Modes of union and interaction between parasite and host in the Loranthaceae. II. Phoradendreae

Abstract

The large genus Phoradendron includes not only species like those of North America already described and Ph. fanshawei from Trinidad, with cortical strands and sinkers comparable especially with those of Viscum capense , but also others with very different modes of attachment. Some have much enlarged primary haustoria, with subordinate cortical strands and sinkers ( Phoradendron trinervium on some hosts) or without them ( Ph. hartii, Ph. flavens ). Ph. trinervium on Calophyllum lucidum , in contrast with its behaviour on other hosts, forms no major wedge but the cortical strands show an exaggerated development with anastomosis of the strands and the formation of numerous sinkers. In Phoradendron crassifolium and Ph. tetrapteron the haustorium simply spreads against the host wood to form a saddle-like attachment. In Ph. perrottetii also the haustorium spreads against the host wood, occasionally girdling it completely, forming downward branches, sometimes twenty or more, which run parallel and in their older parts fuse. Dendrophthora inaequidentata forms a very large primary haustorium, with slender cortical strands. The growth of the haustorium in width is accompanied by conspicuous hypertrophy of host wood on the lower side of it, the vessels and other elements of which abut on the haustorium, linking up with the parasite vascular system and without continuity with host wood above the haustorium. Tangential enlargement of the haustorium with stimulation of the adjoining cambium and annexation of the wood it forms occurs also though less conspicuously in some species of Phoradendron , particularly Phoradendron trinervium (on Rollinia mucosa ) and Ph. flavens .