64 STRUCTURAL BOTANY 



Each stoma consists of a pair of cells, much smaller 

 than those of the epidermis generally, and of quite 

 a different form (see Fig. 20, s, also Figs. 26 and 

 27, s, from leaf). These two cells are called the 

 guard-cells. Each guard-cell is sausage-shaped and 

 curved, the ends of the cells being firmly joined 

 together, while in the middle they are separated a 

 little, leaving a narrow pore or chink between them, 

 which communicates with the intercellular spaces of 

 the ground tissue. The guard-cells of the stomata 

 differ from other epidermal cells in containing chloro- 

 phyll-corpuscles and starch granules. We will defer 

 the more detailed consideration of the structure of 

 the stomata until we come to the leaf, where their 

 more important functions are discharged. 



b. Structure of the Leaf 



We have already learnt so much of the structure 

 of the leaf as can be .made out with the help of a lens 

 only (see Fig. 7). The leaf is built up of the same tissues 

 as we have found in the stem, but with many differ- 

 ences in arrangement and in the details of structure. 



The vascular bundles of the leaf are the direct 

 continuation of those in the stem. We have already 

 traced the general course of the veins of the leaf, and 

 this is identical with the course of the vascular 

 bundles, each vein corresponding to a bundle. In 

 the larger veins, however, such as the midrib, the 

 bundles are accompanied by a thick, enveloping layer 

 of parenchyma, differing from that of the rest of the 

 leaf. The principal bundle traverses the midrib from 



