362 MISS HILDA M. CUNNINGTON ON THE 
The Vascular System of the Leaf. 
The development of the small bundles is easiest to trace, as the large bundles develop 
much earlier and rarely show very early stages. 
The abaxial cells of. the trabecule give rise to the bundles. In transverse section the 
following sequence of cell-division is shown in the formation of a bundle :—Each of the 
long row of cells (parallel with the length of the leaf), which is about to produce 
a bundle, divides by two walls at right angles to the epidermis and parallel to the 
length of the leaf, thus dividing each cell into three (Pl. 36. fig. 8, b). The central 
one of these then divides into two by a similar wall. These two inner cells (P1. 36. 
Fig. 9. 
Fig. 10. 
fig. 11, a, a) so formed divide by walls parallel to the epidermis, forming six cells 
(Pl. 36. fig. 12, 0); and these with the two original end-cells, by further division 1n 
various directions, form the primordia of the bundle (Pl. 36. fig. 13, p), which elongate, 
forming the xylem, sieve-tubes, and parenchyma. 
The bundles arise in every alternate trabecula ; thus there are twice as many air-chambers 
to be seen in transverse section as there are bundles. As the leaf grows in the basal 
region, and the midrib and marginal bundles become larger, the trabeculz below these 
divide and the cells separate, forming two extra air-chambers below them, so that there 
are six extra air-chambers in the leaf. 
Pigment-cells.—In the leaf there are three different kinds of pigment-cell in three 
different tissues :— 
(1) In the trabeculz the pigment-cells are of the same size and shape as the paren- 
chyma-celis of the trabeculze. 
(2) In the subepidermal parenchyma elongated pigment-cells like those of the cortex 
of the stem are present. 
(3) In the bundles very much elongated narrow tubular pigment-cells. 
In the intravaginal scales the pigment-cells are of the same size and shape as the other 
parenchyma-cells composing them. 
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