The strapped-leaved blades of seagrasses always possess three or more parallel longitudinal veins, which are interconnected, by transverse veins at regular intervals. However, Enhalus, unlike other sea-
grasses, has several additional smaller lateral veins forming a separate adaxial and abaxial vascular system. Seagrasses without a strap-leaved blade, such as Halophila, have a midvein and two lateral 'intramarginal veins,' which are joined at the base and top and are interconnected by several 'cross veins' in the blade. Vascular systems of seagrasses are similar in structure and composition to those of other vascular land plants, and all have sieve and xylem elements and vascular parenchyma cells (Fig. 8F-K). However, the so-called companion cells that are usually associated with sieve tubes in terrestrial plants are not distinct (Fig. 10A-D) and thus are referred to as vascular parenchyma cells in this chapter.
A distinct layer of sheath cells always encloses each longitudinal vascular bundle of the strap-shaped leaf and the structure of these sheath cells differs between genera: (a) sheath cells have thin and lignified walls in Posidonia (Figs. 8H and 10B); (b) sheath cells have thick, unlignified walls, but possess a suberized lamella in Thalassodendron and Amphibolis (Fig. 8I and J) (Barnabas and Kasvan, 1983; Kuo, 1983a); (c) sheath cells have thin and unlignified walls but possess a suberized lamella in Syringodium (Fig. 10D and E) (Kuo, 1993a); (d) sheath cells have wall ingrowths in the inner tangential walls in Zostera and Heterozostera (Fig. 10A) (Kuo, 1983a); and (e) sheath cells appear to have no obvious structural specialization in all other genera, e.g. Halodule, Cymodocea (Fig. 10C) andHalophila (Kuo, unpublished). Regardless of the difference in the structure, it has been speculated that bundle sheath cells in seagrasses, as with those in terrestrial plants, might be involved in restricting solute transfer between the mesophyll and the vascular tissue to a symplastic pathway (Kuo and McComb, 1989; Kuo, 1993a). This has been demonstrated by dye transpiration experiments in Thalassodendron and Halodule leaves (Barnabas, 1989, 1994a).
Sieve elements in seagrasses are of three anatomical types. (a) Elements that are thin-walled with a relatively large lumen; these occur in all sea-grass genera (Fig. 10B-D). (b) Elements that are not lignified with an irregular wall thickening and an uneven inner surface; thus having varied lumen dimensions along the element. This type of sieve tube is known as a nacreous-walled sieve element and occurs only in the Zosteraceae (Figs. 8G and 10A and F) and in Halodule of the family Cymod-oceaceae (Kuo, 1983b). (c) Elements that are small with evenly thick walls and a small lumen. This element always abuts with xylem and has only been recorded in Syringodium (Fig. 10D and G) (Kuo, 1993a) and Thalassodendron (Barnabas, 1983). Despite the difference in their locations and appearances, all three types of sieve elements have similar cytoplasmic properties, with a distinct plasmalemma and contain mitochondria, stacked smooth endoplas-mic reticulum, typical monocotyledonous plastids with protein crystalloids, and lack a nucleus and ribosomes at maturity. In addition, there are many symplastic connections from sieve elements to adjacent vascular parenchyma cells via sieve areas along their common walls (Fig. 10G and F). These observations suggest that all three types of sieve elements are functional. However, the precise functions, such as the degree of effectiveness in solute translocation or temporary solute storage as suggested for terrestrial plants, have yet to be determined in the sieve elements of the marine angiosperms. Various types of sieve tube are not unique to seagrasses, as they have also been found in the vascular bundles of terrestrial plants.
Aquatic plants generally are characterized by reduced xylem tissue. This reduction is interpreted as a result of the loss of functional need (mechanical and conductive) in plants with a constant supply of water and supported by the aqueous medium (Sculthorpe, 1967). Vessels are essentially absent and the trac-heary elements are mostly reduced in all submerged species. They may be represented by weakly lignified elements with annular or spiral wall thickening, and minimal amount of secondary wall material (Tom-linson, 1982). In the Zosteraceae, the xylem and phloem are totally separated (Fig. 8F) and the xylem is represented by a single wide lacuna surrounded by a distinct layer of large xylem parenchyma cells with thickened but not lignified walls, a feature that distinguishes them from other seagrasses (Fig. 10H). Ul-trastructurally, the xylem wall of Zostera and Phyl-lospadix appears to be completely hydrolyzed and the so called 'xylem wall' in fact is represented only by the middle lamella, which appears as an electron dense layer, and the thickened wall of the adjacent xylem parenchyma cells (Fig. 10B). On the other hand, in other families, xylem and phloem are together and xylem elements have highly hy-drolyzed walls with little, e.g. Posidonia (Fig. 10G), or no lignification, e.g. Halophila, Cymodocea (Fig. 10C) and Syringodium (Fig. 10E). The reduced xylem has led to a suggestion that there is little xylem transport in seagrasses (Tomlinson, 1982), and an experimental study on this topic is highly recommended.
The structure of the vascular parenchyma cells is very similar in all seagrasses, except for the Zosteraceae, in which wall ingrowths occur in phloem parenchyma cells. Vascular parenchyma cells (Fig. 10F) usually contain several chloroplasts (with poorly developed grana), mitochondria, lipid droplets, free ribosomes and rough endoplasmic reticulum, as well as plastids with electron-opaque bodies resembling protein crystalloids. Branched plasmodesmata with enlarged walls frequently form connections between vascular parenchyma cells.
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