84 REGENERATION 



phenomenon dependent upon the quantity of the material sent 

 by the leaf into the stem or formed in the stem itself. 



The fact that the material sent by an apical leaf into the stem 

 collects in greater quantity on the lower than on the upper side 

 of a stem suspended horizontally finds its explanation in the 

 assumption that under the influence of gravity the tissue sap 

 collects on the lower side of such a stem. 



It is hardly necessary to state that this explanation is intended 

 merely to account for the observations on the geotropic curvature 

 of the stem of Bryophyllum. 



