90 STRUCTURAL BOTANY 



branch. It is clear that the rootlet, owing to its 

 endogenous origin, has the advantage of a more pro- 

 tected position from the first a protection which it 

 needs, as its delicate tissues would otherwise be 

 exposed to injury by contact with the soil. The 

 endogenous mode of branching is one of the most 

 constant characters by which roots may be dis- 

 tinguished from stems. 



f. Secondary Growth in Thickness 



The stem and root of the Wallflower, like those of 

 most members of the Class to which it belongs, go on 

 increasing in thickness as long as the plant lives. 

 This is due to the activity of the cambium, the origin 

 of which, in the vascular bundles themselves, we have 

 already described. It is impossible to draw a sharp 

 line between procambium and cambium ; for the latter, 

 as we have seen, is simply the permanently active 

 middle part of the procambial strand ; but it is to 

 be noticed that while, in the earlier stages of develop- 

 ment, the cell-divisions in the procambium take place 

 indiscriminately in all directions, after a time they 

 become very regular, nearly all the new walls being 

 formed in the tangential direction, i.e. parallel to a 

 tangent drawn to the surface of the stem, while radial 

 walls are formed at long intervals only. This regular 

 tangential division is characteristic of the cambium as 

 distinguished from the procambium (Fig. 21, c, p. 51). 



The cambium of the bundle, then, goes on growing 

 and dividing without limit ; on its inner side it cuts 

 off cells, which, after one or two further divisions, 



