DR. J. D. HOOKER ON WELWITSCHIA. 19 
(figs. 1 & 2 e) having cell-walls without markings, and almost empty cavities, with 
a few spicula, and occasionally groups of cells transformed into amorphous gum 
(collenchyma). 
The strong fibro-vascular bundles of the leaf traverse this tissue parallel to one 
another, and continuously throughout the whole length of the leaf in its medial plane. 
The vascular bundles (fig. 2 f & fig. 3) are of the same character as those of the stock, 
but more symmetrically developed: in a transverse section they are oval or cuneate, 
with the broad end upwards. They are surrounded by a single layer of thick-walled 
pitted cells (fig. 3 a), and consist of, superiorly, a large crescent-shaped bundle of liber- 
cells (fig. 32), which, like those in the stock and root, have thick walls, narrow or no 
cavities, and delicate transverse striæ ; beneath this is a thick crescent-shaped cambium- 
layer (fig. 3 c), marked in a transverse section with radiating lines (fig. 1), due to an 
obscurely radiating arrangement of the cells; beneath this, again, is a bundle of slender 
vessels, of which the uppermost (fig. 3d) are thick-walled, with slit-shaped markings, 
the next (fig. 3e) thinner-walled, with close-set spirals, gradually passing into loose 
spirals. Another crescentic layer of liber-cells, like that above the cambium, but with 
its concavity upwards, closes the vascular bundle below. This second liber-bundle is, no 
doubt, developed from a second narrower belt of cambium-cells, on the lower side of the 
vessels. 
The stomata (Plate XIV. figs. 5—13) are situated in parallel lines on both surfaces of 
the leaf, and occupy the strize, thus overlying the soft cellular tissue (figs. 1 & 20). "They 
do not differ in any essential point from the usual form of these organs; their guard- 
cells lie between the bases of epidermal cells which leave a funnel-shaped depression 
between their contiguous walls. The guard-cells have much-thickened walls, continuous 
with the equally thickened outer walls of the epidermal cells, and, in a transverse 
section (figs. 11 & 12), present interruptions of continuity which are not easily ex- 
plained, except on the supposition that their walls are either perforated or much thinner 
at those points. 
There is much in the permanent character, parallel nervation, and anatomy of the leaf 
of Welwitschia that recalls the foliage of Dammara, Podocarpus, and many parallel- 
nerved Cycadee, and especially of those South African species of the latter Order in which 
a layer of liber-cells underlies the epidermis. After a cursory examination of the leaf- 
structure in many species of these families, I find none of which the anatomy, in point 
of complexity or beauty, approaches that of Welwitschia; nor do I know of any plant 
that is in this respect to be compared with it. It is further to be observed, that in all 
these plants the vascular bundles of the leaf, unlike those of most Monocotyledons, 
nowhere anastomose nor communicate by lateral branches; whence each nerve represents a 
single independent vascular axis, extending from the base to the apex of the leaf, or, in 
Welwitschia, from the axis of the trunk to the apex of the leaf. Such leaves resemble 
more closely a series of parallel uninerved leaves united by cellular tissue, than a foliar 
expansion of parenchyma traversed by one system of inosculating vessels; and the 
frequent presence of many linear cotyledons in these plants seems to favour this view, 
D2 
TS a STE TASA TOP Vom ar‏ 2 وو عون SSS‏ ريده 
a ae 
