84 LEAVES. 



(46) Alexandrian Senna. 



The leaflets of Cassia acutifolia, Delile (N.O. Leguminosae). 



The, epidermis of both surfaces is furnished with stomata and 

 simple hairs. The cells of which it is composed ate polygonal, 

 averaging about 45^ in length, but attaining 70/u ; they have 

 straight walls, and are covered by a rather thick cuticle. The 

 stomata are usually surrounded by two, three or four cells, of which 

 two are parallel to the ostiole. The hairs are one-celled and conical; 

 they may be straight or curved, and are sometimes enlarged towards 

 the middle and slightly contracted at the base; their walls arc 

 moderately thick and warty. 



The mesophyll is heterogeneous and symmetrical, there being 



palisade tissue on both surfaces ; the spongy parenchyma is much 



reduced, and consists of oval or rounded cells. Both these cells 



and those of the palisade tissue contain cluster crystals of calcium 



- oxalate. 



The midrib is biconvex. There is. a layer of collenchyma both 

 above and below the meriptele. There are no fibres in the bast, 

 l)ut there is a well-developed arc of lignified pericyclic fibres, both 

 above and below the wood. The parenchymatous cells next to 

 these contain prismatic crystals of calcium oxalate. 



Indian senna closely resembles Alexandrian in its structure ; 

 it differs in having fewer hairs and larger epidermal cells. 



The diagnostic characters of powdered senna are : 



(a) The characteristic hairs. 



(b) The stomata accompanied usually by two cells parallel to the 

 ostiole. 



(c) The fibrous pericycle, accompanied by crystals. 



(d) The isobilateral structure. 



