42 BOTANICAL GAZETTE [january 



Our investigation shows that this growing region of the cortex 

 coincides with the region where early roots are formed. This 

 suggests the possibility that the geotropic growth of the cortex on the 

 lower side of a horizontally suspended stem is due to a cause which is 

 either closely associated or identical with the cause of root formation. 



If we assume with Sachs that there are specific root-forming 

 substances, then the question presents itself whether we are not 

 forced also to ascribe the geotropic curvature to the existence of 

 specific geotropic substances or hormones; both substances having 

 the tendency to collect on the lower side of a horizontally sus- 

 pended stem; and both substances stimulating growth, the one of 

 roots, the other of the cortex. On the basis of such an assumption 

 we might understand why no or only an insignificant geotropic 

 curvature takes place in a split stem when the cortex is on the 

 upper side, the reason being that the geotropic substances settling 

 at the lower side find no cortex which can grow and cause geotropic 

 bending. This assumption will of course be a mere hypothesis 

 until the existence of such hormones can be demonstrated directly. 



v. Further experiments on the influence of the position 



OF THE LEAF UPON THE GEOTROPIC BENDING OF A STEM 



When we remove the cortex on the upper or lower side of a 

 horizontally suspended stem of Bryophyllum calycinum (without 

 removing the wood and pith), an extensive bending of the stem 

 takes place instantly (fig. 25), the side on which the cortex is 

 removed becoming convex. The mechanism of this phenomenon 

 becomes clear on the assumption that the cortex is under a tension 

 longitudinally which shortens the wood and pith. If this tension is 

 removed on one side of the stem, the wood and pith on that side 

 can stretch, while the wood and pith on the opposite side are held 

 in check by the cortex. This leads to a considerable curvature 

 whereby the side on which the cortex is preserved becomes concave 

 (fig. 25). This curvature due to cortex tension is much stronger 

 than the curvature which takes place instantly w r hen we split a stem 

 longitudinally. In this case not only the cortex but also the wood 

 and pith are removed on one side of the stem, and hence the tend- 

 ency of this side to stretch is considerably less than if only the cor- 



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