OF THE LEAVES OF BRITISH GRASSES. 323 
The bundles of the second order possess the two large lateral vessels, but no inter- 
cellular space. 
The bundles of the ¿hird order possess neither intercellular nor lateral vessels. 
The bundles of the fourth order are similar to the preceding, but have a smaller 
amount of lignified tissue. 
The bundles of the fifth order possess no lignified elements. 
I have never seen a bundle of the fifth order. Moreover, the distinction between 
bundles of the third and fourth orders is so slight that it is deer, useless to 
continue to recognize it. - ; — en 
I have, therefore, only recognized m orders of bundles. Pl. 40. fig. 69 shows a 
bundle of the first order from the leaf of Festuca pratensis; fig. 70 one of the second 
order from the leaf of Elymus arenarius; fig. 71 one of the third order from the leaf of 
Agropyrum repens. In a leaf-section of Agropyrum junceum bundles of all three orders 
can easily be recognized with the aid of a pocket-lens. 
Nearly all grass-leaves possess vascular bundles of each of the three orders. The 
character is, therefore, not of importance in recognizing the leaves in transverse section, 
and I have not thought it necessary to indicate the different orders in the diagrams—no 
further, at least, than can be made out from their relative sizes. 
The bundles, whatever their size, always show both xylem and phloem elements. The 
proportion of fibrous elements varies very largely—in Molinia cerulea, for instance, they 
form the greater part of the bundle. 
When a leaf is ribbed, we nearly always get one vascular bundle in the centre of each 
rib In the enormous triangular ribs of Aira cespitosa (Pl. 37. fig. 20) we get one or 
more subsidiary bundles in addition to a large one in each rib. In Glyceria fluitans, 
again (Pl. 38. fig. 39), we get a number of low ribs on the upperside, but the vascular 
bundles are situated in the thinner parts of the leaf between the ribs, the centre of the 
rib being occupied by a large air-space. 
For leaves where the midrib projects strongly below, the central bundle of the leaf is 
often much larger than any of the others; this is the case in most species of Poa, in 
Arrhenatherum avenaceum, &c. Very often, however, this central bundle is no more 
conspieuous than the other large bundles of the leaf. 
Bundle-sheaths —In nearly all leaf-sections each bundle is seen to possess two or more 
conspicuous sheaths, each one cell thick, surrounding it. 
The inner sheath (Pl. 40. figs. 69-71, i.s.) is always more or less thick-walled and 
lignified. Pée-Laby states (on what evidence I do not know) that it represents the 
endodermis. It is usually more conspicuous in bundles of the first and second than in 
those of the third order, where, indeed, it sometimes can hardly be recognized (fig. 71). 
Raunkier notes that in the small bundles of many grasses the inner sheath is only 
partial, surrounding the phloem, but not the xylem. Generally, the inner walls of the 
cells are much more strongly thickened than the outer, the radial walls in part sharing 
the thickening (P1. 40. fig. 69); the thick inner wall is in these cases provided with 
numerous pits. Sometimes, however, the cell-wall is equally thickened all round 
(Pl. 40. fig. 70). In cases where the bundle is joined to the epidermis by a sclerenchyma- 
band, the inner sheath becomes continuous with the tissue of the girder (figs. 69-71). 
SECOND SERIES.— BOTANY, VOL. VI. 9A 
