24 



XEROPHYTES 



[CH. 



however ; and their relative numbers, heights and breadths, 

 section — acute, rounded, or flattened — furnish valuable 

 characters; as also does the coexistence or absence of 

 hairs, asperities, &c. 



Fig. 17. 



Fig. 20. 

 Fig. 17. Transverse section of the leaf of Festiica elatior, var. pratensis 



(xl2). 

 Fig. 18. Ditto of the leaf of F. ovina ( x 15). 

 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 ( x 35). 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 scorching 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. 



