52
J. Kuo and C. den Hartog
1989a) will not be dealt with again in this review,
unless essential. The taxonomic arrangement of
the seagrasses as given by den Hartog and Kuo,
Chapter 1, is used in this chapter.
II. General Morphology of
Seagrasses—Size and Shape
Seagrasses are herbaceous plants (although stiff and
hard upright stems and rhizomes do occur), and can
be grouped into three main morphological categories
with some taxonomic implications.
a. Plants without strap-shaped leaves but with either a pair of petiolate leaves at the rhizome
node or two or more leaflets on each of the distal nodes of the erect stem. This category is
restricted to Halophila, which has the smallest
shoots among seagrasses. Shoots can be less
than 1 cm in length, as for H. beccarii and H.
minor (Fig. 1B and C), and up to 20 cm long as
for H. australis.
b. Shoots with a distinct erect stem and strapshaped leaves borne at the top of an erect
stem. This group includes Thalassia of the Hydrocharitaceae and all genera of the Cymodoceaceae (Fig. 1F and I).
c. Plants without visible erect stems, but with
strap-shaped leaves derived from the rhizome
nodes. Enhalus of the Hydrocharitaceae, the
Posidoniaceae and all members of the Zosteraceae belong to this group (Fig. 1A, D and
E). The leaves of some members of Zostera
subgenus Zosterella can be as small as 10 cm;
while for Enhalus, Posidonia, Zostera subgenus Zostera and Phyllospadix, it is not uncommon for them to reach 1 m or more.
Vegetative organs of all seagrasses have a welldeveloped creeping rhizome. At each node or at
certain nodes there are one or more branched or unbranched roots and a shoot or an erect stem bearing
several leaves.
III. Vegetative Morphology and Anatomy
A. Roots
1. Root Morphology
The roots of seagrasses are adventitious and arise
from the lower surface of the rhizomes, generally
at the nodes. The external morphology of roots often has characteristic features in different genera,
which do not necessarily completely relate to specific
types of substrates. For example, Enhalus bears several coarse, soft, unbranched roots with a few short
root hairs (Fig. 1A), and grows on muddy substrata.
In contrast, both Thalassia and Halophila (Fig. 1B)
produce unbranched roots with massive root hairs
for penetrating various types of substrata. Roots of
Zostera and Heterozostera in the Zosteraceae are
always arranged in two groups, each bearing 2–
12 unbranched, slender roots with numerous long
root hairs (Fig. 2A, B and J) for growing in muddy
to sandy substrata. On the other hand, roots of
Phyllospadix also occur in two groups at rhizome
nodes, but they are extremely short and covered
with dense root hairs, together with robust rhizomes
(Fig. 2L–N). Unlike other seagrasses, Phyllospadix
can firmly attach to rock surfaces in high-energy environments (Cooper and McRoy, 1988; Barnabas,
1994b). The roots of Posidonia are thick, soft and extensively branched (Fig. 1E), but root hairs are rare
(Fig. 2F), and are usually associated with sandy substrata. The Cymodoceaceae including Syringodium,
Cymodocea and Halodule have, at each rhizome
node, one or more moderately branched roots with
few root hairs (Figs. 1F–H and 2D); they usually
grow on coral sands. The roots of Amphibolis are
wire-like and moderately branched, with no root
hairs, while those of Thalassodendron are woody,
thick and strong with a shiny black surface, rarely
branched, with a few short root hairs only near the
root tip (Figs. 1I and 2C and E). In general, Amphibolis and Thalassodendron are associated with
firm sedimentary and, sometimes, rocky substrata;
the roots of Thalassodendron can penetrate the gaps
in hard calcareous reef floors.
2. Root Anatomy and Ultrastructure
The root tip of all seagrasses has a distinct root cap
protecting meristematic cells, which provides a continuous set of dividing cells. The mature region may
bear root hairs depending on the species and overlying cortical parenchymatous tissues, which enclose
air lacunae and a central stele. However, the details
of root anatomy may be different among seagrass
groups reflecting their root morphology and the substratum.
On the outer-most layer, the epidermal cells usually have thin, unlignified walls and a peripheral
cytoplasm. The walls of these cells are soft in
J. Kuo and C. den Hartog
1989a) will not be dealt with again in this review,
unless essential. The taxonomic arrangement of
the seagrasses as given by den Hartog and Kuo,
Chapter 1, is used in this chapter.
II. General Morphology of
Seagrasses—Size and Shape
Seagrasses are herbaceous plants (although stiff and
hard upright stems and rhizomes do occur), and can
be grouped into three main morphological categories
with some taxonomic implications.
a. Plants without strap-shaped leaves but with either a pair of petiolate leaves at the rhizome
node or two or more leaflets on each of the distal nodes of the erect stem. This category is
restricted to Halophila, which has the smallest
shoots among seagrasses. Shoots can be less
than 1 cm in length, as for H. beccarii and H.
minor (Fig. 1B and C), and up to 20 cm long as
for H. australis.
b. Shoots with a distinct erect stem and strapshaped leaves borne at the top of an erect
stem. This group includes Thalassia of the Hydrocharitaceae and all genera of the Cymodoceaceae (Fig. 1F and I).
c. Plants without visible erect stems, but with
strap-shaped leaves derived from the rhizome
nodes. Enhalus of the Hydrocharitaceae, the
Posidoniaceae and all members of the Zosteraceae belong to this group (Fig. 1A, D and
E). The leaves of some members of Zostera
subgenus Zosterella can be as small as 10 cm;
while for Enhalus, Posidonia, Zostera subgenus Zostera and Phyllospadix, it is not uncommon for them to reach 1 m or more.
Vegetative organs of all seagrasses have a welldeveloped creeping rhizome. At each node or at
certain nodes there are one or more branched or unbranched roots and a shoot or an erect stem bearing
several leaves.
III. Vegetative Morphology and Anatomy
A. Roots
1. Root Morphology
The roots of seagrasses are adventitious and arise
from the lower surface of the rhizomes, generally
at the nodes. The external morphology of roots often has characteristic features in different genera,
which do not necessarily completely relate to specific
types of substrates. For example, Enhalus bears several coarse, soft, unbranched roots with a few short
root hairs (Fig. 1A), and grows on muddy substrata.
In contrast, both Thalassia and Halophila (Fig. 1B)
produce unbranched roots with massive root hairs
for penetrating various types of substrata. Roots of
Zostera and Heterozostera in the Zosteraceae are
always arranged in two groups, each bearing 2–
12 unbranched, slender roots with numerous long
root hairs (Fig. 2A, B and J) for growing in muddy
to sandy substrata. On the other hand, roots of
Phyllospadix also occur in two groups at rhizome
nodes, but they are extremely short and covered
with dense root hairs, together with robust rhizomes
(Fig. 2L–N). Unlike other seagrasses, Phyllospadix
can firmly attach to rock surfaces in high-energy environments (Cooper and McRoy, 1988; Barnabas,
1994b). The roots of Posidonia are thick, soft and extensively branched (Fig. 1E), but root hairs are rare
(Fig. 2F), and are usually associated with sandy substrata. The Cymodoceaceae including Syringodium,
Cymodocea and Halodule have, at each rhizome
node, one or more moderately branched roots with
few root hairs (Figs. 1F–H and 2D); they usually
grow on coral sands. The roots of Amphibolis are
wire-like and moderately branched, with no root
hairs, while those of Thalassodendron are woody,
thick and strong with a shiny black surface, rarely
branched, with a few short root hairs only near the
root tip (Figs. 1I and 2C and E). In general, Amphibolis and Thalassodendron are associated with
firm sedimentary and, sometimes, rocky substrata;
the roots of Thalassodendron can penetrate the gaps
in hard calcareous reef floors.
2. Root Anatomy and Ultrastructure
The root tip of all seagrasses has a distinct root cap
protecting meristematic cells, which provides a continuous set of dividing cells. The mature region may
bear root hairs depending on the species and overlying cortical parenchymatous tissues, which enclose
air lacunae and a central stele. However, the details
of root anatomy may be different among seagrass
groups reflecting their root morphology and the substratum.
On the outer-most layer, the epidermal cells usually have thin, unlignified walls and a peripheral
cytoplasm. The walls of these cells are soft in