SKÄI MR. L. LEWTON-BRAIN ON THE ANATOMY 
The function of this thick-walled sheath is probably, as suggested by Raunkiær, that 
of mechanical protection to the bundle, and more particularly to the soft-walled phloem, 
Great stress is laid by Pée-Laby upon the outer sheath to the vascular bundles. This 
is thin-walled, and in transverse section differs from the ordinary parenchyma of the 
leaf only in the arrangement of its cells around the bundle, and sometimes in their 
smaller size (Pl. 40. fig. 70, 0.s.). In longitudinal section it is easily distinguishable, the 
cells being much elongated in the direction of the long axis of the leaf. 
Like the inner sheath, this thin-walled sheath is usually more easily recognizable in 
bundles of the first and second than in those of the third order (compare fig. 70 with 
fig. 71). This is due, in part, to the fact that in these large bundles the cells lose their 
chlorophyll and become colourless, while in the small bundles they are usually green ; 
sometimes also round these small bundles the cells of the outer sheath are less regularly 
arranged, and are thus hardly distinguishable from the surrounding green parenchyma 
in transverse section (fig. 71). Sometimes, again, the outer sheath is quite conspicuous 
round the small bundles—this is particularly the case in Setaria viridis, for example 
(fig. 65). 
From its position and the arrangement of its cells it would appear probable that this 
outer sheath functions as transfusion-tissue, placing the bundles in more intimate 
connection with the assimilating green parenchyma. 
Mechanical Tissue.—The amount of mechanical tissue present in the leaves of grasses 
varies within very wide limits. In the small leaves of Mibora verna (Pl. 36. fig. 1), &e., 
it is practically absent; while in the leaves of the maritime sand-grasses, for instance, it 
is present in greater amount than any other tissue (Pl. 36. fig. 12 of Psamma arenaria, 
and Pl. 36. fig. 9 of the tip of the leaf of Elymus arenarius). 
The form of the elements of the mechanical tissue is that of De Bary’s * sclerenchyma- 
fibres—that is, much elongated, thick-walled, empty cells with pointed ends (Psamma 
arenaria). 
The stereome is arranged in bands running longitudinally the whole length of the 
leaf. The most usual positions for these bands is just within the epidermis above and 
below the vascular bundles (Pl. 39. fig. 41 &e., and Pl. 40. fig. 62, s/.) and always at 
the margin of the leaf (Pl. 38. figs. 33, 34, &c., and Pl. 40. fig. 68, s/.). Very frequently 
the bands above and below the vascular bundles extend right to the bundles piercing the 
outer sheath; they become continuous with the inner thick-walled sheath, and thus form 
girders to the bundles, joining these to the lower or upper epidermis (Pl. 36. fig. 14, 
Pl. 38. fig. 34, and Pl. 40. fig. 70, st.). Another common feature is for the stereome on 
the lower side of the leaf to be stronger than that on the upper; thus we often find 
bundles girded only to the lower epidermis, and with only a small band of stereome 
above. In many grasses, on the other hand, the stereome is about equal on the two 
sides, or it may even be slightly stronger on the upper (Pl. 38. fig. 37; Pl. 39. fig. 45). 
The tendency for the mechanical tissue to be stronger on the lower than on the upper 
side is much accentuated in some maritime and heath-grasses—for example, Psamma 
arenaria (Pl. 36. fig. 12) and Festuca ovina (Pl. 36. fig. 3), —the reason for which we shall 
consider later, 
* De Bary (1884), pp. 128 &c. 
