Transfer cells in the vascular system of stems: Taxonomy, association with nodes, and structure

Abstract

In a further search for transfer cells the vascular tissues of nodes and adjacent regions of internodes of a wide variety of plants have been examined by light microscopy of serial sections, by electron microscopy of ultra-thin sections, and by high-voltage electron microscopy. The survey included 190 species and the types of node included those subtending scale leaves, floral bracts and bracteoles, as well as true leaves and cotyledons. Transfer cells occur as modified vascular parenchyma in all of these types of node, and more than half of the species examined possess them. They were consistently present in certain large families of flowering plants, and we predict that they exist in the majority of herbaceous angiosperms. They are also found in horsetails, ferns and gymnosperms. Xylem transfer cells are predominantly associated with departing foliar traces, and in this locality their wall ingrowths exceed in number and size those found in any other of the genre. This applies to uni-, tri-, and multi-lacunar systems. Phloem transfer cells also appear in nodes, but less frequently and less conspicuously than the xylem type: they are mainly developed on the margins of the leaf gap and in the traces leading to axillary meristems (if present). Both the xylem and the phloem transfer cells are less well developed, or may even be absent, above and below the region of the leaf gap. A function for the cells in the nutrition of young apices subtended at the node is postulated.