THE DIFFERENTIATION OF THE PLANT-BODY 25 



and twigs are of similar composition, the former being 

 strengthened very materially also by its bark. But there 

 are many smaller trees and shrubby plants, as well as some 

 herbaceous forms, whose requirements are similar, but 

 which for various reasons have not a very great develop- 

 ment of either primary tegumentary tissue or of vascular 

 bundles. With no additional mechanism for support, they 

 would be in great danger of either collapsing or being 

 actually uprooted. In their cases we meet with a sub- 

 sidiary development of supporting tissue, which shows a 

 great variety in its arrangement and distribution. 



We find that the tissue which most frequently subserves 

 this purpose is either collenchyma, sclerenchymatous paren- 

 chyma, or true sclerenchyma. In a few delicate stems 

 these tissues are much more prominent than the vascular 

 bundles. We can notice three regions of the stem or axis 

 where they may appear, and in these places they may take 

 the form of isolated cells, or strands of tissue, or complete 

 sheaths going round either the whole axis or separate parts 

 of it. The first of these regions is the layer underlying 

 the tegumentary tissue, which the new development sup- 

 plements and strengthens. Most moss plants show the 

 hypoderrnal cells of their axis thickened, while such a 

 development is very common in many petioles and leaf- 

 blades. The new development may occur in close relation- 

 ship with the vascular bundles which, in such cases, are 

 found among large-celled somewhat succulent parenchyma, 

 and are not generally very strongly developed. The scleren- 

 chyma by forming a separate sheath round each bundle 

 gives it a rigidity which it could not derive from its own 

 elements, and in addition prevents the whole stem from being 

 crushed. This is seen in the stems of many semi-succulent 

 monocotyledonous plants, such as those of the maize and the 

 asparagus (fig. 31). The sclerenchyma may also occur freely 

 in the ground tissue, at some distance from both tegumentary 

 and vascular structures. The bands of it which occur in 

 the rhizome of the bracken fern are good illustrations of 



