362 PHYSIOLOGICAL BASIS OF POLARITY. 



SUMMARY. 



1. In Bryophyllum calycinum two apical leaves suppress the shoot 

 formation in all the dormant buds situated basally from the leaf; one 

 apical leaf suppresses the shoot formation in the basal buds situated 

 in the same half of the stem where the leaf is, and, if one-half of 

 the petiole of such a leaf is removed, the growth of basal buds in one 

 quadrant of the stem is suppressed. 



2. This inhibitory influence of a leaf upon shoot formation in' the 

 basal part of a stem is diminished or disappears when the mass of the 

 leaf is reduced below a certain limit. 



3. The inhibitory influence of an apical leaf upon the growth of 

 shoots in horizontally suspended stems is greater when the leaf is 

 on the upper than when it is on the lower side of the stem. 



4. All these facts suggest the possibility that the inhibitory influence 

 of the leaf upon shoot formation is due to inhibitory substances 

 secreted by the leaf and carried by the sap from the leaf towards 

 the base of the stem. 



5. An apical leaf accelerates root formation in the basal part of a 

 stem and this accelerating effect increases with the mass of the 

 leaf. 



6. This inhibitory influence of a leaf upon shoot formation and the 

 favoring influence upon root formation in the more basally situated 

 parts of the stem is one of the factors determining the polar character 

 of regeneration. 



