Various Physical Factors 357 



counteract, by its upward push, the downward pull caused by the 

 weight of the branch. A bent-over terminal shoot develops reaction wood 

 on its lower side and thus pushes the axis up to vertical. If the terminal 

 shoot is removed, an excess of reaction wood begins to develop on the 

 under side of the lateral branches just below the tip and one of them 

 is finally pushed up to the vertical and becomes a new "leader." Some- 

 times two share this leadership, neither becoming quite vertical. 



Fig. 16-7. Development of reaction wood in pine in three branches which had been 

 bent upward artificially. Reaction wood normally is formed on the lower side but in 

 the new growth on these branches it develops on the upper side after bending and 

 thus tends to push them back to their normal orientation. (From Sinnott.) 



Angiosperm branching is regulated in much the same way except that 

 the reaction wood here is normally on the upper side of a branch and 

 acts by producing tension rather than compression, thus pulling the 

 branches into place instead of pushing them. Wardrop ( 1956 ) believes 

 that the distribution of tension (reaction) wood in Eucalyptus is regula- 

 tory and operates to maintain normal tree form much as reaction wood 

 does in the conifers. 



That compression itself is not responsible for the formation of this 

 wood is shown by the fact that when a terminal shoot is bent around and 



