n] 



RIDGES ON LEAVES 



23 



to the height of these ridges the thinner parts between 

 look like deep or shallow furrows (cf Figs. 8 — 16 and 

 Chapter IV.). If the leaf is held up to the light the 

 ridges appear dark in proportion to their opacity — i.e. 

 height or thickness — and the furrows light in proportion 

 to the thinness of the tissues there. If the contrast is 

 very great, as in Aira ccespitosa (Fig. 23), the furrows 

 seem like transparent sharp lines, and when, as in Poa, 

 which is practically devoid of ridges, the difference of 

 thickness is small they appear merely as fine striae. 

 These characters must be determined on the fresh leaves, 

 however, because the contraction in drying draws the 

 ridges closer together and tends to obliterate the lines. 

 The ridges are almost always evident — Catabrosa, Poa, 

 and Avena furnishing the chief exceptions — and are nearly 

 invariably on the upper surface : they are below in Melica, 



(z?5p: 



r^-*- an.?. -^' e-i-t ryTT-T^-T-T-T-T-'^ — t-t* 



Fig. 14. Digraphis arundinacea. Transverse section of mid-rib and half 



the leaf ( x about 6). 

 Fig. 15. Holeus lanatus. Transverse section of leaf-blade ( x 10). 

 Fig. 16. Cynosurus eristatus. ■ Transverse section of the leaf-blade 



( X 20). Stebler. 



