8 STRUCTURE 
Hairiness or pubescence of the leaves is not a constant character 
in some species, but is less variable in respect to the lower sheaths. 
The leaves are often rough to the touch (scabrid) on the upper or 
both surfaces, and on the margins ; this is most readily detected 
by the lips or tongue. Ifa scabrid leaf-margin be examined under 
the microscope it will be seen to be finely serrulate or sawlike, the 
asperities or minute teeth pointing towards the apex, The various 
shapes of the ligule sometimes help us to discriminate allied 
species ; it may be acute, or its apex may appear as if cut off 
Fic. 4.—Outline transverse section (enlarged) of leaf of Lolium perenne, showing ribs 
and venation. The dots represent fibrovascular bundles in cross-section. 
abruptly (truncate) ; sometimes it is split down the middle (bifid), 
or torn into shreds (laciniate) ; at other times it is scarcely more 
than a margin at the mouth of the sheath, or it may be modified 
as a ring or tuft of hairs. 
The flowering stem of grasses, usually spoken of as the culm, 
or popularly the straw, is quite distinctive of this order of plants, 
being hollow except at the knots or nodes which are a solid mass 
of fibres dividing the culm into a series of chambers. The culms 
are solid in the young state, but as they grow in length, the ground 
tissue of the centre ruptures, leaving only a thin ring to carry on 
the elongation of the internodes. From this period, therefore, the 
culm is fistular. In other respects, the structure of the culm is 
similar to that of the stems of other monocotyledons. If we make 
a very thin cross section through an internode of a culm and place 
it under the microscope, we see (fig. 5) a large central cavity bounded 
by cellular tissue (parenchyma) which is traversed longitudinally 
by isolated strings of woody tissue (fibro-vascular bundles), often 
disposed in two or more circles. Towards the outer part of the 
stem the cells are modified in having their walls thickened and 
hardened (sclerenchyma) ; these are often termed mechanical cells 
because of their function, namely to give strength and rigidity to 
the stem. A sheath of sclerenchyma surrounds each fibro-vascular 
bundle. The latter, once formed, is not able to grow further in 
thickness, having no longer any cambium or dividing-cells ; the 
culm therefore does not increase in diameter. As the student is 
no doubt aware, these definite or closed bundles, as they are 
termed, are characteristic of all monocotyledons. Silica enters 
largely into the composition of the superficial tissue of the stems 
and leaves of grasses, as any one may demonstrate by using a 
blowpipe, which will fuse the silica into vitreous globules. In the 
mechanics of their structure, fistular culms exhibit much more 
specialization than the hollow iron pillars of man’s contrivance for 
