Modes of union and interaction between parasite and host in the Loranthaceae - VI. A general survey of the Loranthoideae

Abstract

The available information on the structure of the attachments of epiphytic Loranthoideae to their hosts and their mode of formation is summarized, arranged according to the current classification. With few exceptions the haustorium penetrates to the host cambium; the subsequent behaviour shows variations which bear little relation to taxonomic grouping based on floral structure, some of them recurring in widely separated sections, while some genera exhibit a range of types. In the simplest type the haustorium establishes contact with the wood and thereafter host and parasite grow conformably, with some stimulation of cambial activity, forming together a haustorial wedge in a xylem socket, the outer tissues also showing mutual accommodation. The stimulation of the cambium varies in intensity and in range: when the gradient is steep the host wood spreads out till its contact surface is convex. Marginal proliferation of the cambium may also result, as in wound reactions, causing upgrowth of host wood around the haustorium and leading to elaborations, including typical ʻwood roses’, in which parasite tissue interlocks with lobes of host wood and rind. Outstanding modifications of the simple type include (1) the penetration of the haustorium under the cambium, forming a flange which is later overlaid with wood: (2) the maintenance by such flanges of contact with host phloem locally by the formation of radial shafts which continue growth in the cambial region; (3) the formation of a succession of flanges on alternate sides; (4) the partial or complete ringing of the host by the haustorium, interrupting the cambium, followed by reactions below and, when complete, by failure of the host distally. Among unique types are the longitudinal strands of Loranthus europaeus and the still longer strands of Tapinanthus capitatus ; also the hyphal clusters, actually penetrating the wood, formed by the haustoria of Struthanthus spp. and Phthirusa spp. in America and by the haustorial flanges of Ileostylus micranthus in New Zealand. The need is emphasized for investigation of terrestrial species of Gaiadendron and Helixanthera , including their seedlings and those of their epiphytic relatives.