VIII] SAND DUNES 135 



furrows. The behaviour of these leaves under vary- 

 ing circumstances of wind and moisture is interesting. 

 It may be noted on any breezy day that the leaves 

 present their smooth and convex surface to the wind, 

 so that not only is their mechanical resistance to it 

 the most eifective, but also any sand it may convey 

 glances off instead of catching in the grooves of the 

 concave face. Another point which is certainly an 

 adaptive one is the capacity of the leaf for rolling 

 in its margins in dry weather till it looks like a length 

 of green wire. But when moisture is plentiful it 

 flattens out so that the smooth lower surface is almost 

 flat, and the grooved upper face is widely exposed. 

 This automatic change of form, which quickly alters 

 the general appearance of the grass after a shower, 

 is brought about by the contraction or expansion of 

 certain water-containing cells on the upper surface. 

 As these swell with moisture the leaf flattens, and 

 evaporation of water-vapour from the upper surface 

 is easy. As the moisture becomes deficient the leaves 

 contract, the margins of the blade are drawn together, 

 and evaporation is checked. Such automatic control 

 is one of those features which make life possible for 

 the plant under the exacting conditions of the sandy 

 Dune. The inflorescence appears as a dense white 

 panicle in July, but it presents no characters of 

 special interest. This much may be observed without 

 digging j^tlie plant up. But below the surface of the 



