24 XEROPHYTES [CH. 
however ; and their relative numbers, heights and breadths, 
sectlon—acute, rounded, or flattened—furnish valuable 
characters; as also does the coexistence or absence of 
hairs, asperities, &c. 
Fig. 20. 
Fig. 17. Transverse section of the leaf of Festuca elatior, var. pratensis 
(x12). 
Fig. 18. Ditto of the leaf of F. ovina (x15). 
Fig. 19. Ditto of the leaf of F. ovina, var. rubra (x 35). 
Fig. 20. Festuca ovina, var. rubra. Transverse section of the blade of 
an upper leaf (x35). Stebler. 
A very interesting anatomical adaptation is met with 
in the leaves of many grasses which grow in dry situations 
(xerophytes) such as on sandy sea-shores,exposed mountains 
and so forth. When the air is moist, in wet weather or in 
the dews, and the sun’s rays not too powerful, the leaf is 
spread out with its upper surface flat or nearly so, but | 
when the scor¢hing sun and dry air or winds prevail, 
the leaves fold or roll up, with the upper sides apposed 
or overlapping inside the hollow cylinder thus made. 
