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

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. Lengteverlopende are in die blaarlamina van die seegras Thalassodendron ciliatum (Forssk.) Den Hartog bestaan uit ’n skede van dikwandige selle wat die xileem en floëem omsluit. Alhoewel die xileem gereduseerd is, is die floëem goed ontwikkel en bestaan uit sifvate, begeleidende selle en floëemparenchiem. Twee tipes sifvate kan onderskei word: dié met dik en dié met dun selwande. Begeleidende selle het oor die algemeen ’n digte sitoplasma- inhoud en is deur middel van vertakte plasmodesmas met die sifvate verbind. Die vaatparenchiemselle bevat ’n organelryke sitoplasma met baie chloroplaste. Talryke plasmodesmas verbind die aangrensende vaatparenchiemselle onderling, terwyl plasmodesmas wat vaatparenchiemselle met sifvate verbind, minder dikwels waargeneem is. Elke vaatbondelskedesel is in ’n meerlagige suberienlamella ingebed. Wande van die bondelskedeselle bevat prominente stippelvelde met plasmodesmas. Die plasmodesmas deurboor die suberienlamella en verbind die skedeselle met die aanliggende vaatparenchiem en mesofilselle. Bondelskedeselle bevat oor die algemeen ’n sitoplasma-arme inhoud maar met goed ontwikkelde chloroplaste. Die suberienlamella vorm waarskynlik ’n apoplastiese versperring tussen die mesofil en vaatweefsel. Die strukturele kenmerke van die are word met dié wat in soortgelyke are in die blare van terrestriële eensaadlobbige plante voorkom, vergelyk.