28 



VEGETABLE PHYSIOLOGY 



in all three regions, sometimes the bands being all inde- 

 pendent, sometimes united in various ways. In Cladium 

 Mariscus (fig. 32, 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. 



FIG. 31. VASCULAR BUNDLE OF STEM OP MONOCOTYLEDONOUS PLANT. 

 (After Kny.) 



pTi, 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 



