172 ELEMENTS OP STRUCTURAL BOTANY. 



293. An examination of the under surface of almost 

 any leaf will show the presence of large numbers of oval 

 openings, somewhat similar to that shown in Fig. 227. 

 These are stomata. They are formed by two epidermal 

 crescent-shaped cells with a space between them, and 

 these have the power of separating or closing together 

 according to circumstances ; separating in the light, in 



moist weather, and closing in dry. The 

 openings communicate with intercellular 

 spaces in the body of the leaf, a number of 

 which are seen in Fig. 226. In ordinary 

 leaves with an upper and a lower surface, 

 the stomata are far more numerous on the 

 lower side ; indeed, many such leaves are 

 entirely without stomata on the upper sur- 

 Fig. 227. face. Vertical leaves have them rather 

 equally distributed on both surfaces. Immersed leaves 

 and underground stems have hardly any at all, and they 

 are never found 011 roots. The use of the stomata will 

 be referred to presently. 



294. The stems of Dicotyledons lose their epidermis at 

 a comparatively early period, and a tissue consisting of 

 cells of cork, filled with air, takes its place. These cork- 

 cells are modifications of the cells beneath the epidermis, 

 and they form an effectual protection to the tissues 

 within. The skin of the Potato-tuber exhibits this corky 

 layer very clearly. The special tissue from which the 

 cork is developed is called pliellogen. 



295. In the fibre- vascular System, different plants 

 exhibit a very different arrangement of the component 



Fig. 227. Stoma from the urface of a leaf, showing the crescent-shaped 

 guard -cells. 



