Page:The Botany of the Antarctic Voyage.djvu/334

302 external to it being more or less accessory. The above may be considered the first stage of developement, when the pericarp and albumen have fallen away; the outer coat of the radicle is expanded into a horizontal membrane, firmly attached to the cuticle of the Faff us, and the cushion-shaped body is pressed against the stem or twig of the tree, at that point where the process of penetrating the integuments will begin.

The second period commences by the cushion destroying the cuticle beneath it and reaching the epiphlceum. Still pushing onwards, it raises the membranous expansion and causes the surrounding cuticle, with or without some of the subjacent tissue, to be also detached with this membrane, for a considerable distance round the base of the radicle. The lesion to the plant is now extensive, and a diseased action, favourable to the progress of the parasite is induced, for the whole bark opposite the Myzodendron is raised from the wood (Plate CVI. /. 8). The cushion, which may now be called a true root, is protruded rapidly, and the disorganization of the tissues it meets in its progress is equally rapid, a cavity is formed in the bark and the edges of the fleshy sheath that surrounds the root commence generally to form a close adhesion with the lips of the wound, (Plate CVI./. 10 and 11). At last the root, having penetrated the bark, arrives at the space left between the latter and the wood (Plate CYI. /. 9), and, from the perpendicular direction, assumes a horizontal one. Previous to this there has been a deposition of much disorganized cellular tissue between the bark and the wood. The spiral vessels that descend from the plumule have followed the course of the root, which has gained the point where an abundance of nutritive matter is retained by the disorganized cellular tissue, and where a further supply is ensured for the use of the now fully established Myzodendron.

If at this time, the end of the second period, a vertical section of the Myzodendron and Fagus together be made, the appearance will be that represented at Plate CVI./. 10, where the plunmle is seen fully developed into two opposite leaves, ready for protrusion, and where also a large space is left that almost isolates the axis of the caulicule from what remains as an investing membrane. The true root, or cushion-shaped body, at no time appears to form an adhesion with the bark; that being effected by the thick and fleshy sheath through which the radicle is protruded.

The union between the sheath and bark is more or less complete in different specimens that I have examined, according to their stage of growth. At Plate CVI. /. 9, though the radicle has pierced the bark, the sheath had hardly united with the edges of the wound, and the young Myzodendron was removed without force from the Faff us; at /. 10 the union is complete, probably from the stem of the Beech being younger, and affording less resistance to the attack, and lastly, / 11 represents a still more close union, the circumference of the sheath passing along the inner walls of the bark and its revolute edges, thus completely enveloping all the diseased part.

The third period is marked by the exsertiou of the plumule from a dehiscence of the two coherent cotyledons, the consequent elongation of the ascending axis or stem of the Myzodendron, and the deposition of the liber and scalariform tissues in their proper positions ; a process I have never had the opportunity of following.

A branch attacked by the Myzodendron suffers no apparent change below their point of union, but all beyond it being insufficiently nourished, does not increase in proportion, and after a time dies from atrophy. The whole juices of the branch being suddenly arrested at its abrupt termination, this dilates into the form of a cup, which, from the turning of the Myzudendroti towards the light assumes an erect position. This cup (Plate CVI./. 12) is more or less broad (sometimes three inches in diameter) and shallow, uniformly covered with cuticle and bark, and has inflexed, wrinkled edges. On a vertical section (Plate CVII. / 1) it is seen to present a shallow, conical hollow, in which the Myzodendron rests, and to be formed of as many layers of wood as years have elapsed since the germination of the parasite. The equally expanded base of the latter completely fills this cup in the form of a disk, whose edges overlap those of its support, and swell out into a prominent ridge at their point of union. At Plate CVII. /. 1, a vertical section of the cup of the Beech and still attached disciform base of the Myzodendron is given. The union of the cellular tissues of the bark of the two is very intimate, but in no case and no stage of their growth have I been enabled to trace any interlacing of their tissues, or any union of the one with the other.