Page:The Botany of the Antarctic Voyage.djvu/330

298 Anatomy of the stem. A branch of this species, after attaining the age of two years and upwards, consists principally of a soft white cellular tissue, occupying the axis of the plant and communicating with the thick bark by means of broad medullary rays. The latter are separated by woody plates, disposed in two concentric series, and formed almost entirely of scalariform tissue with sometimes pleurenchyma. Cuticle. This is very stout in texture: in a first developed branch it consists of only one row of small cells (Plate CYII. bis, f. 11) these must be rapidly added to, for after another year the cuticle of the same branch is of much greater density and formed of many series of cells, much blended together, though not so completely as to assume the appearance of a homogeneous tissue without any trace of cellularity, which it afterwards attains (Plate CVII. fig. 4, 5, and 6). The cuticle is devoid of stomata commonly so called, but furnished with numerous longitudinal prominences, each marked by a fissure. A transverse section of one of these is given at Plate CV1I. f. 4, where the appearance is as of several layers of cuticle superimposed and forming the prominence, becoming cellular towards the centre, and depressed, pushing the subjacent epiphlceurn before it. There is no actual stoma or communication between the external atmosphere and tissue of the bark, further than what may be supposed to be afforded by cellular tissue, which is a rapid conductor of moisture. These are very evident in the branches of the second year, no doubt answer to stomata, whether performing the same functions or no, and are an instance either of the cuticle retaining its originally cellular organization at the point where they occur, or reverting to that structure.

Bark. This is composed almost entirely of a mass of cellular tissue, shrinking much when the stem is dry. The epiphlocum is formed of several rows of transversely elongated thick-walled cells, it occasionally contains air-cavities, but these are not so numerous or conspicuous as in M. punctulalnm. The vessels of the liber are disposed about half way between the cuticle and wood, are often very inconspicuous and formed of scattered bundles of fibres (Plate CVII. f. 5 and 6 a.) protected by very thick-walled cells, as in most, if not all, the Loranthacea, at other times they are in two series or variously disposed. This tissue does not appear to pass from one internode to another, but to be' interrupted at each articulation, as M. Decaisne found to be the case in Viscum The parenchyma between the vessels of the liber and wood is often dense, sometimes but rarely these vessels are seen to be immediately in contact with the wood as at Plate CVII. f. 5 and 6 b. Wood. Within the bark are arranged two concentric series of woody plates or wedges, these two series are separated by a zone of cellular substance, and are generally arranged with tolerable precision: besides these the pith of the plant is intruded upon by other wedges or bundles of vascular tissue, unsymmetrieally disposed, one of them often occupying the axis itself. Each wedge or plate is composed principally of concentric layers of very large vam scalariformia, becoming more densely packed and much smaller in diameter towards the axis of each layer, where they are almost invariably furnished with a spiral filament. Between the layers of the first three or five years there is generally deposited two bundles of pleurenchyma similar to that of the liber, one on each side (Plate CVII. ter, f. 1. f) but between the more recent layers there intervenes only the more delicate vascular tissue (f. 1 arid 2. e): as mentioned above, however, pleurenchyma is sometimes more copiously deposited between every layer, as at Plate CVII. f. 5 and 6, b. The narrow portion of each wedge invariably rests on a mass of pleurenchyma (Plate CVII. ter, f. 1. y,) deposited at the same time as the fibres of the liber c, that is during the first year, as in the common Misseltoe. The wedges of wood belonging to the second series are smaller than those of the first, but similarly formed in all respects, and consisting of as many layers, though the inner are very inconspicuous.

The pith consists of cellular tissue similar to that of the liber, and is very lax even in the older stems.

The transverse section of this stem, appears at first sight to differ very remarkably from that of most exogenous plants ; this arises from the wood being deposited in two concentric series, separated by a broad zone of parenchyma, from the great breadth of the medullary rays, the irregular distribution of the fibres of the liber