266 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



Thus Rochea falcata, a South African plant, is armed with a siliceous coat of 

 mail. A cross-section of the leaf shows that the small cells of the epidermis 

 are overlaid with a continuous layer of large, bladder-like cells (Fig. 114), the 

 walls of which are richly impregnated with silica. These siliceous cells are filled 

 with water, which is replaced by air only when they become old. As long as 



Fig. 114. — Section through leaf of Rochea falcata, showing siliceous cells of upper epidermis. 



these cells contain water they behave like reservoirs from which the deeper- 

 lying cells of the leaf draw their supply. 



The leaves of Stipa capillata (Fig. 115) furnish an example of characteristic 

 arrangements that prevent excessive transpiration. Fig. 115, A' shows a cross- 

 section of a leaf of this plant under normal conditions. When drought begins, 



Fig. 115. — Cross-sections through leaves of Stipa capillata {A, A') and of Festuca alpestris (B). 



however, the stomata not only close but the leaf also rolls and forms a tube 

 (Fig. 115, A), so that only one of its surfaces — and indeed the surface that 

 possesses thick cuticle and is quite devoid of stomata — is exposed to the outer 

 air. All the stomata are then on the inner surface of the leaf. Fig. 115, B, 

 represents a cross-section of a similarly rolled leaf of Festuca alpestris. 



