~ DR. J. D. HOOKER ON WELWITSCHIA. 13 
Fibro-vascular system.—This consists primarily of a thin, elongated or cup-shaped 
stratum, crossing the axis of the stock (to the surface of which it is parallel), just beneath 
the crown, and uniting the bases of the leaves. This gives origin to, secondly, an ascend- 
ing system of definite isolated vascular bundles, which traverse the crown, and termi- 
nate in its ridges; and, thirdly, to a descending system of bundles, which in the axis of 
the stock and root are collected into wedges, but elsewhere in the stock are for the 
most part definite and isolated, and lose themselves in its cambium-layer. 
Owing to the depression of the centre of the crown, or, more strictly speaking, to the 
upward growth of the lobes of the stock, the vascular stratum, which always keeps at a 
uniform distance from the periderm of the crown, is never horizontal, but becomes more 
and more cup-shaped as the plant grows older; the concavity varies, however, extremely 
in depth in different individuals (Plate XI. figs. 1, 5,6). The circumference of this 
stratum (being coincident with the bases of the leaves) is the focus of greatest vitality in 
the plant; from it the innumerable fibro-vascular bundles which traverse the leaf are 
given off, and through it that stimulus to growth which is set up by the leaves is trans- 
mitted (so to speak) to the living tissues throughout the crown, stock, and root. At 
the margin of this stratum the cambium-layers of both the stock and crown meet, and 
having formed the soft walls of the leaf-groove, are continuous with the parenchyma of 
the leaves. This, then, is the point of greatest vitality in the whole plant; and, as we 
have seen, the delicate and important tissues here concentrated are protected from ex- 
ternal influences by the tightly compressed lips of the groove, described at p. 9. 
The vascular stratum itself forms neither a very conspicuous nor well-defined system, 
though necessarily rendered evident in the sections figured in Plate XI.; but it is suf- 
ficiently distinguishable when once understood. The bundles that compose it do not 
arrange themselves in groups, nor unite into a compact mass, but are arranged side by side, 
forming a fibrous layer not more than 3-4 lines thick in the largest specimens (Plate XI. 
fig. 2). They contain more vessels and comparatively less liber than the bundles in the 
root. In young and middle-aged specimens they run with great regularity; in old ones 
they sometimes branch, and present many anomalies in their course and arrangement. 
This fibro-vascular stratum consists of two very ill-defined parts, in which the bundles 
are somewhat differently arranged, viz., a central and a circumferential portion. Of 
these the central is much the smallest ; it occupies in the largest specimen a circular 
area of about 2 inches diameter, and consists of a matted fibro-vascular plexus, in which 
bundles of liber, barred vessels, spirals, &c., are all confusedly mixed. In a vertical 
section of the stock through the base of both leaves (é. e. at right angles to the lobes) 
(Plate XI. fig. 1), this central system is very manifest, though merging into the better- 
defined circumferential portion; and in a vertical section taken in a line exactly parallel 
to the lobes, and between them (fig. 5), it is hardly apparent. The circumferential por- 
tion consists of innumerable, very closely set, laterally compressed bundles, radiating out- 
wards from the central portion to the entire semicircular base of each leaf. The struc- 
ture of this portion is best seen in a vertical section through one of the lobes, halfway 
between the leaf-base and medial depression of the crown, and parallel to its long axis 
(Plate XI. figs. 2 & 6), which section intersects transversely all the bundles. 
