Modes of union and interaction between parasite and host in the Loranthaceae. VII. Some Australian Loranthoideae with exceptional features

Abstract

Specimens received from Australia during 1961 and 1962 included types of attachment additional to those described in part VI (Thoday 1961). In particular Diplatia maideni from the Northern Territory, on species of Acacia , had formed longitudinally running haustorial strands in the outer phloem of the host. From these parallel strands, slender branches penetrated radially, with further ramifications, reaching the host cambium in many places and forming ‘sinkers’ grafted into the wood. On some hosts the parasite had formed strands upwards as well as downwards. Another species of Diplatia , D. grandibractea , had formed on Eucalyptus microtheca a large attachment bearing a group of shoots. The parasite extended downwards as a haustorial strand longitudinally against the host wood. Between the dark surface tissues of the parasite and the smooth pale bark of the Eucalyptus was a zone marked by transverse fissures and ridges where host and parasite tissues were interlocked, the parasite having formed marginal tongues growing out actively into the host, with some response on the part of the host. In the strands of D. maideni , groups of stone cells and xylem are differentiated and the prominent xylem strands increase radially by an external meristematic layer. Parenchy­matous rays are formed in the process and in them stone-cell masses are differentiated, supplementing the larger masses formed in the rest of the strands. These strands together form a tangential zone in the outer part of the host, which with the xylem becomes increasingly prominent, so that the surface becomes ribbed and in parts fissured by the resulting tension. Some additional examples of longitudinal strands against the host wood were also included among the specimens, as well as of tangential flange formation. Specimens of Amyema sp. increased the range of attachment structure in that genus. A. pendula had longitudinal strands against the host wood. A. gibberula in particular was noteworthy as comparable with Diplatia maideni , but with coarser strands in Grevillea striata . Longitudinal strands against the wood were also found in A. congener on Alphitonia excelsa , Acacia cunninghamii , Acacia fasciculifera , Casuarina littoralis and Geijera parviflora , but not on Acacia melanoxylon or Flindersia australis ; also, less regularly, in A. lucasii on Flindersia sp. and sometimes in A. quandang , on Acacia harpophylla and Acacia kempeana . Other specimens of A. pendula had formed them on Eucalyptus melliodora , E. propulnea , hemiphloia and haemastoma , but not on E. Stuartiana or Acacia dealbata .