which are sometimes biserial, and the disproportionate amount of scalariform tissue. The structure of M. punctulatum is however far more abnormal, fibres of pleurenchyma being deposited in the axis of the stem, thus replacing the pith, and forming very obsolete rays, and all future increment of the stem being effected by an addition of layers of variously marked scalariform tissue alone, as far as I have been able to observe.
Formation of wood. I shall next describe the course the vascular tissue pursues in the newly formed buds and branches, and thus attempt to explain the origin of the two series of woody plates which this species and M. quadrifiorum DC. possess.
A transverse section of the stem of a flower- or leaf-bud made in the first year of its formation, (Plate CVII. bis, f. 10 and 11), presents a mass of globular utricles, covered with a delicate cuticle (a) formed of one moniliform row of cells, and traversed by one series of twenty or thirty vascular bundles (b). These bundles descend from the base of each leaf, traverse the branch and enter the stem. A transverse section of the stem again from which the bud or branch is given off, and below the point of attachment of the latter, presents two concentric series of vascular bundles (CVII. bis, f. 12. b, c), besides an imperfect third consisting of a few scattered promiscuously in the axis of the stem ; the outer series was formed in the former, the inner is derived from the buds and branches of the present year.
A longitudinal section through the axis of the stem, so made as to pass also through the axis of the branch, clearly shews that it is due to the position in which the buds are developed that a second series of wedges of wood is deposited. The buds originate towards the axis of the stem, within the vascular bundles of the previous year, (Plate CVII. bis, f. 10. b), and opposite the insertion of the petiole (f). The whole of the vascular tissue descending from a bud is consequently deposited within the wood of the former year (f. 9 e.) generally each bundle on entering the stem from the branch divides, one portion joining the old wood, the other, remaining free and descending the stem, forms the second or inner plate of wood. The course of the bundles is however very uncertain, sometimes they do not divide, but either join the old vascular tissue, or continue free, and at others one portion crosses to the opposite side of the stem. Figures 9 and 10 of Plate CVII. bis, shew various modifications of the course these vessels pursue, the uniform result being, that in the internode of the second year all the wedges of wood are formed, though these become lower in the stem multiplied by division.
As each bud gives off thirty to forty bundles of vessels, and these being superadded to those of the branch, such a plexus arises at the contracted junction of the second year's branch and that of the third year that their course can no longer be followed. Each of the woody plates however, continues to receive accessions throughout the life of the plant, those of the inner series containing as many layers as those of the outer. It is hence evident that the bundles first arranged in the branch of the second year (f. 10 A), on entering that of the third year (f. 10, B), must present a very complicated arrangement of tissues. The increase of the stems in diameter being, however, effected throughout the length of the plant by an addition of matter to the outside of both concentric series of wedges, it follows that the growth is in one sense at the same time Exogenous and Endogenous. However complicated the nature and disposition of these tissues may cause the developement of the stems to appear, the order in which each wedge of wood and its layers of pleurenchyma are deposited in the first year is tin-same as in Viscum; nor are the tissues themselves very different from those of that plant. This is seen by comparing the figure of a first year's branch of Viscum as given in M. Decaisne's elaborate essay[1] pl. iii. f. 4, or in Link's superb 'Icones'[2] pars iv. t. 8. f. 1, with that of .1/. brachystachyum, (Plate CVII. bis, f. 13).
In both M. brachystachyum and Viscum album two bundles of pleurenchyma are first deposited, one anterior and
