CHAPTER IV 



THE INHIBITING ACTION OF THE RAPIDLY GROWING 

 NOTCHES ON THE OTHER NOTCHES OF A LEAF 



In Figs. 14 to 17 it is obvious that only those notches of a leaf 

 which dip into the water give rise to new growth of shoots and 

 roots, while in the other notches no growth is noticeable. This is 

 not so at the beginning of the experiment; at first, growth of 

 tiny roots and even of shoots may commence in many notches 

 of the leaf, but soon the growth stops in all the notches except 

 those dipping into water or which are close to the water. When 



Apex in WdXep 



Fig. 19. — I and la, sister leaves; I, dipping with the apex into water; la, 

 suspended entirely in moist air. Formation of roots and shoots commences in 

 many notches in both leaves simultaneously. Drawn after 12 days. 



the roots or shoots in these latter notches begin to grow, the tiny 

 air roots already formed in the other notches shrink, dry out, and 

 disappear; and the buds in the notches which upon further growth 

 would have developed into shoots stop growing, so that after a 

 few weeks no further growth is noticeable, except that in the 

 notches which are in the water or near the edge of the water. 

 This can be easily illustrated with large leaves or with the succu- 

 lent leaves as they develop in the greenhouse during the winter. 

 Figure 19 represents such an experiment. On Feb. 12, 1921, two 

 sets of sister leaves were cut off and one of each pair was sus- 

 pended entirely in air, while the other dipped with the apex into 

 water. Figure 19 shows the appearance of the leaves on the 



19 



