It] 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 (Mestuca), 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. 
Elymus. Alopecurus. 
Psamma. Glyceria fluitans. 
Aira cespitosa. Keleria. 
Lolium. Festuca elatior. 
Cynosurus (Fig. 16). Festuca Myurus (var. sciwroides). 
Agrostis. Melica has ridges on the lower 
surface. 
Ridges are less prominent in Phleum pratense, Briza, 
Agropyrum, Triodia, Arrhenatherum avenaceum. 
Leaves practically devoid of ridges. 
Poa—all common species. 
Glyceria aquatica. 
Catabrosa aquatica. 
Avena pratensis. 
