308 THE FOLIAGE LEAF 



escaping from the leaf, and the accumulation of this 

 round the leaves checks further diffusion from their 

 intercellular spaces, and thus enables the turgor of the 

 leaf cells to be recovered by the retention in the leaf 

 of the water coming from the root. 



Most land plants possess structures which tend to 

 produce the same sort of effect as the bell jar placed over 

 the plant, i.e. the prevention of the rapid removal by 

 air currents of the water vapour escaping through the 

 stomata. The commonest of these is the sinking of the 

 stoma in a pit (vestibule), the bottom of which, occupied 

 by the guard cells and pore, is sunk below the general 

 level of the cuticle (Fig. 51, C). Sometimes there is a 

 deep groove or cavity in the leaf containing several 

 stomata. In other cases the leaf rolls or folds up in dry 

 weather, thus protecting the surface bearing the stomata. 

 In other cases, again, there is a thick covering of hairs 

 either over the whole surface bearing stomata or confined 

 to the cavities in which the stomata are sunk. All 

 these structures serve the same purpose to keep the 

 air just outside the stomata relatively still and thus 

 prevent the rapid removal of water vapour by currents 

 of air, which is a very much quicker process than the 

 slow diffusion, which continues in any case. The drying 

 effect of wind is well known, and the shoots of plants 

 exposed to strong, and especially of course to dry, 

 winds, very quickly wilt and even die (through loss of 

 water greater than they can recover) , unless they are very 

 well protected in one of the ways described. It is 

 among plants living in very dry climates, or frequently 

 exposed to strong winds during the growing season, or 

 growing in dry soil, that the structures described are 

 most developed, for it is only such plants (xerophilous I 



1 Greek t]po^, dry ; <f>t\oz, friend. 



