Ultrastructure of differentiating graniferous tracheary elements in the haustorium ofExocarpus bidwillii (Santalaceae)

Abstract

The haustorium of the root hemi-parasiteExocarpus bidwillii has tracheary elements that contain protein granules suspended within the lumen of the cell. The differentiation of these graniferous tracheary elements has been studied by transmission electron microscopy based mainly on tracheary elements formed during secondary growth of the haustorium. The vascular cambium of the haustorium is unusual in differentiating tracheary elements and some parenchyma centripetally and a few parenchyma cells centrifugally but no phloem. The cambial initials contain the usual complement of organelles and in the active state vacuoles are small and the groundplasm of the cell is rather opaque. Differentiating tracheary elements are distinguished from developing parenchyma by the early appearance of granules within the cytoplasm and the presence of small vacuoles and only a few lipid bodies. The granules arise amid local masses of endoplasmic reticulum (ER) and are initiated as small swellings of the cisternae in which the matrix material of the granule accumulates. Continuity between the membrane sac of the granule and ER is often maintained as small tubular connections. By the stage the cell is fully expanded the granules are well developed and during the subsequent phase of secondary wall formation they undergo only a small amount of growth. The secondary wall is initiated on the primary wall as low ridges that soon expand circumferentially into the stalked bands of the mature cell. Lignification begins early and spreads progressively centrifugally throughout the band. Microtubules are closely associated with the developing bands and dictyosomes are usually also common in the vicinity. Once the secondary wall thickenings are developed the cell enters a phase of senescence and the components of the protoplast, with the exception of the granules, become smaller and eventually disappear. Disintegration of cell contents occurs rapidly on disappearance of the tonoplast and the release of the contents of the vacuole into the cytoplasm. The granules remain unchanged throughout senescence and on death of the cell they persist as naked structures in the lumen.