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 
Tig. 31.—Vasounar Bunp_e or Stem or Monocoryneponous PLANT. 
(After Kny.) 
ph, phloom ; «, xylem vessels; p ph, protophloem. The bundlo 
is surrounded by a small-celled sheath of sclerenchyma. 
similar character (lig. 81); in Pennisetum (tig. 382, 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. 82, 5) there is a development of 
