Il] INFOLDING OF LEAVES 25 



In such leaves some of the upper epidermal cells, 

 either next the mid-rib {Sesleria &c.) or between the other 

 ribs {Festuca &c.) are large and very thin-walled, full of 

 sap when distended, and so placed that as they lose water 

 by evaporation they contract, and so draw together 

 the two halves of the lamina (Sesleria) or each ribbed 

 segment (Festuca), thus causing the infolding or in- 

 rolling (see Chapter IV.). Not only from the structure 

 and actions of these motor-cells, but also from the fact that 

 the stomata are on the upper surfaces and thus protected, 

 and that the lower surfaces which alone are exposed to 

 the drought are defended by hard and impenetrable 

 tissues, we must look upon these as adaptations to the 

 xerophytic conditions. 



Leaves prominently ridged. 



Elynvm. Alopecurus. 



Psamma. Glyceria fluitans. 



Aira ccespitosa. Koeleria. 



Lolmm. Festuca elatior. 



CyTiostirus (Fig. 16). Festuca Myurus (var. sciuroides). 



Agrostis. Melica has ridges on the louver 



surface. 



Ridges are less prominent in Phleum pratense, Briza, 

 Agropyrum, Triodia, Arrhenatherum avenaceum. 



Leaves practically devoid of ridges. 



Poa — all common species. 

 Olyceria aqaatica. 

 Catahrosa aqvMtica. 

 Avena pratensis. 



