Iv] HETEROPHYLLY 71 
tions of grass leaves. The results, which are based on the 
elaborate investigations of Duval Jouve, Schroeter, Pée- 
Laby and Grob, as well as on my own observations, are 
Fig. 28, Transverse section of subulate leaf of Aira flexuosa (x about 
50), the upper surface represented by a mere ridge with two flanking 
grooves each with but traces of motor-cells below. One large 
vascular bundle and four much smaller ones are seen. There are no 
girders, but slender bands of sclerenchyma at the lower surface 
nearly join into a continuous sub-epidermal sheath. This type is 
the extreme form of that in Fig. 26. 
not complete in all respects, and much more should be 
done to extend the theme, but the account given will 
serve to show the student how such results may be 
employed. It is as yet impossible to decide how far these 
characters are constant—they are known to be fairly so in 
many cases—but several grasses cannot yet be distin- 
guished by them alone. 
It should also be added that some grasses develope 
two types of leaves (heterophylly), solid or subulate below, 
flat or shghtly inrolled above—e.g. Festuca heterophylla— 
and the following arrangement is intended to apply to the 
vegetative lower leaves and not to those on the upper 
parts of the flowering specimen. Moreover the sections 
should be cut from the basal third of the lamina, and not 
from the tip of the leaf. 
