Chapter 3 Seagrass Morphology, Anatomy, and Ultrastructure
83
den Hartog, 2000). The relationship between seagrass morphology/anatomy and water depth also has
been discussed by Larkum et al. in Chapter 14.
VII. Discussion and Concluding Remarks
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 strapshaped 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
−
3 in
seawater into CO 2 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.
Acknowledgments
Thanks to Hugh Kirkman for his professional comments on the earlier version of the manuscript, and
to editors and anonymous reviewers who made further valuable and constructive suggestions on the
manuscript. Special thanks are due to L.-Y. Kuo
for photographing and scanning assistance, and to
J. Murphy for computer assistance. Thanks also
go to those colleagues who provided seagrass materials over many years for anatomical and ultrastructural studies, in particular Hugh Kirman,
Marion Cambridge, Mike Fortes, Warren Lee Long,
Rob Coles, Z. Kanamoto, Hitoshi Izumi, Hiroshi
Mukai and Keiko Aioi.
References
Abal EG, Loneragan N, Bowen P, Perry CJ, Udy UW and
Dennison WC (1994) Physiological and morphological responses of the seagrass Zostera capricorni Aschers. to light
intensity. J Exp Marine Biol Ecol 178: 113–129
Ackerman JD (1993) Pollen germination and pollen tube growth
in the marine angiosperm, Zostera marina L. Aquat Bot 46:
189–202
Ackerman JD (1995) Convergence of filiform pollen morphologies in seagrasses: Functional mechanisms. Evol Ecol 9: 139–
153
Aioi K, Komatsu T and Morita K (1997) The world’s longest
seagrass, Zostera caulescens from northern Japan. Aquat Bot
61: 87–93
Arai M, Pak JY, Nomura K and Nitta T (1991) Seawater-resistant,
non-spherical protoplasts from seagrass leaves. Physiol Plant
83: 551–559
Arber A (1920) Water Plants. A Study of Aquatic Angiosperms.
Cambridge University Press, Cambridge
83
den Hartog, 2000). The relationship between seagrass morphology/anatomy and water depth also has
been discussed by Larkum et al. in Chapter 14.
VII. Discussion and Concluding Remarks
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 strapshaped 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
−
3 in
seawater into CO 2 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.
Acknowledgments
Thanks to Hugh Kirkman for his professional comments on the earlier version of the manuscript, and
to editors and anonymous reviewers who made further valuable and constructive suggestions on the
manuscript. Special thanks are due to L.-Y. Kuo
for photographing and scanning assistance, and to
J. Murphy for computer assistance. Thanks also
go to those colleagues who provided seagrass materials over many years for anatomical and ultrastructural studies, in particular Hugh Kirman,
Marion Cambridge, Mike Fortes, Warren Lee Long,
Rob Coles, Z. Kanamoto, Hitoshi Izumi, Hiroshi
Mukai and Keiko Aioi.
References
Abal EG, Loneragan N, Bowen P, Perry CJ, Udy UW and
Dennison WC (1994) Physiological and morphological responses of the seagrass Zostera capricorni Aschers. to light
intensity. J Exp Marine Biol Ecol 178: 113–129
Ackerman JD (1993) Pollen germination and pollen tube growth
in the marine angiosperm, Zostera marina L. Aquat Bot 46:
189–202
Ackerman JD (1995) Convergence of filiform pollen morphologies in seagrasses: Functional mechanisms. Evol Ecol 9: 139–
153
Aioi K, Komatsu T and Morita K (1997) The world’s longest
seagrass, Zostera caulescens from northern Japan. Aquat Bot
61: 87–93
Arai M, Pak JY, Nomura K and Nitta T (1991) Seawater-resistant,
non-spherical protoplasts from seagrass leaves. Physiol Plant
83: 551–559
Arber A (1920) Water Plants. A Study of Aquatic Angiosperms.
Cambridge University Press, Cambridge