DR. J. D. HOOKER ON WELWITSCHIA. 15 
the axis are always larger than those towards the circumference, the outermost of all 
being extremely small. The individual wedges are radially very much elongated, taper 
outwards, and are often waved, and split from the centre nearly to the tip by intruded 
parenchymatous wedges; they are, for the most part, formed of liber, which, in a trans- 
verse section of youngish specimens, presents a dense, somewhat horny, pale brown 
mass; this is succeeded, towards the axis of the root, by a very inconspicuous cambium- 
layer, and this by a bundle of slit-marked vessels. In old specimens the liber pre- 
ponderates so inordinately over the parenchymatous and other tissues, that, when riven 
open, the interior of the axis and root presents the appearance of tangled hemp-yarn, 
the masses of which, when torn out, are as thick as the little finger, and are matted 
together with dry parenchyma, full of yellow spicular cells and fragments of vessels. 
The whole forms a tissue so loose and tough, that an old root can neither be sawn nor 
eut clean aeross. 
In some specimens, the wood-wedges of the root are obviously arranged round two 
eentres (Plate XI. figs. 8 & 9), a vertical plane passing through both corresponding 
with the depression between the bases of the leaves. 
The elements of the vascular bundles above described are singularly uniform through- 
out all parts of the plant, and consist of:—First, a great proportion of filiform liber- 
cells ; these are very slender, terete, white, 1 to 2 or more inches long; their cavity is 
all but obliterated, and their surface marked with very fine transverse strize (Plates VII. 
figs. 11, 12; VIIL.fig. 25; XII. .ع8‎ 14: XIII. fig. 14); they sometimes, but very rarely, 
are unequal in diameter and branched (Plate XII. fig. 12). Second, slit-marked vessels, 
or pitted vessels, with thick walls (Plate XIII. figs. 10, 15; Plate XIV. figs. 14, 15): 
in some of these the whole interior is filled with secondary deposit, except opposite the 
slits or pits, giving rise to various modifications (Plate XIV. fig. 14); in others the pits 
seem united by a faint spiral line traversing the interior of the tube (Plate XIV. figs.14 
& 15). These pass into—Third, tubular vessels, with more or less thickened walls, in 
which the deposits are spirally arranged (Plate XII. fig. 4; Plate XIII. fig. 10). The 
peculiar disc-bearing tissue which abounds in all other gymnospermous plants seems to 
be totally absent in this. 
On a comparison of the stem of Welwitschia with that of other vascular plants, its 
development seems to be referable to the exogenous plan, of which it is à remarkable 
and hitherto unique modification. Except, indeed, in the fact of so many of the 
descending bundles of the stock and root remaining isolated and closed, and in those of 
the stock losing themselves in the periphery, I do not see any analogy with an endo- 
genous mode of development. The first of these characters is very common in many 
Orders of Exogens with anomalous wood; and the second is aecounted for by the exi- 
gencies of the growth of the stock, Nor should I have expected it to be otherwise; the 
embryo being dicotyledonous, and giving origin (it is to be assumed) to a primary root 
continuous with its radicle, lays the foundation, as it were, for an exogenous arrange- 
ment of the vascular tissues within it, cellular tissue in all cases preceding and regula- 
ting the time of appearance, amount, and disposition of the vascular, which is formed as 
