Anatomy of Rhizome and Stem

The internodes of rhizomes and erect stems are similar in transverse section, and have a similar tissue arrangement as those in roots. The epidermis is normally distinct and covered by a cuticle, and may contain tannin cells (Fig. 4C). Walls of epidermal cells are usually thickened and lignified. The ex-odermis is distinct and usually has more than one layer of cells with a thickened wall and a suberized middle lamella (Fig. 5A and B) (e.g. Posidonia and Halophila). Cortical parenchyma cells are larger in the outer region than in the inner one. Sometimes these two regions are very distinct, for example, in Amphibolis and Thalassodendron. Cells of the outer cortical parenchyma have thick, lignified walls, while those of the inner cortical parenchyma have thin, non-lignified walls. In this cortical tissue, large lacunae may develop, as in Enhalus, Syringodium, Cymodocea (Fig. 4C, D and H) Zostera and Halophila and the number of lacunae appears to be constant within genera. Unlike root tissues, beside the central stele, there are always two or more vascular bundles and numerous fiber bundles scattered among the outer cortical tissues in the rhizome and the erect stem (Fig. 4A-D). In addition, starch accumulates in the cortical parenchyma cells in some genera, e.g. Enhalus (Fig. 4G), Thalassia, Zostera and Halodule, but not in others (Amphibolis and Thalassodendron).

A conspicuous endodermis encloses the central stele and may become suberized (e.g. Posidonia, Halodule) or develop thickened and lignified walls (e.g. Amphibolis, Thalassodendron). The central stele has one or more central protoxylem elements surrounded by sieve tubes, but the pericycle is not distinct.

Underground rhizomes are a common feature of many monocotyledons and account for distinctive ecological features (Harper, 1977). The main functions of the rhizome in seagrasses are for anchoring, mechanical support, nutrient storage, and regulation and maintenance of vegetative growth. Tomlinson (1974) considered that vegetative propagation of rhizomes is probably of greater importance in the maintenance and spread of seagrasses than seed production.

The rhizomes of the larger species may, together with roots and leaf sheath remains, form dense mats. The seagrass Posidonia oceanica in the Mediterranean Sea usually grows on the thick 'matte' with a thickness of up to 4 m locally, constituting

Fig. 4. Anatomy of rhizome and stem internodes. A. Zostera caulescens. Rhizome internode has a central stele (S), two opposite cortical vascular bundles (V) and a rather uniform cortical tissue (C). Scale = 700 |m. B. Halodule uninervis. Rhizome internode has a central stele (S), two opposite cortical vascular bundles (V) and a rather uniform cortical tissue (C). Scale = 200 |im. C. Syringodium isoetifolium. Rhizome internode has very distinct large tannin-containing cells (T) near the surface, and a ring of cortical vascular bundles (V) between the outer and inner cortical tissue (C). A few small air lacunae (A) occur near the stele (S). Scale = 100 | m. D. Cymodocea nodosa. Rhizome internode also has a ring of cortical vascular bundles (V), the compacted outer cortical tissue (C) and large air lacunae (A) occurring in the inner cortical tissue. Scale = 500 |im. E. Posidonia australis. Rhizome internode has numerous fiber bundles (F) scattering among the cortical tissue (C) almost reaching the centre stele (S). Scale = 100 |im. F. Posidonia sinuosa. Rhizome internode has smaller fiber bundles scattered in the cortex (C) and larger bundles (F) are restricted to the inner cortex and near the central stele (S). Scale = 500 |im. G. Enhalus acoroides. Rhizome internode has an outer cortex (C) rich in starch and an inner cortex (C) also having smaller starch grains and a few tannin cells (T). Scale = 500 |m. H. Cymodocea nodosa. Stem internode has a central stele (S) containing four distinct vascular poles and large air lacunae (A) occurring in the cortical tissue. Scale = 200 | m.

Fig. 4. Anatomy of rhizome and stem internodes. A. Zostera caulescens. Rhizome internode has a central stele (S), two opposite cortical vascular bundles (V) and a rather uniform cortical tissue (C). Scale = 700 |m. B. Halodule uninervis. Rhizome internode has a central stele (S), two opposite cortical vascular bundles (V) and a rather uniform cortical tissue (C). Scale = 200 |im. C. Syringodium isoetifolium. Rhizome internode has very distinct large tannin-containing cells (T) near the surface, and a ring of cortical vascular bundles (V) between the outer and inner cortical tissue (C). A few small air lacunae (A) occur near the stele (S). Scale = 100 | m. D. Cymodocea nodosa. Rhizome internode also has a ring of cortical vascular bundles (V), the compacted outer cortical tissue (C) and large air lacunae (A) occurring in the inner cortical tissue. Scale = 500 |im. E. Posidonia australis. Rhizome internode has numerous fiber bundles (F) scattering among the cortical tissue (C) almost reaching the centre stele (S). Scale = 100 |im. F. Posidonia sinuosa. Rhizome internode has smaller fiber bundles scattered in the cortex (C) and larger bundles (F) are restricted to the inner cortex and near the central stele (S). Scale = 500 |im. G. Enhalus acoroides. Rhizome internode has an outer cortex (C) rich in starch and an inner cortex (C) also having smaller starch grains and a few tannin cells (T). Scale = 500 |m. H. Cymodocea nodosa. Stem internode has a central stele (S) containing four distinct vascular poles and large air lacunae (A) occurring in the cortical tissue. Scale = 200 | m.

dead or living rhizomes, leaf sheath remains together with the sediment, which fills the interstices (see Fig. 13, Mateo et al., Chapter 7). Thus, rhizomes of this species grow in a horizontal (plagiotropic rhizomes) and a vertical (orthotropic rhizomes) plane (see Pergent, 1990). These leaf sheath remains in the matte can persist for more than 4600 years and show cyclic variation depending on environmental conditions such as light, temperature or water movement (Pergent, 1990; Di Dato, 2000) (see also Mateo et al., Chapter 7 and Gobert et al., Chapter 17). On the other hand, none of the Australian Posidonia species appears to be growing in a similar condition, although

Fig. 5. Anatomy of rhizome internodes (A, B) and sheaths (C-H). A, B. Posidonia australis. Walls of the rhizome epidermal (E) and exodermal (Ex) cells are lignified and autofluorescent, a few tannin cells (T) occur in the cortical tissue (C). Scales = 50 p.m. C. Zostera marina. Several large air lacunae (A) occur between two sheath longitudinal vascular bundles (V). Scale = 250 |im. D. Posidonia kirkmanii. Numerous smaller lacunae (A) occurring in the entire mesophyll tissues even near the longitudinal vascular bundle (V). Note that large fiber bundles (F) occur near the adaxial epidermal cells. Scale = 400 | m. E. Posidonia australis. Fibre cells (F) occur in the mesophyll tissue (M) near the adaxial epidermal cells (E) and walls of fiber cells are lignified. Scale = 50 |im. F. Posidonia australis. A thin, uniform electron transparent cuticle (arrows) and the outer tangential wall of the epidermal cell (E) has outer (OW) and inner (IW) regions. Scale = 1 |im. G. Phyllospadix scouleri. The adaxial sheath epidermal cell contains a nucleus (N), several chloroplasts (C), mitochondria (M), vacuoles (V) and wall ingrowths. Scale = 2 | m. H. Phyllospadix scouleri. The abaxial sheath epidermal cell contains a large central vacuole (V) and a few small plastids (C). The cuticle appears as a thin electron transparent layer (arrow), and the wall has outer wall (OW) and inner wall (IW) regions. Scale = 1 | m.

Fig. 5. Anatomy of rhizome internodes (A, B) and sheaths (C-H). A, B. Posidonia australis. Walls of the rhizome epidermal (E) and exodermal (Ex) cells are lignified and autofluorescent, a few tannin cells (T) occur in the cortical tissue (C). Scales = 50 p.m. C. Zostera marina. Several large air lacunae (A) occur between two sheath longitudinal vascular bundles (V). Scale = 250 |im. D. Posidonia kirkmanii. Numerous smaller lacunae (A) occurring in the entire mesophyll tissues even near the longitudinal vascular bundle (V). Note that large fiber bundles (F) occur near the adaxial epidermal cells. Scale = 400 | m. E. Posidonia australis. Fibre cells (F) occur in the mesophyll tissue (M) near the adaxial epidermal cells (E) and walls of fiber cells are lignified. Scale = 50 |im. F. Posidonia australis. A thin, uniform electron transparent cuticle (arrows) and the outer tangential wall of the epidermal cell (E) has outer (OW) and inner (IW) regions. Scale = 1 |im. G. Phyllospadix scouleri. The adaxial sheath epidermal cell contains a nucleus (N), several chloroplasts (C), mitochondria (M), vacuoles (V) and wall ingrowths. Scale = 2 | m. H. Phyllospadix scouleri. The abaxial sheath epidermal cell contains a large central vacuole (V) and a few small plastids (C). The cuticle appears as a thin electron transparent layer (arrow), and the wall has outer wall (OW) and inner wall (IW) regions. Scale = 1 | m.

there is plenty of 'marine fiber' from leaf sheath remains deposited on the ocean floor. However, Davies (1970, Fig. 12) illustrated a tidal channel on the Wooramel Bank on the eastern margins of Shark Bay about 3 m deep cutting through Posidonia rhizome mesh (termed rhizome and root mesh) and deposits under living seagrass reached a maximum thickness of more than 7 m. The rhizomes of Thalassodendron ciliatum appear also to be very persistent forming extensive mats. As the decomposition of the material is very slow, these mats can attain a considerable thickness. Lip-kin (1988) found living rhizomes up to 25 cm down in the mat, although it was very difficult to distinguish the old dark living rhizomes from dead material. In the Banda Sea, the mats were up to ca. 70 cm thick, and living rhizomes were only found in the upper 10 cm (Brouns, 1985). Further studies on the structure of the underground rhizome-root systems of the various seagrass species are recommended.

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