the other posterior to the wood, the former constituting the fibres of the liber, between which and the wood all new layers of the latter are placed. The three or four succeeding layers of wood are accompanied in Myzodendron with bundles of pleurenchyma (Plate CVII. ter, f ,f), and Link figures occasional fibres similar to those of the liber situated within the wood of Viscum. In many other particulars the structure of the wood of this genus and Viscum is very similar, as in the form the alburnum assumes (f. 1, c) ; in the narrow tubes containing a spiral vessel that occurring at the inner margin of the layers, which they thus separate, forming in Viscum, as here, an obscure medullary sheath. The tissue of the wood itself, (which is much modified in the different species of this genus, as I shall hereafter shew,) in the present species consists of longer tubes, whose walls, though very much more delicate than those of Viscum, probably are similarly perforated.
In some respects this disposition of tissues may be compared, though not strictly, with that of Menispermacea, so beautifully illustrated by M. Descaisne.* After the stem of Cocculus laurifolius has attained a certain age, a second deposit of wedges of wood is formed externally to the liber, but without any additional liber. In Myzodendron the two wedges are deposited within one year of each other, the second within the first, and both are annually augmented by new matter ; in both wedges however, the deposit of pleurenchyma similar to that of the liber, which accompanies every layer for the three or four first years, is withheld from all future layers.
Wood of other species. The above-described structure of the wood is, with slight modifications, common to all the species of the group Eumyzodendron. Though I am quite unable to make so satisfactory dissections of that of M. punctulatum, from the very remarkable density and minuteness of its tissues, I still am inclined to consider that its greatest peculiarity, the occupation of the position of the medulla by pleurenelrynia, is a modification of what occurs in many Loranthacea, and is owing to the existence of a second or inner deposit of pleurenchyma similar to that of the liber, which in this species instead of being arranged in separate concentric bundles, is collected into one in the axis of the stem. Hence in the section of the stem of a leaf or flower-bud (Plate CVII. ter,f. 8.), this tissue is seen to be present instead of the pith, and the great density of the wood of the older stems may, in a measure, be due to the incompressible nature of this tissue, and to the rays (which cannot be called medullary) being also formed, not of cellular tissue, but of pleurenchyma deposited with the scalariform in very small quantities.
Of the other Eumyzodendrons there are three whose tissues I have examined, and added figures of all, except M. ollongifolium, which hardly differs from M. brachystacliyum.
In M. quadrifiorum, DO, (Plate CVII. ter, f. 6 and 7,) there is generally but one series of wedges of wood, the second being reduced to a single wedge, lying obliquely across the axis of the stem ; the vessels of the liber are exceedingly inconspicuous (/. 6, b) the cellular tissue, bark, and pith large and loose, and the tubes forming the wood very large and few in number ; the smaller tissue interposed between the layers (e) is conspicuous. In the old stems I cannot detect the inner layer of liber. The alburnum (c) has the appearance of compressed cellular tissue. I have not met with woody cells in this species.
In M. linearifolium, DO, there is but one series of woody plates (Plate CVII. ter, f. 3, 4 and 5). The cellular tissue is large and loose, and contains both in the bark and medullary rays, numerous conspicuous masses of woody cells (/. 3 and 4, a). Both series of vessels of pleurenchyma (/. b and y) are large. Those of the scala- riform tissue are of a greater diameter than in any of its congeners, cylindrical, constricted, (as in bothrenchyma) and septate at intervals equal to their breadth (/. 5. a) : the tubes of the medullary sheath and those interposed between the layers of wood are of very unequal diameter and constricted here and there (/. 5. b). Wood of Fagus. To Plate CVII. is added a sketch of the wood of Fagus Forsteri; — /. 11 represents a transverse slice from a branch five years old, the letters denote the same tissues as in /. 5, 6, 8, 9 and 10. From
- Decaisne, Memoire sur la famille des Lardizabale'es ; iii
Archives du Museum d'Hist. Nat. vol. i. t. 10.
