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 ag 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 
