74 REGENERATION 



leaf on the polar character of the regeneration in a vertical stem 

 of Bryophyllum is due not to any difference in the chemical con- 

 stitution of the ascending and descending sap from a leaf, but is 

 due to the fact that the descending sap in a stem suspended 

 vertically reaches primarily the anlagen for root formation while 

 the ascending sap reaches primarily the anlagen for shoot forma- 

 tion. Before going into the details of these experiments it will be 

 necessary to discuss some other effect of gravity on the stem; 

 namely, the geotropic curvature of a stem of Bryophyllum. placed 

 in a horizontal position. When stems of Bryophyllum are placed 

 in a horizontal position and kept in moist air, the originally 

 straight stem undergoes a curvature whereby the lower side of the 

 stem becomes convex and the upper concave (Fig. 54, a). The 

 rate of this curvature increases with the mass of the leaf. In 

 order to demonstrate this fact a few remarks about the mech- 

 anism of this curvature and the method of its investigation may 

 be desirable. 



2. Influence of the Leaf on Geotropic Curvature. — In these 

 experiments it is necessary to remember that the geotropic curva- 

 ture of a stem is the resultant of two opposing forces. One is, 

 as we shall see, the excess of longitudinal growth of cortical tissue 

 on the lower side of the stem suspended horizontally over that of 

 the rest of the stem; the opposing force is the rigidity of the upper 

 layers of the stem, chiefly the wood. When the wood is too hard 

 the stem cannot bend. The influence of equal masses of an apical 

 leaf on the rate of geotropic curvature of two stems can only be 

 equal if the rigidity of the wood is identical in the two stems, a 

 condition which cannot often be fulfilled. Quantitative work of 

 this kind must therefore be statistical; but it is only intended here 

 to prove in a semi-quantitative way that the rate of geotropic 

 curvature of a stem increases with the mass of the apical leaf. 

 It is necessary to select for experimentation only the more apical 

 parts of the stems of young plants where the wood is still soft or 

 not too rigid, otherwise little or no curvature is possible. The 

 method of the experiments is illustrated in Fig. 54. Stems of 

 about equal flexibility were selected and defoliated with the 

 exception of one leaf at the apex. With the aid of a string 

 around the petiole of the leaf, the stem was suspended in moist 

 air. In order to secure a horizontal position of the stem, the lat- 

 ter was put on a wire netting bent in the shape of a U, on the bot- 

 tom of which the stem was allowed to rest (Fig. 54). The meshes 



