CLIMATIC AND EDAPHIC FACTORS 



4G1 



closing movements. We have still to discuss a number of structural 

 features, which have no direct connection with the mechanism of the 

 stoma, but which are nevertheless of great physiological importance. 

 Most of these features have been developed in adaptation to particular 

 climatic or edaphic factors of the environment. 



Under what may be termed average conditions of water-supply, 

 transpiration and hence gaseous interchange in general can be suffi- 

 ciently regulated by the movements of the guard-cells alone. In such 



Fio. 175. 



Sunken stomata. A. Amherstia nobilis (upper side of leaf). B. Hakea suaveoleiis; 

 i, internal, a, external air-chamber. 



cases the front- and back-cavities are never greatly developed : the 

 entrance to the front-cavity is often very wide, and the stoma as a 

 whole lies at the same level as the surrounding epidermal cells. 



Among plants which grow in a dry climate, or in arid localities, 

 the greater risk of desiccation renders it essential, that transpiration 

 should be further restricted by special arrangements. In such cases 

 the stomata are generally sunk below the level of the surrounding 

 epidermal cells, so that each appears to be situated at the bottom of a 

 funnel, cup- or saucer-shaped depression, the so-called external air- 

 chamber (Fig. 175). Not infrequently, the entrance to this chamber 

 is partially blocked by ridge- or wall-like outgrowths of the neighbouring 



