64 OUTLINES OF EVOLUTIONARY BIOLOGY 



shall see at once that it is made up of three principal tissue- 

 systems, the epidermis, the mesophyll and the vascular bundles. 

 The epidermis (epd.) covers the upper and lower surfaces in the 

 form of two single layers of cells, the mesophyll (mes.) occupies the 

 space between these two layers, and the vascular bundles (r.fc.) 

 corresponding to the veins of the leaf are imbedded in the 

 mesophyll at fairly regular intervals (only one is shewn in the 

 figure). We must examine each of these tissue-systems sepa- 

 rately, and in order to gain a correct idea of the form and 

 arrangement of the cells of which they are composed it will be 

 necessary to study them from various points of view. 



The epidermis may be stripped off bodily from the surface of 

 the leaf and then exhibits under the microscope the appearance 

 shown in Fig. 26, B. It is composed of elongated cells, placed 

 side by side in a single layer. The amount of protoplasm which 

 these cells contain is very small, but the nucleus (nu.) is 

 frequently conspicuous. Their external walls are specially 

 thickened in relation to their protective function, a feature 

 which can only be seen in sections (Fig. 26, A). At frequent 

 intervals little slit-like openings occur in the epidermis. These 

 are the stomata (st.) which lead into air-spaces in the mesophyll. 

 Each stoma is bounded by a pair of specially modified epidermic 

 cells known as the guard-cells (g.c.) the only epidermic cells 

 containing chlorophyll which have the power of opening and 

 closing the stoma like a pair of lips and thus regulating the 

 amount of aqueous vapour which passes through the stomata in 

 the process of transpiration. On the outer side of each guard- 

 cell lies another epidermic cell of much smaller size than the 

 ordinary kind, and these, together with the guard-cells, form a 

 kind of roof (or floor) to the air-cavity (Fig. 26, A, a.c.). 



The mesophyll contains the chlorophyll-bearing cells, by 

 which the assimilation of carbon dioxide is effected and which 

 are at once recognized by their green colour. They appear more 

 or less round or oval in transverse sections (Fig. 26, A, ex.], but 

 are really considerably elongated, parallel to the length of the 

 leaf, as shown in Fig. 26, C. They come in contact with one 

 another by numerous short protuberances, between which lie the 

 spaces in which the air, containing carbon dioxide and aqueous 

 vapour, circulates. Each contains a nucleus (nu.) and numerous 

 small, biscuit-shaped green bodies, the chloroplastids or chloro- 

 phyll corpuscles (cp.), imbedded in the cytoplasm. 



