THE DIFFERENTIATION OF THE PLANT-BODY 25 
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 colenchyma, sclerenchymatous par- 
enchyma, 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 
hypodermal 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. ‘lhe scler- 
enchyma 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 
this mode of disposition. The two main ones form an 
interrupted cylinder (fig. 29), so arranged as to protect the 
delicate vascular tissue, which is in great part placed either 
