156 THE PHYSIOLOGY OF PLANTS 



is at a lower level than the middle of the shoot, the buds at 

 the highest point of the arch formed by the shoot will be the 

 most favoured. If the whole shoot is laid horizontally, the 

 supply of water to the whole branch is diminished, as the bend 

 at the base of the shoot retards the current of water which is 

 directed towards the apex. 



If you examine the under surface of a greatly bent shoot 

 at the bend, you will see that the outer tissues are compressed 

 into considerable wrinkles. This is explained by the fact that 

 the cortical tissues have had to be reduced within the bend to a 

 smaller area. These outer wrinkles are, however, only an external 

 manifestation of an internal damage which consists in the split- 

 ting away of cortex and bast from the internal wood. The space 

 thus formed between the wood and the bast is, however, not 

 apparent from the outside, and later on a microscopic investi- 

 gation does not reveal the existence of a split within the bast, 

 but shows a whitish hardened tissue. This is newly-formed, 

 short-celled, and parenchymatous wood, which is rich in starch. 

 Any considerable bending of a branch entails, therefore, at the 

 bend a loosening of the tissues in the lower side of the branch 

 and an increase of tension on the upper side, so that the cells 

 on this side are stretched and consequently become narrower. 

 By this lateral compression of the cells of the upper side the 

 conducting tissue, both for the plastic material descending from 

 the leaves and also for the rising sap, has its activity diminished. 

 The kinking of the tissues on the under side produces a similar 

 interruption of the flow of substances. Considerable bending 

 of a shoot always diminishes the flow of sap towards the apex, 

 and also the conduction of assimilated material towards the 

 base of the shoot. The latter accumulate, therefore, at the 

 upper portion of the bend ; the raw sap, on the other hand, is 

 stopped on the lower side. 



Every bend acts, therefore, as a barrier to the ascending and 

 descending currents. Above the bend the plastic material 

 accumulates and causes a more ready development of the buds 

 at that point into flower-buds. Below the barrier the buds are 

 more liberally supplied with water, and develop, therefore, into 

 strong leafy shoots. 



The effect of the bending depends chiefly on the mechanical 



