32 VEGETABLE PHYSIOLOGY 



sclerenchyma is developed in connection with only one of 

 the three regions already alluded to. The stem of Equi- 

 setum and the leaves of Conifers are furnished with a layer 

 of thick-walled cells immediately under the epidermis 

 (fig. 30) ; the vascular bundles of many Monocotyledons 

 are surrounded separately by a sheath of small cells of 





Eia. 31. VASCULAR BUNDLE OF STEM OF MONOCOTYLEDONOUS PLANT. 

 (After Kny.) 



ph, phloem ; x, xylcm vessels ; p ph, protophloein. The bundle 

 is surrounded by a small-celled sheath of aclerenchyma. 



similar character (tig. 31) ; in Pennisetum (tig. 32, 4) a 

 sheath is developed round the stem in the form of a 

 hollow cylinder which lies between the bundles and the 

 epidermis. 



More frequent instances occur in which two of the 

 regions in question are strengthened simultaneously. In 

 the stems of Scirpus (fig. 32, 5) there is a development of 



