Composition and fine structural features of longitudinal veins in leaves of Thalassodendron ciliatum

Abstract

Longitudinal veins in leaf blades of the sea-grass Thalassodendron ciliatum (Forssk.) Den Hartog consist of a sheath of cells with thickened walls which encloses xylem, phloem and vascular parenchyma cells. While the xylem is reduced, phloem is well developed and consists of sieve tubes and companion cells. Two types of sieve tubes are distinguishable: those with thick walls and those with thin walls. Companion cells generally have dense cytoplasmic contents and are connected to sieve tubes by branched plasmodesmata. Vascular parenchyma cells possess an organelle-rich cytoplasm with chloroplasts as plastids. Numerous plasmodesmata interconnect contiguous vascular parenchyma cells. Plasmodesmata connecting vascular parenchyma cells to sieve tubes and companion cells are observed less frequently. Each bundle sheath cell is encased within a multilayered suberin lamella. Bundle sheath cell walls have prominent pit fields which are traversed by plasmodesmata. The plasmodesmata perforate the suberin lamella and interconnect sheath cells with contiguous vascular parenchyma and mesophyll cells. Bundle sheath cells generally possess sparse cytoplasmic contents but have relatively well developed chloroplasts. The suberin lamella appears to form an apoplastic barrier between mesophyll and vascular tissues. Structural features of the veins are compared with those found in similar veins in leaves of terrestrial monocotyledonous plants.