THE INFLUENCE OF GRAVITY 



25 



left was intact while the lower edge of the leaf on the right was 

 cut off. The intact leaf on the left formed roots and shoots 

 almost exclusively on the lower side, the sister leaf only on the 

 upper side. The drawing was made on the thirty-third day of 

 the experiment. 



Fig. 24. — Sister leaves suspended sidewise in moist air. Left leaf intact, 

 while the lower edge is cut off in leaf to right. In left leaf shoots and roots 

 develop on lower edge, and the rapid development of roots and shoots on the 

 lower side retards or inhibits the development of roots and shoots on the upper 

 side of the leaf. In leaf to the right, where the lower edge is cut ofT, the devel- 

 opment of roots and shoots on the upper edge is neither restricted nor retarded. 



The explanation is as follows: The effect of gravity consisted 

 in a greater collection of sap on the lower edge of the fleshy parts 

 of the leaves. As a consequence the growth on the lower edge of 

 leaves suspended sidewise is accelerated and this now leads sec- 



FiG. 25. — Quantitative difference in regeneration when a whole leaf and a half 



leaf are suspended sidewise in air. 



ondarily to a flow of the sap of the whole leaf to the lower edge. 

 This suppresses the growth in the upper notches. If the lower 

 edge of a leaf suspended sidewise in moist air is cut off a collec- 

 tion of sap will also occur in the lower edge, but since no growth 

 can occur there it will not result in the flow of the sap of the whole 



