THE STRUCTURE OF THE LEAF 393 



leaves are frequently supplied with bands of solerenohynia, which 

 extend from the epidermis inwards, often forming a thick sheath 

 round the bundles, and reaching from one epidermis to the 

 other. 



In the centric leaf there is not much differentiation of the 

 mesophyll, but the internal tissue is more or less homogeneous. 

 This type is generally found in succulent leaves, such as those 

 of Crassula. The cells are usually elongated like those of pali- 

 sade parenchyma, and have between them narrow intercellular 



Fi'/.77is. Centric leaf of Pinits. ep. Epidermis. }iij. Hypoderma. t'ti. Eu- 

 dodermi^. v.h. Vascular bundle, r.d. Resin-duct. 



spaces. The chlorophyll is most abundant in the outer layers. 

 Frequently tannin sacs are present. The oeUs are all very 

 turgid, from the presence of water. 



A particular form of the centric leaf is found in some of the 

 Conifers. It is somewhat awl-shaped, andin section appears plano- 

 convex (fig. 778). The epidermis is thick-walled and has stomata 

 on all its surfaces, the guard-cells of the latter being usually de- 

 pressed below the surface. The mesophyll contains a layer of 

 sclerenchyma on its exterior, which at the margins of the leaf 

 consists of a number of cells filling up the angle of the section. 

 The other cells of the mesophyll are sinuous in outline, and their 



