76 LEAVES. 



(42) Orange Leaves. 



Orange leaves are obtained from the bitter orange tree, Citrus 

 vulgaris, Risso (Citrus aurantium, var. Bigaradia, Hook, f.), 

 belonging to the natural order Rutacese. 



The epidermis is glabrous and covered with a rather thick 

 cuticle ; it is composed of polygonal cells, and is free from hairs of 

 any kind, simple or glandular. Stomata occur on the under surface 

 of the leaf only, and each one is surrounded by from four to six 

 cells rather smaller than the other epidermal cells. 



The mesophyll is heterogeneous and asymmetrical. The upper 

 part is composed of two or three rows of palisade tissue, some of 

 the cells of which are conspicuous by reason of their larger size ; 

 each of these cells contains a large octohedral crystal of calcium 

 oxalate ; this is a distinguishing feature of leaves belonging to the 

 genus Citrus. 



The spongy parenchyma of the leaf is composed of cells of 

 varying shape, and contains large internal oil glands. 



In transverse section the midrib appears nearly flat, or slightly 

 convex towards the upper surface of the leaf, but strongly convex 

 towards the lower surface. The wood, which has a similar shape, is 

 covered with a layer of bast, and this again by a pericycle consisting 

 of strongly lignified and thickened fibres. The cells next to the 

 pericycle (endodermis) contain octohedral crystals of calcium 

 oxalate, and similar crystals are also found in the cortical tissue 

 of the midrib. 



The diagnostic characters of powdered orange leaves are to be 

 found in : 



(a) The large crystals of calcium oxalate. 



(b) The absence of hairs. 



(c) The cliaracters of the epidermis. 



