82 



REGENERATION 



Figure 62 is a photograph of marked whole stems 9 days after 

 the beginning of the experiment. The stems had been suspended 

 horizontally in the aquarium ; all had one apical leaf. That part of 

 the cortex which was below had stretched, while the cortex above 

 was shortened. The India ink marks were made at the beginning 

 of the experiment with a distance of 1 centimeter from each other. 

 The photograph shows the change in the position of the marks 

 on the convex side in the bent region of the stem. 



Fig. 62. — Stems suspended horizontally but bent. Marks in India ink show 

 that in the bent region the lower side of the cortex grew in length while the 

 upper side did not. June 3 to 12. 



It is highly probable, if not certain, that the increase in length 

 on the lower side of the horizontally placed stem takes place pri- 

 marily in the cortex of the bending region, not in the wood. 

 This follows from the behavior of these two parts when the cortex 

 of a bent (split or whole) stem is removed, and the I'igidity of 

 the cortex is compared with that of the pith and wood taken out. 



If we remove the cortex on the lower (convex) side of a split 

 geotropically bent stem, like that in Fig. 60, we find that the 

 rigidity of the cortex in the bent region is much greater than that 

 of the wood; the latter appears soft in comparison with the cortex 

 of the bent region on the convex side of a geotropically bent stem. 

 It is possible also that the increase in the rigidity of the cortex 



