68 SCLERENCHYMA [CH. 
elements are always present. Contrary to the general 
assumption, there are frequent though minute transverse 
bundles joining the parallel veins. 
The rule is that one vascular bundle runs up each 
mid-rib or ridge, but exceptions occur—e.g. in Arundo 
several bundles run up the mid-rib, and in Azra cespitosa 
(Fig. 23) and others even the strong ribs may have two 
or three bundles. 
Each vascular bundle has its own sclerenchyma sheath, 
and very often the stronger veins are accentuated owing 
to the vascular bundle having a girder-like band of 
sclerenchyma running conjointly with its sheath and 
joining the latter above and below—or below only—to 
the epidermis (Figs. 24 and 25). In many cases these 
lower girders spread out laterally below—fan-shaped in 
section—and nearly jom the neighbouring girders. 
In other cases the strands of sclerenchymatous sup- 
porting tissue do not join the bundles, but run parallel to 
them, above or below, as separate strands just beneath the 
epidermis. 
Finally, these strands may separate from the bundles, 
and fuse below into a continuous layer under the epi- 
dermis; this occurs especially in leaves of xerophytes 
where the cuticle is well developed—e.g. in varieties of 
Festuca ovina (Fig. 18), Aira flecuosa (Fig. 28). 
The distribution of the strands of isolated sclerenchyma 
affords good characters. While there are none in Mibora, 
we find one large strand at the ridge of the keel and one 
at each margin, in addition to smaller ones subtending 
each vascular bundle, in Avena pubescens, Sesleria, Poa 
