Vegetative body of the parasitic angiosperm Ombrophytum Subterraneum (Balanophoraceae)

The vegetative body of Ombrophytum subterraneum is an irregularly spherical structure, called a tuber, consisting of two portions: one composed of host cells, the other of O. subterraneum cells. All O. subterraneum nuclei are much larger than those of the host, and the vessels have distinctive secondary wall ingrowths, thus each tuber cell can be identified as belonging to either host or parasite. Infection by O. subterraneum causes host root cells to proliferate outward, forming a structure that initially resembles a lateral root, having a central mass of xylem surrounded by phloem, but differing from the host's normal root tissues in having narrow vessels and few or no fibers. The O. subterraneum portion of the tuber consists of a mass (up to 933 grams fr. wt.) of starch-filled parenchyma penetrated by vascular bundles containing both xylem and phloem. All cells in the O. subterraneum portion of the tuber are O. subterraneum cells-host cells were not mixed with parasite cells and there are no chimeral meristems as reported for Balanophora. The interface between host and parasite is sinuous but generally smooth, not a mixture of host and parasite cells. The tuber has no apical meristem; no leaves or axillary buds; no eustele; no epidermis. It produces long (up to 20 cm), slender (4 mm diam) runners that have an apical meristem with no root cap; no leaf primordia; a protoderm and epidermis but no stomata, no trichomes and no root hairs. In some places the runners' stele is rootlike (a protostele), in other places it is stemlike (with pith and collateral bundles).