IT] 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—e. 
height or thickness—and the furrows light im proportion 
to the thinness of the tissues there. If the contrast 1s 
very great, as in Aira cespitosa (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 striz. 
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 Mela, 
Fig. 14. Digraphis arundinacea. Transverse section of mid-rib and half 
the leaf (x about 6). 
Fig. 15. Holcus lanatus. Transverse section of leaf-blade (x 10). 
Fig. 16. Cynosurus cristatus. Transverse section of the leaf-blade 
(x20). Stebler. 
