The Haustorial System ofViscum album

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Abstract
The strands of Viscum album in the rind of the host are bilateral, even at the apex. Most commonly the tip is triangular in section, with the broad side outwards; some are broader and more massive, others more slender and less clearly dorsiventral. The forwardly directed outer and lateral marginal cells grow out into elongated hyphal or club-shaped cells with swollen laminated walls. In advance of them a space opens in the host tissues, filled with mucilage probably produced in part by the Viscum but with host cell-contents lying in it. A small-celled meristem, naked, lies on the inner side, towards the host wood. Between this and the hyphal fringe is a transitional region where the cells are larger and more elongated in a direction obliquely forwards and inwards. A procambium strand near the inner side can be traced nearly to the small-celled tissue. The whole apex can be interpreted as a terminal meristem, the different parts of which respond to a gradient of conditions, from the host cambium, in contact with which a sinker is eventually initiated, to the cortex, where growth is aggressive and the digestive hyphal cells are formed: a few of these may become separated, but many are incorporated in the outer tissues of the strand. On germination of the ‘seed’, the tip of the hypocotyl responds to contact by unequal tangential growth of the sides, to flatten the convex meristematic face against the host and to produce a dome-shaped holdfast. Copious secretion of a viscous sealing fluid, giving the reactions of cutin, follows near the margin of the holdfast and the epidermal cells of the face grow out into papillae which adhere to the host surface. Further growth of the walls of the holdfast dome, the growth of new papillae, thrusting outwards underneath, and the collapse of a subcortical zone of cells, which may have a contractile effect as in the contractile roots of Oxalis spp., result in the progressive lifting and disintegration of the host periderm till a slit is opened by which the primary haustorium penetrates. The latter has the form of a narrow oval wedge; in it a median plate of xylem is differentiated. In the host its behaviour depends on its orientation and on the conditions it meets. In bottle cork it remains a simple wedge. Its edge is slightly papillate and probably secretes a pectic enzyme. It divides to pass strands of fibres in its inward passage to the host cambium, and it forms the cortical strands as lateral outgrowths, running chiefly parallel to the axis of the host branch. Xylem is initiated as a median plate or, in narrower branches, as a central strand. For the formation of the holdfast, close contact and firm pressure with exclusion of air are required. In response to yielding pressure the face of the hypocotyl tip bulges and although papillae arise a holdfast is not formed. In the absence of a contact stimulus, as in air, on agar, or on a slippery glass surface, the face grows out and no papillae are formed. It is clear that in these haustorial organs structure and behaviour are closely interconnected and highly specialized in relation to the functions of the system.

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