68
J. Kuo and C. den Hartog
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 strapshaped 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) and Halophila
(Kuo, unpublished).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 seagrass 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 Cymodoceaceae (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 endoplasmic 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 tracheary 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 (Tomlinson, 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). Ultrastructurally, the xylem wall of Zostera and Phyllospadix 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 hydrolyzed 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
J. Kuo and C. den Hartog
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 strapshaped 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) and Halophila
(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 seagrass 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 Cymodoceaceae (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 endoplasmic 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 tracheary 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 (Tomlinson, 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). Ultrastructurally, the xylem wall of Zostera and Phyllospadix 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 hydrolyzed 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