210 
REVIEWS. 
histology of Welwitschia. Dr. Hooker regards the structure of the 
stem as presenting a modification, hitherto unique, of the exogenous 
plan. This modification, which gives it, prima facie , somewhat of 
a monocotyledonous character, consists in a large proportion of the 
descending bundles remaining closed, and thus isolated in the largely 
developed parenchymatous matrix of the stock. This isolation, 
together with much irregularity in the course of the bundles arises, 
no doubt, from the very peculiar conditions of the plant, the ‘ stock’ 
of which we regard with Dr. Hooker, as the enlarged tigellum of the 
embryo. 
As the Vegetable Kingdom does not afford us a parallel case 
of a tree thus reduced to its £ simplest expression,’ we cannot 
institute comparisons, nor even indicate how far, under like con¬ 
ditions, any other woody perennial might be similarly affected. 
Dr. Hooker illustrates his view of the probable mode of de¬ 
velopment of Welwitschia by a reference to the curious case of 
Streptocarpus , a biennial or annual, with one persisting cotyledon 
and a ‘ plumulary’ formation apparently analogous to that of the 
‘ crown’ of Welwitschia. But Streptocarpus is herbaceous, and the 
arrangement of its vascular cords has not been described. It is 
remarkable, however, that the best known cases of life-long persist¬ 
ing cotyledons should both be furnished by South African plants. 
We have alluded to the extraordinary development of the liber 
or bast element of the vascular cords. The cells of this liber exhibit 
a kind of marking, indicating some peculiarity in the deposition of 
their thickened walls, which we have not seen elsewhere. The 
markings are directly transverse and exceedingly close together. 
The structure of the vascular cord is most readily examined in the 
leaf, and a cross section, showing a few of these side by side in the 
median line of the leaf is a most beautiful microscopic object. In 
respect to their composition, Dr. Hooker notes in the leaf, that they 
are bounded both above and below by a liber-crescent. The liber on 
the lower side is apparently developed from a narrow belt of ‘ cam¬ 
bium’ cells on the same side of the vessels, which latter occupy 
nearly the centre of the bundle. 
Perhaps the rigid spicular cells, which occur scattered through 
the parenchyma of almost the entire plant, afford its most remark¬ 
able histological feature. They are usually very thick-walled, much 
larger than the surrounding cells in diameter, often branched or 
hooked, and their external surface is more' or less clothed with 
minute rhomboidal, or shortly prismatic crystals of undetermined 
composition. This occurrence of crystals on the outer walls of cells 
forming part of a tissue is extremely rare. But a single case has 
come to our knowledge, in a fragment of wood, of unknown origin, 
formerly in the possession of the late Prof. Quekett. We shall be 
curious to learn from the chemists what these crystals are com¬ 
posed of. # 
* Since the publication of Dr. Hooker’s Momoir, Col. Yorke has ascertained 
that these crystals consist of carbonate of lime. The circumstance that they are 
