THE DIFFERENTIATION OF THE PLANT-BODY 29 



each other and are disposed in a ring comparatively near 

 the periphery. They thus afford an effective resistance 

 to such a lateral strain as they would be subjected to during 

 a high wind. This is very conspicuous in hollow herbaceous 

 stalks. Later on, if the shoot increases in dimensions, the 

 ring of separate strands is replaced by a central core not 

 unlike that of the root. 



The distribution of the woody elements in the leaf 

 is different from either. They exist in the form of the 

 so-called veins, which constitute a network of very tough 

 fibrous bands upon which the delicate tissue, easily tearable, 

 is supported. These strands usually strengthen particularly 

 the margins and apex of the leaf blade and protect it from 

 being torn. The blade, therefore, when acted on by wind is 

 made to play as' a single rigid piece moving up and down 

 without losing its flatness for a moment. 



There are, however, many herbaceous forms, whose re- 

 quirements are similar to those of the very young axis of 

 the woody plant, but which 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 subsidiary develop- 

 ment 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 par- 

 enchyma, or true sclerenchyma. Of these collenchyma alone 

 is capable of elongating as the growth in length of a member 

 containing it proceeds. It is rather tough than rigid, and 

 offers a very great resistance to any force tending to tear 

 it in any direction. Sclerenchyma is not extensible, but 

 is extremely hard and rigid. 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 



