GRASSES 267 



require. Nearly all grass-leaves are rolled up in their 

 early stages of growth, and even when full-grown they 

 may require to be rolled up as a temporary protection 

 against hot sun and dry air. Some of our native grasses, 

 growing on dry pastures, such as Sesleria, can roll or unroll 

 in a few minutes. It is enough to put a bell-glass over 

 the growing plant to cause the leaf to open widely, as it 

 always does when the air contains much moisture. If we 

 remove the bell-glass and expose the plant thereby to 

 the warm, dry air of an ordinary room, the leaf will roll 

 up again, and expose a diminished evaporating surface. 

 Some grasses, like the mat-grass of our moors (Nardus), 

 or the sheep's fescue, grasses which inhabit places where 

 there is no shelter from the sun and wind, are permanently 

 inrolled. Others, which grow in damp meadows or shady 

 woods, never roll up when they have once expanded. A 

 few grass-leaves are flat, and have no ridges at all. 



The stomates of a grass, that is the pores by which 

 water- vapour is given off and air taken in, often lie only 

 on the upper surface of the leaf, within the grooves between 

 the ridges. Hence they are well protected from too dry 

 air, especially when the leaf is wholly or partially rolled 

 up. If the leaves are flat, the stomates are usually found 

 on both surfaces. 1 In certain cases this concealed position 

 of the stomates protects them against an opposite but 

 equally dangerous accident, that of being choked by water, 

 which would prevent gas or vapour from passing in or out. 

 You have no doubt often seen the float-grass (Glyceria 

 fluitans), rooted in the mud, and spreading out its leaves, 

 which are sometimes yards long, upon the surface of a 

 pond or a slow stream. The leaves of float-grass lying 

 flat on the water could not, if they possessed the ordinary 

 leaf-structure, drain off the rain, and if they happened to 

 get splashed or drawn beneath the surface by a current, 

 we might suppose that they would find it very hard to 

 get dry again. But no such difficulty is met with. The 



1 Lewton-Brain in Linn, Ti ans., 1904. 



