DR. J. D. HOOKER ON WELWITSCHIA. 11 
crown,—the external portion sometimes breaking up on the stock into distant angular 
areol», as seen in Plates 111. & IV. There is no lamination in the substance of the peri- 
derm, nor any trace of periodic growth. The epidermal cells, as long as retained, have 
thicker walls than the subjacent ones; the outer of all (Plate XII. fig. 3) are very large, 
radially elongated, and their outer walls are excessively thick, hard, and dark brown, 
almost as if charred by fire. The subjacent tissue (Plate XII. fig. 2) is a transversely 
elongated parenchyma, full of peculiar rigid spicular cells, hereafter to be described. 
This periderm is not an independent growth, but is the outer portion of the paren- 
chyma of which the mass of the plant consists, and which, having lost its vitality, forms 
a. durable integument, in some parts of almost stony hardness, to the tender tissues 
beneath. In the root, but probably only in a dried state, this periderm is often separable 
from the subjacent tissues; this is in part owing to the vascular bundles which traverse 
the root running parallel to the periderm, not more or less transversely to it, as in the 
stock and crown. 
Within the groove that holds the base of the leaf, the periderm is replaced by an 
exceedingly soft and tender tissue, which in Mr. Monteiro's specimens was still fresh and 
living. It consisted of a bright yellow-green parenchyma, covered with a delicate mem- 
branous transparent glistening layer of epidermal cells, with walls of extreme tenuity, 
without stomata, and without any appreciable thickening of the outer cell-wall, The 
cells were loosely filled with a watery grumous endochrome. 
Parenchyma.—A. cambium-layer * of soft parenchyma occurs beneath the periderm, 
investing nearly the whole trunk of Welwitschia: it is in all respects identical in struc- 
ture with the subjacent parenchyma; but the cell-walls are more delicate, abounding 
also in the before-mentioned rigid spicular cells. This layer is best developed on the 
periphery of the crown, where the flower-buds originate, and it also forms the uppermost 
(outermost) of the ridges of the stock just below the leaf-base, whence it passes inwards 
surrounding the groove. It is least developed towards the depression in the long axis of 
the crown, and is perhaps absent under the periderm in its disc; but it otherwise invests 
the whole trunk with a layer of tissue, endowed with more or less vitality. 
Beneath this cambium-layer a cellular tissue, of the ordinary hexagonal form in section, 
forms the chief substance of the crown, stock, and root. It is most developed in young 
and middle-sized specimens, and becomes gradually more intruded upon by the fibro- 
vascular tissues. In the freshest of Mr. Monteiro's specimens, this parenchyma was 
quite soft, and of a pale straw-colour, due chiefly to the abundance of large yellow spi- 
cular cells; it had a faint odour, something like that of fresh-boiled meat t, and contained, 
scattered through its substance, small cavities full of transparent gum. The cells of this 
tissue (Plate XII. fig. 1) are thin-walled; they have no markings, and are apparently not 
nucleated; their contents are insignificant, and present nothing worthy of notice. 
The cellular and vascular tissues, though soft, are so interlaced, and spicular cells 
* I apply this term here to the stratum of thin-walled cells, capable of multiplication by division, such as is to be 
found in Dracena, and which is wholly distinet from the closed cambium of the vascular bundles. It is a persistent 
layer of the “meristem ” of Nägeli (Beitr. z. wiss. Bot. i. 3). 
+ This I have not perceived in other equally fresh specimens; and it may be attributable to decay. 
c2 
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