THE STIOI 



103 



g, they have already become elongated and pointed, and their 

 pointed ends are penetrating between the upper and lower 



cells, so that the „_^„,.„ 



whole tissue be- mmmMm^ ^^^^^^Wi^^^^WW^ 



comes considerably 



firmer. 



If we imagine a 

 large number of 

 such segments as 

 Fig. 1 3 represents 

 fitted together later- 

 ally, we have a pic- 

 ture of the trans- 

 verse section of our 

 branch with its 

 cambial layer caus- 

 ing the increase in 

 thickness. 



§ 17. What is the 

 function of the cam- 

 bium in the ordinary 

 course of growth ? 



The ring of 

 meristematic cell 

 situated between 

 the wood and the 

 bast is placed in a 

 very advantageous 

 position for further 

 growth. It can 

 obtain from the 

 adjoining wood cells 

 as much of the 

 unelaborated sap as 

 it needs, while in the 

 bast at a very little 



distance from the cambium are the sieve-tubes carrying down 

 from the leaves the fully elaborated plastic materials. The 



