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 
Fic. 3.—EpmrrmMis, witH STOMATA, FROM THE UNDER SIDE oF A Lear, 
(Hieuty 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 
