OF THE LEAVES OF BRITISH GRASSES. 317 
with some reference to their biological importance; and in Chapter V. a classifi- 
cation of the more common grasses based on the anatomical characters of the leaf. 
The form of classification adopted is that of a continuously dichotomizing key. "The 
main characters made use of are, as with Pée-Laby, the arrangement of the chlorophyll- 
tissue, the presence or absence of conspicuous ribs on the upperside of the leaf, the 
relative number of stomata on the two sides, and the position and arrangement of the 
motor cells. Other characters made use of are the degree of development and the 
arrangement of the mechanical tissue and the hairiness. He claims, however, only to 
have **brought tegether some of the principal anatomical features in such form that 
their characters may be employed in checking other determinations of grass-leaves." 
My own acquaintance with the subject began by my testing, at Professor Marshall 
Ward's request, his classification of leaf-sections. The object of the present research 
has been twofold. In the first place, I hoped that, by accurate diagrams and short 
deseriptions of a number of transverse sections of the leaves, some further help might be 
given towards identifying grasses from their vegetative characters. In the second place, 
l wished, after plaeing the grasses in groups according to habitat, to see how far in each 
case the structure of the leaves is adapted to the environment. 
In Pée-Laby's work, the leaves used were taken from plants growing in the Botanical 
Gardens at Toulouse, and which had either been raised from seed or had been planted 
there for a long time. This plan is open to the obvious objection that cultivation 
in a soil differing from that of their natural habitat, and otherwise under unnatural 
conditions, may very possibly influence the growth of the plants, so that the structure of 
leaves taken from such plants may be obviously different from the normal one. I had 
hoped to make some experiments to discover what changes, if any, are produced in 
the structure of the leaf by alterations in the environment. The leaves for my own 
work were in nearly every case taken from plants growing under natural conditions in 
their natural habitat; in one or two cases only, where I have been unable to obtain such 
leaves, have I made use of cultivated plants. 
The leaves were fixed in absolute alcohol and sections cut by hand. The diagrams 
were all drawn by means of a camera lucida, thus securing identity of magnification and 
rendering comparison more easy. 
I propose to divide the remainder of this paper into three parts :— 
I. A short account of the general anatomy and histology of grass-leaves, so far as is 
necessary to explain the following parts (iI. & IIL). 
II. Brief descriptions of the leaf-structure of all the grasses I have examined, arranged 
in an artificial key for purposes of classification. 
III. A grouping of the grasses according to habitat, and a discussion of the question 
how far a similarity in environment entails a similarity in leaf-structure. 
222 
