66 
Rev. T. A. Jefferies. 
and they are peculiar also in that they stand beneath rows of motor 
cells, thus occupying hollows, like the bundles in the leaves of 
Glycerin fluitans and Dactylis glomerata, instead of the usual 
position under the ridges ; in this position they strengthen the leaf 
at the point where it is weakened by the development of air spaces. 
Fig. 7. Transverse section of a main lateral bundle of the leaf. Letter¬ 
ing :—A., intercellular air-space; G., stereome girder; I.S., inner sheath; 
O.S., outer sheath ; P., plate-cell ; V., large lateral vessel, x 400. 
All the vascular bundles of the leaf have both an outer and 
an inner sheath (Fig. 7): the outer is composed of large cells, very 
large in the middle of the section, with walls thickened especially 
on the inner and lateral sides; the inner sheath is thickened and 
lignified throughout, and becomes continuous with the stereome 
of the girders, into which the outer sheath also merges. Pee-Laby 
distinguished five orders of bundles in grass leaves, which Brain (3) 
reduces to three, namely (1) bundles containing two large lateral 
vessels and a prominent intercellular air space, (2) bundles with 
large lateral vessels but no intercellular space, (3) bundles with 
neither lateral vessels nor intercellular space. In Molinia the 
midrib and main lateral bundles are of the first order, the secondary 
lateral bundles near the midrib of the second order, while the more 
remote secondary lateral bundles and the supporting bundles are 
of the third order. In all the bundles the phloem is surrounded 
