THE INHIBITORY ACTION OF APICAL LEAVES 



105 



experiment lasted from Jan. 12 to Feb. 8, 1923. Stem a, without 

 a leaf, formed two tiny shoots at the apex. Stem h formed roots 

 at the base on that side only where the leaf is, and a large shoot 

 at the apical end of the stem on the opposite side of the leaf. This 

 shoot exceeded in mass the two tiny shoots formed in a. The 

 determinations of the dry weight confirmed this (Table XXII). 

 The mass of shoots formed by the stems alone was only 44 milli- 

 grams, while the stems with an apical leaf formed shoots with a dry 

 weight of 358 miUigrams. Hence more than seven-eighths of 

 the mass of shoots in b was furnished by the descending sap from 

 the leaf at the apex. This sap inhibited the shoot formation on the 

 side where the leaf was and at the same time increased the mass 

 of shoots regenerated by about 700 per cent; hence it cannot 

 be said that the inhibiting action of the descending sap sent 

 out by the leaf is due to an inhibitory substance in the leaf. 



The dry weight of the 6 apical leaves of Set I was 3.388 grams. 



In the preceding experiments the stems dipped into water. 

 The results remain the same when the stems are entirely in air. 

 Three sets of 4 small stems each were split longitudinally and 

 suspended horizontally in moist air (Fig. 82). Set I had a large 

 leaf at the apex, Set II a reduced leaf, and Set III had no leaf. 

 All produced shoots at the upper apical node, but, as Fig. 82 

 shows, the size of the shoots increased with the size of the apical 

 leaf. In this case, the sap from the leaf had to travel in the 

 descending current to the base of the stem and then on the other 

 side of the split back to the apex. There can be no doubt that the 

 descending current from the apical leaf favored shoot formation. 

 In Set I, 1 gram dry weight of stem produced 57 milligrams 

 dry weight of shoot, in Set II, 37 milligrams, and in Set III, 

 which had no leaf, only 12 milligrams. 



