Modes of union and interaction between parasite and host in the Loranthaceae - I. Viscoideae, not including Phoradendreae

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Abstract
The structure and development of the haustorial systems found in Old World genera of the Viscoideae are surveyed, with observations on species of Viscum and Korthalsella not previously investigated. The implications of the differences observed are discussed. In the genus Viscum a common general plan is discernible, with variations ranging from V. minimum, with abortive plumule and numerous adventitious shoots, to V. articulatum, in which only the primary graft has been observed and no cortical strands. Arceuthobium resembles Viscum minimum, differing in the filamentous structure of the young strands. In Korthalsella the haustorium is of a different type; on reaching the host wood it spreads in all directions, the meristematic margin separating still functional cambium from the wood. In Viscum, active penetration, whether of primary haustorium or cortical strand, is in one direction and is limited to a growing point or short margin, which ceases its separating activity on reaching host wood or is diverted; the part surrounded by host cambium grows in conformity with it, becoming embedded, so forming a graft-like union, either primary attachment or sinker. In Korthalsella, parts of the haustorium sooner or later become overlaid by host wood, the separated cambium resuming its centripetal activity.

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