Modes of union and interaction between parasite and host in the Loranthaceae - IV. Viscum obscurum on Euphorbia polygona

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Abstract
A unique specimen of Viscum obscurum on the succulent Euphorbia polygona is described and its structure compared with that of specimens of similar size on two woody hosts, Brachylaena elliptica and Sideroxylon inerme. On Euphorbia, the primary haustorium was embedded in host wood, but it was ribbed and had produced narrow branches at various points. The structure can be related to the normal types on woody hosts as the response of the parasite to initially separate bundles, by production of branches growing along them inwards. The cambial activity of the host was greatly stimulated, groups of bundles united to form steles, these merged more or less as they increased in diameter. At the margin of contact in the cambial region, protrusions formed the ribs, more or less embedded in host xylem. Some of these extended inwards along the host cambium; a few were separated from the main cortex as lateral branches of the haustorium, with separate cortical tips and cambium. Against the haustorium and its branches internal patches of periderm were frequent and the extent of periderm at the surface was greatly increased. Comparison of the different specimens and of a range of seedling forms leads to an emphasis of the dorsiventral character of the haustorial system and an attempt to distinguish the reactions expressed in its structure.
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