36 GENERAL BOTANY 



becomes cylindrical, for that is the form which exposes 

 the least possible surface. 



But the lower end of a tree-trunk, where it enters- 

 the ground, is very often not round, but very ir- 

 regular in form and very much expanded, so that 

 a cross-section would be anything but circular. This 

 is particularly the case in very high trees, and 

 these expansions serve to make the stem much more 

 rigid. The extra wastage in dead bark-tissue to 

 cover the expansion is therefore compensated by 

 the increased strength and thickness of the trunk at 

 this part ; for it is just near the ground where it 

 is fixed firmly by the roots that the trunk of a tall ; 

 tree might be broken in a high wind. It may be 

 asked why, if this is the case, the upper part of the 

 stem and branches have not expansions to make them 

 more rigid. The answer is that it is no disadvantage,, 

 rather it is an advantage, if the branches bend and 

 yield to a strong wind. It is only at the base where: 

 the trunk is fixed by roots firmly into the ground,, 

 and therefore cannot give, that it needs extra strength 

 to prevent its being broken. 



