this is the side where the descending sap flows 

 from the apical leaf. On the lower side it is inhibited because 

 there the tissue sap collects. This inhibition of the shoot forma- 

 tion by the tissue sap is, however, less complete than that by the 

 sap flow in the regular vessels carrying the descending sap. 



Some new light is thrown upon the inhibiting effect of an 

 apical leaf on shoot formation in the basal part of the stem by the 



Fig. 102. — Shoot formation in stem a, with whole leaf at apex has not yet 

 begun; in stem 6, with half leaf, a shoot formed in the second node below; also 

 shoots in stem c, without leaf. Apr. 25 to May 10, 1923. 



fact that the shoot formation on the side of the stem opposite 

 from that of the leaf is at first inhibited and later increased in 

 proportion to the mass of the leaf. 



Young stems of Bryophyllum each possessing 4 nodes were 

 suspended horizontally in air. One set of stems had a large leaf 

 at the apex on the upper side of the stem (Fig. 102, a) and a 

 second set (102, b) had a leaf of about one-third the size of the 

 leaf in a, also on the upper side of the stem; while a third set of 

 stems (Fig. 102, c) had no leaf. In a and h the anlagen for shoots 

 opposite the leaf had been cut out. The drawing was made on 

 the fifteenth day. None of the stems with a large leaf («, Fig. 

 102) had formed a shoot, only a root on the lower side in the second 

 node; three among four stems with reduced leaf (6, Fig. 102) had 

 each formed a shoot on the lower side of the stem (opposite from 

 that of the leaf), and three among four stems without a leaf 

 had formed apical shoots. Hence a large leaf at the apex at 



