26 



REGENERATION 



leaf to the lower edge of the leaf. Hence nothing prevents the 

 shoots in the upper edge from growing out. As soon as this hap- 

 pens all the sap from the leaf will flow to the notches of the upper 

 edge. The correctness of the explanation of the results in Fig. 

 24 can be proved quantitatively. 



A number of pairs of sister leaves were svispended vertically 

 and sidewise in air, one leaf of each pair remaining intact, while 

 the upper half of the sister leaf was cut away (as shown in Fig. 

 25). The whole leaves formed practically twice as great a mass 

 of shoots and roots on the lower edge as the half leaves and this 

 difference is noticeable in Fig. 25. Thirteen whole leaves and 

 their thirteen half sister leaves were chosen for comparison in 

 Table VIII. The experiment lasted from May 8 to 29. 



Table VIII 



The result proves that the excess regeneration in the whole 

 leaves was produced by material furnished by both the lower and 

 the upper half of the leaves, regeneration in the upper half being 

 as a consequence impossible, since almost all the material avail- 

 able for regeneration in the upper half was consumed for regen- 

 eration in the lower half. 



In the case just discussed, the regeneration in the lower half 

 of the leaf is accelerated, since owing to the action of gravity the 

 liquid in the leaf collects in the lower half. This happens when 

 the leaf is suspended sidewise in the air but only rarely when it is 

 suspended in the air with the apex down. The reason for this 

 difference is probably that the apex of the leaf is very thin in 

 comparison with the fleshy middle part of the leaf so that under 

 the influence of gravity hquid cannot collect as abundantly in 

 the apex of the leaf as in the lateral parts. 



This leads to a new conception of the nature of the influence of 

 gravity on the formation of organs in plants. All that gravity 

 does and need do is to cause a collection- of the sap of a plant 



