26 BRITISH PLANTS 



Transpiration. The transpiration-current starts in the 

 roots and ends in the leaves. So long as the roots absorb, 

 this stream is fed and kept in motion. What happens, 

 then, to the water at the end of its journey ? The leaves 

 must get rid of the water which the plant does not require, 

 otherwise the current would stop, and the plant would 

 become surcharged with water and suffocated. The 

 nutritive substances which the current carries are left 

 in the cells, and the excess of water is evaporated away. 

 The doors of exit are the stomata (Gr. stoma, a mouth ; 

 plural, stomata), small pores or openings which are 

 found in enormous number on the surface of the leaves 



FIG. 3. EPIDERMIS, WITH STOMATA, FROM THE UNDER SIDE OF A LEAF. 

 (HIGHLY MAGNIFIED. )- 



(Fig. 3). The water escapes in the form of vapour, and 

 not as liquid drops. In the leaf the cells are not packed 

 closely together, but are loosely arranged, with the 

 cavities between them filled with air. All these cavities 

 communicate with one another, and ultimately focus on to 

 the large air-spaces which occur below the stomata. The 

 plant has the power of varying the size of these openings 

 according to the amount of moisture in the atmosphere. 

 The aperture of the stoma is bounded by two modified 

 epidermal cells called guard-cells (Fig. 4). When these are 

 turgid, the stoma is wide open ; when, through excessive 

 loss of water, they lose their turgidity, they fall together 



