MORPHOLOGY AND ANATOMY OF THE HAUSTORIUM OF THE ROOT HEMIPARASITE OLAX-PHYLLANTHI (OLACACEAE), WITH SPECIAL REFERENCE TO THE HAUSTORIAL INTERFACE

Gross morphology and internal structure of haustoria of Olax phyllanthi are described in parasitism with a range of hosts, including roots of woody and herbaceous dicotyledons and certain monocotyledons, and occasional instances of autoparasitism and haustorial formation on monocotyledon rhizomes. Successful penetration to xylem occurs on virtually all hosts across broad diameters, ages and anatomies of host root, but anatomical impediments to haustorial establishment and penetration are recorded for certain host taxa. Each haustorium is a comparatively simple and ephemeral structure. Its developing sucker (endophytic region of the haustorium) spreads laterally around the surface of the host xylem, yet never completely encircles the host stele. Damage to hosts is minimal and secondary thickening (of hosts) continues on the side of a host root opposite to a haustorium. The haustorial sucker lacks phloem and its interface with host xylem is comprised almost entirely (more than 98.7%) of parenchyma. The few terminating tracheids at an interface lie in very close proximity to or occasionally directly against exposed xylem vessels, but lumen to lumen continuity between tracheary elements of the partners is not achieved. Three dimensional reconstructions based on serial transverse sectioning indicate that well defined files of tracheids connect back from an interface to the core of graniferous tracheary elements in the external body of the haustorium, and thence to the xylem of the parent parasite root. The findings are discussed in relation to existing studies on haustorial anatomy. © 1990 Annals of Botany Company.