28 VEGETABLE PHYSIOLOGY . 
in all three regions, sometimes the bands being all inde- 
pendent, sometimes united in various ways. In Cladium 
Mariscus (fig. 82, 14) those of all the regions are united 
into a continuous system which goes from the tegumentary 
region towards the interior of the stem, embracing the 
vascular bundles and attaching them to each other. 
Fre. 31.—Vascvunsn BunpLE oF STEM OF MoNnocoTyYLEDONOUS PLantT. 
(After Kny.) 
ph, phloem; x, xylem vessels; p ph, protophloem. The bundle 
is surrounded by a small-celled sheath of sclerenchyma. 
Similar differentiation of the supporting system is found 
in many leaves, in which it subserves the same purposes. 
In many cases the veins afford sufficient protection against 
tearing or rupture in consequence of violent winds. The 
methods of their arrangement in many cases subserve 
this purpose very completely. In other leaves of tough 
leathery habit the delicate tissue of the mesophyll is fre- 
quently protected from crushing by isolated thick-walled 
