Ultrastructural characteristics of Hydrilla leaf tissue.

The general anatomy and leaf ultrastructure of Hydrilla verticillata (L.f.) Royle, a submersed vascular hydrophyte, are reported. The transversely sectioned leaf consists of only two contiguous epidermal layers, and the single midvein is composed of three to four concentric layers of cells. There are numerous intercellular spaces throughout the tissue. Electron microscopy confirmed the presence of sieve elements in the mid-vein area. P-protein, plastid inclusions and branched plasmodesmata leading into the companion cells are all considered indicative of phloem-type cells. Plastid inclusions are cuneate and lack associated starch granules, characteristics typical of monocots. The outer wall of the lower epidermis protrudes into the cell. Plant cells with wall ingrowths are termed 'transfer cells' and, in aquatics, function in the absorption of solutes from the external environment. In addition to the lower epidermis, spiny projections of the leaf margin could be included in this category. There is a thick fibrous layer which appears to be part of the outer wall of the upper epidermis. Secretory vesicles containing polysaccharide empty into this layer and into the wall. Cell nuclei are distinctly polymorphic. The granal arrangement, presence or absence of starch and peripheral reticulum in chloroplasts varies.