14 DISEASES OF TROPICAL PLANTS CH. 



W 



le 



FIG. 1. Cross section of leaf. 

 c, Cuticle ; e, upper epidermis ; p, palisade cells ; 

 f.v.b., flbro-vascular bundles; w, mesophyll; I.e., 

 lower epidermis. 



the other two sets of organs and which may also serve 

 other purposes. 



If we cut through a leaf (Fig. 1), and then examine 



the cut edge with a micro- 

 scope we find the most 

 common structure as fol- 

 P lows : both the upper and 

 / v ./j the lower surfaces are 

 composed of a single layer 

 of cells which are usually 

 clear and have very thick 

 walls, the outer or ex- 

 posed walls being thicker 

 than the inner walls. 

 This layer of cells is 

 known as the epidermis, 

 and the thickened outer wall as the cuticle. The 

 epidermis is in reality the skin of the leaf. Just below 

 the upper epidermis are the palisade cells which are 

 elongated, somewhat cylindrical in shape, and placed 

 at right angles to the upper epidermis. Between the 

 palisade and the lower epidermis is the mesophyll, or 

 spongy cells, among which are intercellular spaces or 

 tunnels which ramify among the cells and have external 

 openings called stomata. These external openings may 

 be readily seen by peeling a little of the epidermis (Fig. 2) 

 from the leaf and examining it under 

 the microscope. They are always 

 between two crescent-shaped cells, 

 which are known as guard cells, and 

 are much more abundant on the 

 lower than on the upper surface of 

 the leaves. The palisade and meso- 

 phyll cells contain the chlorophyll 

 or green colouring matter, which is 

 confined to definite bodies known 

 as chloroplasts. These chloroplasts 

 are the real laboratories of the plant, 

 and it is here that the raw food material is transformed 



FIG. 2. Lower surface of 

 leaf showing stomata. 



