This chapter has attempted to illustrate how all modern seagrasses have several morphological and anatomical modifications, both vegetative and reproductive, which differentiate them from terrestrial plants. But many of these morphological and anatomical modifications also occur in freshwater plants (Arber, 1920; Sculthorpe, 1967) and must be interpreted as adaptations to the aquatic environment. These features include leaves having a thin cuticle; small epidermal cells with thick walls; concentrations of chloroplasts in the epidermal cells; a lack of stomata, an enlarged aerenchyma system and a reduced xylem accompanied sometimes by reduced mechanical tissues. In many cases, seagrasses retain many functional, morphological and anatomical features of terrestrial plants. These features include the presence of suberin or lignin or similar structures that restrict apoplastic pathways in bundle sheath cells in the leaves, and having a hypodermis and an endodermis in the rhizomes and roots. While there are few or no particular structures in seagrass that can be identified as unique in terms of structural adaptation to the marine environment, there is a suite of characters, which together can be taken as representative of seagrasses. These include strap-shaped leaves and anatomical reinforcement to resist wave action, adaptation of leaves to carry out photosynthesis in a seawater environment, osmotic adjustment and other adaptations within the leaf blade and leaf sheath, modifications to rhizomes and roots for different substrata, pollination by hydrophily, reduction in the layers of the pollen wall and several unique features associated with seed formation and dispersal mechanisms.
Furthermore, the morphology and anatomy of vegetative and reproductive organs also varies among different taxa suggesting that seagrasses probably neither evolved from a common ancestor nor through the same evolutionary pathways, nor in the same geological period. Most of those freshwater or terrestrial cousins no longer exist. On the other hand, after establishment in marine habitats, 'seagrasses' had little pressure to modify further their morphological and anatomical structures to meet new physiological or biochemical requirements (Larkum and den Hartog, 1989).
The greatest physiological and biochemical adaptation is probably the conversion on HCO- in seawater into CO2 presumably by anhydrase enzymes at the outer tangential walls of epidermal cells and also the presence of a proton pump at the plasmalemma of seagrass leaves. However, this applies also to most freshwater plants.
So far, the salt-tolerant H+-ATPase has only been demonstrated biochemically and physiologically in the leaf plasmalemma of Z. marina that possesses wall ingrowths in the blade epidermis. It will be interesting in future to see whether this pump is a common feature of seagrasses and whether it is connected to the control of sodium concentration.
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