40 DR. J. D. HOOKER ON WELWITSCHIA. 
the case, the secondary sac of Gnetwm is impregnated below the cone of the nucleus, and 
externally to the embryo-sac, it will complete the network of cross affinities thus afforded 
by the behaviour of the pollen-tube and embryo-sac respectively in plants which, in 
many other respects, differ very widely. 
Summary. — 
From the anatomy of the trunk of young and middle-aged specimens of Welwitschia, 
it appears to be the only perennial flowering-plant which at no period has other vegeta- 
tive organs than those proper to the embryo itself,—the main axis being represented by 
the radicle, which becomes a gigantic caulicle, and developes a root from its base and 
inflorescences from its plumulary end, and the leaves being the two cotyledons in a 
very highly developed and specialized condition. The consequence of the persistence 
of the cotyledons, and of their performing all the functions of leaves, is the arrangement 
of the principal vascular system in one horizontal stratum, which increases at its peri- 
phery, and which transmits vascular bundles upwards and outwards to the inflorescence, 
and downwards to the stock and root. | 
Owing to the centrifugal direction of growth in the stock and crown, above, below, 
and beyond the leaf-insertions, these enclose between them a deep slit or groove in 
which the bases of the leaves are lodged and protected from external injury through- 
out the life of the plant. 3 
There is no proper separable bark to the plant, which is invested by a hard periderm, 
formed by the indurated outer cellular system of the stock, crown, and root, and is absent 
only over the growing tissues at the circumference of the trunk, and within the groove 
between the crown and stock. A general cambium-layer underlies the periderm every- 
where, and there are special cambium-layers within the vascular bundles throughout the 
plant. 
The parenchyma is crowded with rigid spicular cells of great size, covered with 
crystals, and which, by their arrangement, form, and surface, give solidity to the 
delicate cellular tissue of which the main part of the plant is composed,—acting in a 
manner analogous to the siliceous spicula of sponges. The vascular system is refer- 
able to the exogenous plan; but its arrangement into concentric wood-wedges is very 
rude, and confined to the axis of the stock and root. In the root the wood-wedges are 
frequently disposed round two cellular axes, a plane passing between which is continuous 
with that passing between the leaf-bases. | 
The gum which flows freely from various parts of the plant is due to a collenchyma- 
tous swelling of the cell-walls of the parenchyma, and sometimes also of those of the 
spicular cells. 
Occasionally flower-buds are developed in the periphery of the stock, below the inser- 
tion of the leaves. This is an analogous case to the recorded ones of the formation of 
buds on the caulicle of species of Euphorbia and Linaria. 
The venation of the leaf is strictly parallel and free, like that 
general appearance ; but there is a tot 
between the bundles, 
of Monocotyledons in 
al absence of lateral vascular communications 
in which respect it more closely resembles the venation of many 
