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 Aira ccespitosa 

 (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 join 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 flexuosa (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 pvhescens, Sesleria, Poa. 



