688 PHYSIOLOGICAL BASIS OF POLARITY. II 



also shown a dominating influence of the mass of the leaf.^ Thus in 

 Bryophyllum calycinum a leaf at the apex of a piece of stem will in- 

 hibit the formation of shoots in the basal part of the stem and this 

 inhibitory influence disappears when the mass of the leaf is diminished 

 below a certain limit. This inhibitory influence of the leaf upon 

 shoot formation follows the path of the conducting vessels from the 

 leaf into the stem. Hence a leaf favors shoot formation at the apex 

 of a piece of stem and inhibits shoot formation at the basal parts 

 of the stem, and both phenomena are a function of the mass of the 

 leaf. 



This influence of the leaf exists in the normal plant as well as in 

 the regenerating plant; i.e., in a piece cut out from the plant. The 

 normal stem of Bryophyllum calycinum has two dormant buds in each 

 node capable of growing into shoots, though they never do so in a nor- 

 mal plant. This is partly due to the influence of the apical leaves and 

 this influence is responsible for the fact that the normal stem of 

 Bryophyllum is unbranched. We say partly due to the leaves since 

 the growing region at the apex acts in a way similar to a leaf .^ 



We intend to show in this paper that the leaf not only influences 

 shoot formation but also root formation. 



//. Influence of the Mass of an Apical Leaf on the Mass of Air Roots 



Formed. 



Pieces containing a number of nodes were cut out from healthy 

 plants of Bryophyllum calycinum. All the leaves except the two 

 apical ones were removed and the stems were split longitudinally so 

 that each half of stem possessed one apical leaf. One leaf was usu- 

 ally left intact; the mass of the other leaf was reduced by cutting 

 away part of the leaf. The two sets of half stems were suspended 

 horizontally in a moist aquarium with the cut surface above (Figs. 

 1 and 2). On the lower side of the stems roots soon formed, first 

 generally at the second node behind the leaf, and later at the base 

 and at other nodes. At the base the formation of roots was most 

 abundant. Still later roots were also formed in the internodes, 

 especially in the region of geotropic curvature. 



3 Loeb, J., Science, 1917, xlvi, 547; J. Gen. Physiol., 1918-19, i, 337. 



