Growth and anatomy of the vegetative body of the parasitic angiosperm Helosis cayennensis (Balanophoraceae)

The parasitic plant Helosis cayennensis (Swartz) Sprengel is composed of a sub-spherical tuber (diameter up to 6 cm) and slender runners (1-7 mm in diameter) which bear inflorescences. The tuber has no typical stem or root organization, no apical meristem or leaf primordia, no typical epidermis or stomata. The tuber consists of a parenchyma matrix and a network of vascular bundles, with sclereids especially abundant 1 to 2 mm below the tuber surface. At the interface between host root and parasite tuber, H. cayennensis parenchyma cells are large (mean diameter = 41.6 ± 6.5 μm [SD]) and cytoplasmic, with large nuclei (mean diameter = 15.2 ± 1.6 μm [SD]), which are larger than those of the host (29.1 ± 6.2 μm [SD] and 4.5 ± 0.8 μm [SD], respectively). Host and parasite vessel members can be distinguished by secondary wall ingrowths: host vessels have smooth secondary walls whereas H. cayennensis vessels have irregular secondary wall ingrowths. Vessel-vessel contact occurs at the host/parasite interface. We detected no parasite sieve plates at the host/parasite interface. Beyond the host/parasite interface, no host tissue can be seen in the parasite portion of the tuber. Vascular bundles branch in all directions in the tuber and undergo secondary growth. Primary xylem vessels have numerous wall ingrowths; secondary xylem vessels have only a few small peg-like ingrowths or have completely smooth walls. Sieve plates are simple, located at the end walls, and compound sieve areas occur on the side walls of the sieve tube members. There are many densely cytoplasmic cells, with large nuclei (mean diameter = 22.2 ± 3.5 μm [SD]), around or within young vascular bundles. Tubers grow by the proliferation of parenchyma cells, which occurs diffusely throughout the ground matrix. Tubers produce runners that are cylindrical and branch irregularly. Runner primordia, initiated either in tubers or in older runners, are rootlike but with no root cap and no endodermis. There is no volva or significant rupturing of the parental tuber or runner. The runner apical meristem consists of a tunica of 3-6 layers of tannin-filled cells surrounding a corpus of tannin-free cells. The vascular tissue of runners is a eustele of 5-7 bundles. The pith of the runner is composed of fiber-like sclereids, and medullary rays differentiate into a sclerenchyma bridge between the pith and the brachysclereid bundle caps. The highly reduced vegetative body, which is a mixture of various characters of d