234 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



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

 Ijdng 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, ^' shows a cross- 



PiG. 114. — Section througli leaf o£ Rochea falcala, showing siliceous cells of upper epidermis. 



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

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

 (Fig. 115, 4), 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 



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



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. 



Other arrangements are exhibited by Dischidea rafflesiana, a climbing plant 

 with two kinds of leaves; some of the leaves have the usual form but others are 



