30 VEGETABLE PHYSIOLOGY 



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, 



hy 



FIG. 29. TKANSVEHSE SECTION OF RHIZOME OF THE BBAOKEN FEKN. 

 X 10. 



sc, bands of sclerenchyma ; hy, hypodermal sheath of sclerenchyma ; 

 st, steles ; ep, epidermis. 



and are not generally very strongly developed. The 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 arrangement 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 



