Feddes Repertorium 2013, 124, 50–60
DOI: 10.1002/fedr.201300020
RESEARCH PAPER
Leaf micromorphology and leaf glandular hair ontogeny
of Myoporum bontioides A. Gray
Sauren Das1, Chiou-Rong Sheue2, Yuen-Po Yang3, 4
Agricultural and Ecological Research Unit, Indian Statistical Institute, 203 Barrackpore Trunk Road,
Kolkata 700108, India
2
Department of Life Sciences & Research Center for Global Change Biology, National Chung Hsing University,
250 Kuo Kuang Road, Taichung 40227, Taiwan
3
Department of Biological Sciences, National Sun Yat-sen University, Kaohsiung 804, Taiwan
4
Department of Bioresources, Dayeh University, Changhua 515, Taiwan
1
Myoporum bontioides A. Gray (Myoporaceae), a red list plant in Japan, is restricted to only Submitted: November 14, 2013
Revised: March 8, 2014
a few East Asian countries like China, Japan and Taiwan, associated to some true manAccepted:
April 30, 2014
groves. The leaf is isolateral and has a thick cuticle; stomata are anomocytic, sunken and
have a beak-shaped cuticular outgrowth at the inner and outer side of the stomatal pore
(ledges); profuse glandular hairs are scattered on both leaf surfaces of young leaves. The
mesophyll is compact with palisade and spongy parenchyma cells in the young stage, but
at maturity profuse intercellular spaces can be observed. Secretory ducts occur in young
leaves. Pear-shaped glandular hairs protrude from the epidermal layer. Hair primordia are
well distinct by their larger size and undergo divisions to produce two laterally placed basal
cells, two stalk cells and four radiating terminal cells. The cuticle layers of the terminal cells
are often separated from the cell wall to form a space, in which ions accumulate for excretion. Inner walls of the basal cells are connected with the mesophyll. Though ontogeny and
structure of glandular hairs have resemblance to typical mangroves, considering leaf
micromorphology, this plant is better termed as “mangrove associate” instead of “true mangrove”.
Keywords:
Glandular hairs, leaf anatomy, morphology, Myoporum bontioides, ultrastructure
1
Introduction*
Myoporum Banks & Sol. ex G. Forst is a figwort plant
genus, formerly placed in Scrophulariaceae (Gray 1862).
Olmstead et al. (2001) opined that there is a close relationship of Myoporaceae with two other families, Buddlejaceae and Scrophulariaceae s. str. They further considered the genus as paraphyletic and included both
Buddlejaceae and Myoporaceae in the family Scrophulariaceae. But recently it has again been segregated as
a separate family, Myoporaceae (Mabberley 2000; Ma
Correspondence: Dr. Das, 203, B.T. Road, Kolkata,
West Bengal 700108
E-Mail: [email protected]
Phone: +91 33 2575 3220
Fax: +91 33 2577 3049
50
et al. 2013). There are about 32 species within the genus, which is spread from Mauritius, across Australia to
the Pacific Islands and up to China. Myoporum bontioides (Sieb. & Zucc.) A. Gray, was considered as synonymous to Pentacoelium bontioides by Siebold & Zuccarini (1846). Later on, this taxon was classified as
Myoporum bontioides A. Gray under the genus
Myoporum. The genus includes about 30 species which
are distributed from Eastern Asia to the Pacific Islands
and Australia (Gray 1862).The plants are mostly distributed in wet places near the sea in islands of Kyushu
(western coast and Tanegashima) and Ryukyus of Japan, Formosa and China. The geobotanical and ecological descriptions of this species were done by Hiroki
(1985), considering it as a semi-mangrove. This species
is regularly recorded amongst the typical mangroves,
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Leaf anatomy and leaf gland ontogeny of Myoporum bontioides
along the sandy seashores in Vietnam, particularly in the
north-eastern zone along with Scyphiphora hydrophyllaceae (Hong & Hoang 1993). They also recorded this
plant from South Japan, Taiwan, Central to Southern
China and Northern Vietnam. The family Myoporaceae
was monographed, and Pentacoelium was re-installed
as a genus distinct from Myoporum Banks & Sol. ex G.
Forst. by Chinnock (2007). Pentacoelium is similar to the
species of Myoporum in respect to vegetative characteristics, though the flowers are much larger and the fruits
are quite different (Chinnock 2007). The fruits of Pentacoelium are large and structurally more complex than in
the other members of this family (Li 2002). The family
Myoporaceae was documented by Li (1998), and he
concluded that the genus Myoporum has about 30 species distributed from Eastern Asia to the Pacific islands
and Australia, and Myoporum bontioides (Sieb. & Zucc.)
A. Gray being the only species reported from Taiwan.
Hongbin et al. (2011) reported Myoporum bontioides
(Sieb. & Zucc.) A. Gray (synonym M. chinense (A. Candolle) A. Gray; Polycoelium bontioides (Sieb. & Zucc.)
A. Candolle; P. chinense A. Candolle) from the sandy
sites along the coasts, near the sea levels of Eastern
Fujian, South Guangdong, South Guangxi, Tainan (West
Taiwan) and East Zhejiang (South Japan). In Taiwan,
the plant was reported from two places, I. Tungshih
(Ton-Shou) mangrove wetland, a west coast of Chiayi
County (Chen & Lee 1978; Wu 1986; Lin et al. 1987a)
and II. Tatu Estuary, northwest of Changhua County
(Karpowicz 1985; Lin et al. 1987b). Theisen & Fischer
(2004) concluded that the family Myoporaceae included
four genera with ca. 330 species, with a center of diversity in Australia, extending to the islands of the Pacific
and Indian ocean, and few species in tropical America,
Hawaii, Caribbean islands and Japan. According to
Global Biodiversity Information Facility (GBIF 2013),
M. bontioides A. Gray is distributed only in Taiwan
(23.5° N 121° E), two regions of Japan, Kagoshima
(30° N 130° E) and Nagasaki (36° N 138° E), and northwest China (Fig. 1A). This species has been considered
as a red list plant by the Threatened Species Committee,
Japan Society of Plant Taxonomists (Yahara et al.
1997). According to the IUCN (1994), M. bontioides
A. Gray has been declared as a globally threatened
(EW, CR, EN or VU) Red List Category plant.
M. bontioides is widely considered as a folk medicinal
plant in the northwest of China and known as a superficies-syndrome drug (Wang et al. 2000). Deng et al.
(2008) reported that the stem and leaf extracts of
M. bontioides show inhibitory effects against seven
fungi, with >58% of inhibition at 10 g L−1 after 72 h of
incubation, and the compound was identified as epingaione. Myoporum bontioides is a rare coastal species
and can be used as potential soil binder. Though a number of papers have been published on bioactive mole© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
cules extracted from this plant (Fraga 2001; He et al.
2004; Kanemoto et al. 2008), a detailed anatomical
investigation of this rare species is insufficient. Anatomical and morphological features serve as a trustworthy
pointer towards the understanding of the ecological
adaptation of a plant. The influence of different ecological factors (microclimate) on plants is best reflected by
its leaf morphology and anatomical organization, as
compared to the other vegetative organs (Krstić et al.
2002). M. bontioides grows in saline/estuarine soil associated to some mangroves, and thus the leaf micromorphology resembles some typical mangrove leaf architectures and signifies the ecological niches of the taxa. The
leaf micromorphology of this taxon is still pertinent to
plant biologists. The present work deals with the detailed
leaf anatomical account to find out the degree of similarity with typical mangrove species. Structural architecture
and developmental pathway of glandular hairs on leaves
are of special interest in comparison to other saltsecreting plants (especially mangroves). The structural
resemblance of leaf glandular hairs often signifies the
harmony in the mechanism of secretion (Levering &
Thomson 1971; Oross & Thomson 1982). Hence, the
ontogeny of glandular hairs with the aid of ultramicroscopy (SEM and TEM) was also carried out for
better understanding.
2
Material and methods
Myoporum bontioides was collected from the Tungshih
mangrove wetland at the west coast of Chiayi County in
southern Taiwan (Fig. 1B). The location comprises several small patches of mangroves with adjacent intertidal
mudflats. The vegetation is a mixture of typical mangroves like Avicennia marina, Kandelia obovata and
Rhizophora stylosa, with stray communities of Atriplex
nummularia and Myoporum bontioides. The climate is
humid tropical with an average annual rainfall of about
2,000 mm.
For anatomical investigation, leaf samples were cut
into small pieces (2 × 2 mm2 approximately) and fixed in
2.5% glutaraldehyde in 0.1 M sodium phosphate buffer
(pH 7.3) for 4 h at room temperature. After washing
thrice in buffer for 30 min, the specimens were secondarily fixed in 1% OsO4 in the same buffer for 4 h. Dehydration was carried out through ethanol series starting
from 30%, the material was infiltrated for 3 days and
embedded in Spurr’s resin (DER = 6.0) (Spurr 1969).
The embedded material was then polymerized in an
oven at 70 °C for 12 h. Semi-thin sections (1 μm) were
cut with a MTX Ultra microtome (RMC, Tucson, Arizona,
USA). The sections were stained with 1% toluidine blue
for light microscopy (Olympus BH-2, Tokyo, Japan). Leaf
surfaces were examined by scanning electron micro51
S. Das et al.
Feddes Repertorium 2013, 124, 50–60
Figure 1. Distribution of Myoporum bontioides A. Gray. (A) Global distribution (source: Global Biodiversity Information
Facility, 2013) [http://www.discoverlife.org/mp/20m?kind=Myoporum+bontioides.]; (B) Collection site of the species
(map of Taiwan in inset). (Source: Google Earth).
52
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Leaf anatomy and leaf gland ontogeny of Myoporum bontioides
Figure 2. Photographs of the species. (A) natural habit; (B) closer view of a twig; (C) twig bearing seeds; (D) closer view
of a flowering twig.
scopy (SEM, Hitachi S 2400, Tokyo, Japan) following the
protocol by Bozzola (2007) and were photographed.
Ultra-thin sections (about 75 nm) were cut and sections
were stained (Ellis 2007) with uranyl acetate (5% in 50%
methanol) and lead citrate (1% in water) for transmission
electron microscopic examination (TEM, Hitachi H 600,
Tokyo, Japan) and then photographed.
3
Results
3.1 Plant morphology
Plants of Myoporum bontioides are evergreen, erect
shrubs, 1–2 m tall. Branches are fleshy, with swollen leaf
scars. Leaves are alternate, coriaceous, fleshy; petioles
are 1–2 cm long; blades are oblong to elliptic, 5–12 cm
long, 2–4 cm wide, the apex acute or acuminate, base
acute, gradually tapering into the petiole, entire; upper
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
surface lustrous, midrib slightly projected on lower surface, lateral nerves obscure. Flowers appear from January to May, 2–4, fascicled in leaf axils. Pedicel slender,
2 cm long at flowering, up to 3 cm long in fruiting stage.
Calyx campanulate, 10 mm long, deeply 5-lobed; lobes
lanceolate, acuminate, 5 mm long. Corolla campanulate
to funnel-shaped, white, with purple spots, 2–2.5 cm
long, more or less fleshy; tube stout, 10–15 mm long,
5-lobed; lobes elliptic, obtuse or acute, 10 mm long,
reflexed. Stamens 4, slightly didynamous, exert, 18–
25 mm long; filaments glabrous, divergent. Style filiform,
18–25 mm long. Fruits are drupe, ovoid to globose,
acute, 10–15 mm long, 8–10 mm in diameter, reddish
brown at ripening (Fig. 2A–D).
3.2 Micromorphology of the leaf
In transverse sections, the leaves are isolateral; mature
leaves have profuse intercellular spaces in the mesophyll
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S. Das et al.
region (Fig. 3A). Mesophyll tissues of young leaves are
compactly arranged and have spherical secretory cavities
lined with an epithelial layer of small, isodiametric paranchymatous cells (Fig. 3B, C). Glandular hairs are present on both the adaxial and abaxial surface of the young
leaves, but at maturity they are absent (Fig. 4A, B). Mature glandular hairs appear as pear-shaped structures,
encrypted in the epidermal layer (Fig. 3H, I; 4D). Mature
leaves are 0.605 mm ± 0.011 thick; the cuticle is prominent, wavy and more or less equal in thickness on both
surfaces of the leaf (0.039 mm ± 0.004). Leaves are
amphistomatic, but the frequency of stomata is lower on
the adaxial surface (19.7mm–2 ± 1.16 and 25.8 mm–2
± 1.81 on the adaxial and abaxial surface, respectively).
The sunken stomata are anomocytic, surrounded by 3–4
epidermal cells of varying size (Fig. 3D; 4A–C). The anterior walls of the stomatal guard cells are much cutinized
(Fig. 3D; 4C). In transverse sections of the leaf, mature
stomata show a distinct beak-shaped cuticular outgrowth
overarching the stomatal pore at the outer and inner
side, termed as ledges (Fig. 3E). The epidermal layer is
0.107 mm ± 0.016 thick on the upper surface and
0.074 mm ± 0.14 on the lower surface, more cutinized at
the outer tangential wall than at the lateral wall (Fig. 3E;
4F). The epidermal cells are mostly rectangular, and the
outer wall is more cutinized and sinuous at the tangential
wall (Fig. 3A, E).
The mesophyll is composed of thin-walled chlorenchyma, differentiated distinctly into one or two layers of
elongated, anticlinally extending palisade cells on both
adaxial and abaxial sides, and between the adaxial and
adaxial palisade there are 4–6 layers of isodiametric
spongy parenchyma cells (Fig. 3A). In young leaves, the
mesophyll is compact without any intercellular space
(Fig. 3B), but at maturity, palisade and spongy cells are
loosely arranged with profuse intercellular spaces. Veins
are placed in the middle of the transversal leaf section.
Younger leaves show numerous circular secretory cavities with a layer of narrow epithelial cells (Fig. 3B, C; 4E)
and occur at any level within the mesophyll. In mature
leaves, these cavities are absent. Vascular bundles are
sheathed by one layer of small polygonal sclerenchyma
cells. Xylem strands are present on the abaxial side, and
Feddes Repertorium 2013, 124, 50–60
the thin-walled phloem occurs adaxially. Metaxylem is in
the anterior and protoxylem in the posterior position
(Fig. 3A). The leaf anatomy shows that the mesophyll
zone lacks any hyaline water storage tissues in the
endodermal region as commonly found in typical mangrove leaves; bundle sheath extensions and terminal
tracheids are also absent.
3.3 Ultrastructure
The leaf surface is rough and undulating; in younger
leaves, numerous glandular hairs are present on both
the adaxial and abaxial surface, the frequency is higher
on the adaxial side. The anterior wall of anomocytic
stomata is more cutinized than the posterior wall
(Fig. 4A–C). Glandular hairs are encrypted within the
epidermal layer, the basal cells (two in number) are
embedded in the leaf epidermis, and the inner walls of
the basal cells are connected with the mesophyll. The
stalk cells and radiating terminal cells protrude from the
leaf surface (Fig. 4D). Large secretory cavities, encircled
by a single layer of barrel-shaped, thin-walled epithelium
cells, occur in the upper part of the mesophyll zone in
young leaves (Fig. 4E).
The transmission electron microscopy observations
also confirm that the glandular hairs differ prominently
from the surrounding epidermal cells as they protrude
from the latter. They lack chloroplasts, have centrally
located vacuoles and a large nucleus. Abundance of
other cell organelles like mitochondria, golgi apparatus
and endoplasmic reticulum are noticed (Fig. 5A–D).
Based on the ultra-structural studies, each mature gland
consists of two basal cells, two stalk cells and four radiating terminal cells. The basal cells are large, embedded
in the leaf tissue, and assumed to be the collecting cells
for the excretory substances (mainly excess salts). Radiating terminal cells protrude from the laminar surface
and have a secretory function (Fig. 5A–C). In longitudinal section, the cuticle of the terminal cells gets separated from the cell wall and forms a space, probably
serving towards the accumulation of secretory products
prior to excretion (Fig. 5D). The extreme outer portion of
the periplasm of the terminal cells appears as a darker
Figure 3. Photomicrographs of semi-thin transverse sections of leaves. (A) mature leaf; (B) young leaf; (C) enlarged !
secretory cavity; (D) stoma with subsidiary cells (top view); (E) stoma with cuticular outgrowth forming a beak-like structure on both inner and outer side of the stomatal pore (ledges); (F)–(I). Developmental stages of leaf glandular hairs.
(F) Single glandular hair primordial cell with dark nucleus and upwardly elevated from the normal epidermis; (G) primordial cell undergoes a periclinal division to form one terminal cell and one basal cell. It is a dividing cell, after division, the
two nucleus are placed at the two opposite poles (indicated with double arrow); (H) basal cell divides longitudinally to
form two basal cells, and terminal cell divides to form one stalk cell and one terminal cell; (I) basal cells encrypted within
the epidermis, after division two stalk cells and four radiating terminal cells are produced. (Bc – basal cells, Cu – cuticle,
Ep – epidermis, Et C – epithelium cell layer, Gc – guard cells, Gh Pr – glandular hair primordial, Gl H – glandular hair,
L – ledges, Mx – metaxylem, Ph – phloem, Pp – palisade parenchyma, Px – protoxylem, Sc – secretory cavity, Sp – spongy
parenchyma, St – stomata, St C – stalk cell, Su C – subsidiary cell, Tc – terminal cell, Vb – vascular bundle).
54
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Leaf anatomy and leaf gland ontogeny of Myoporum bontioides
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S. Das et al.
layer, the electron-dense portion of the periplasm of
terminal cells (Fig. 5D).
3.4 Ontogeny of glandular hairs
The glandular hair primordia differ remarkably from the
other neighboring epidermal cells in their level of elevation from the epidermal cells, they are larger in size
with an enlarged, deeply stained nucleus, lack of chloroplasts and a central vacuole in the cytoplasm as well as
abundant mitochondria and other cellular organelles
(Fig. 5A–C). Transverse sections show that the mature
hair consists of four radiating terminal cells, two stalk
cells and two basal cells (Fig. 3I; 4F; 5A–C). The hair
primordia protrude from the epidermal layer (Fig. 3F).
The first division of the primordia takes place in periclinal
plane to form a terminal and a basal cell (Fig. 3G). The
basal cell undergoes another periclinal division to form a
stalk cell and a basal cell, while the terminal cell remains
undivided. The stalk cell then undergoes one transverse
division to produce two stalk cells; the basal cell undergoes a longitudinal division, and two basal cells are
formed. The terminal cell undergoes two longitudinal
divisions to form four radiating cells (Fig. 3H, I). At maturity, the basal cells are embedded in the leaf epidermis, and the inner parts remain attached to the mesophyll (Fig. 3H, I; 4F; 5A–C). The mature glandular hairs
remain enclosed within the uninterrupted cuticular layer.
From the surface view, the four terminal cells appear as
a radiating fan (Fig. 3I; 4F; 5C).
4
Discussion
The presence of a considerably thick cuticle on the leaf
can be attributed to a constraint device of the plant
against non-stomatal water loss, which is a typical halophilic character (suffice by its natural habitat). Waisel
(1972) deliberated that the thick cuticular layer is an
adaptive feature of mangroves. The stomata are anomocytic and sunken. Distinct beak-like cuticular outgrowths
occur on both inner and outer side of the stomatal pore
(ledges). Das (2002) reported that this structure is a
common character in true mangroves and provides an
extra protective means against water loss through the
stomatal pore during transpiration. Considering the leaf
anatomical features, M. bontioides was described as
‘mangrove shrub’ by Toyama & Itow (1975). Though the
leaf micromorphology of M. bontioides has some resemblances with typical mangroves, the present observation reveals some disparities, too. Earlier works (Tomlinson 1986; Das 1999) described four unique leaf
anatomical features of typical mangroves, such as i)
presence of colorless water storage tissue in the hypodermal layer, ii) short terminal tracheids at vein endings,
56
Feddes Repertorium 2013, 124, 50–60
iii) absence of sclerotic bundle sheaths and iv) presence
of sclereids of different shapes. In mature leaves, the
investigated taxon has a loosely arranged mesophyll
tissue. The complete absence of any definite water storage cells, sclereids and terminal tracheids can be ascribed towards the non-mangrove characters of the
taxon, but some other leaf anatomical features of the
studied taxon surely can be considered as associated to
mangroves. Terminal tracheids and branched sclereids
in mangroves provide mechanical support and capillary
water storage function (Zimmermann 1983; Tomlinson
1986). The present taxon shows secretory cavities in the
mesophyll region of young leaves, lined with a layer of
epithelium cells which are absent in the mature leaf.
Karrfalt & Tomb (1983) opined that all members of
Myoporaceae possess secretory cavities, except Oftia.
They observed that the secretory cavities in Bontia
daphnoides (Myoporaceae) are homologous to the air
spaces in Leucophyllum minus, where cellular separation and disintegration are not preceded by cell division
(lysigenous origin). They also concluded that the air
space in Leucophyllum differs from the secretory cavities of Bontia, principally in the lack of secretory function
and absence of cell division during their development.
Later, Lersten & Beaman (1998) did not notice any anatomical evidence of transitory epithelial cells or lysis of
cells in developing air spaces in five species of Leucophyllum, thus the hypothesis that air spaces are transformed secretory cavities is not supported. They also
reported epithelium-lined cavities scattered abundantly
in leaves from three species of Myoporaceae (Bontia
daphnoides L., Eremophila longifolia (R. Br.) F. Muell.
and Myoporum sandwichense A. Gray). Hence, the
origin of this secretory cavity is yet to be resolved.
Theisen & Fischer (2004) also described the secretory
cavities in younger leaves of some members of Myoporaceae.
The longitudinal sections of mature glandular hairs
are composed of three different types of cells – radial
terminal cells, lateral stalk cells and laterally placed
basal cells. The entire glandular hair is encircled by
cuticular layer. These features are similar to the glandular hairs of some true mangrove species such as
Avicennia sp. and Aegiceras corniculatum and a mangrove-associated taxon, Acanthus ilicifolius (Das 1999;
2002). The salt glands in all salt secreting mangroves
show some structural similarities in having basal cells,
stalk cells and a capitate group of radiating terminal
cells, and all these can be correlated to the convergent
evolutionary character among the different mangrove
taxa (Das & Ghose 1993). Fahn & Shimony (1977) studied the ontogeny of glandular and non-glandular hairs of
Avicennia marina and opined that the both types of leaf
hairs follow the same developmental pattern, at least up
to the three-celled stage. Metcalfe & Chalk (1950), in
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Leaf anatomy and leaf gland ontogeny of Myoporum bontioides
Figure 4. SEM images of the leaf. (A) lower leaf surface; (B) upper leaf surface; (C) stomata with subsidiary cells;
(D) single glandular hair, (E) transverse section of leaf showing secretory cavity; (F) longitudinal section of glandular
hair. (BC – basal cells, Cu – cuticle, E – epidermis cell, Et C – epithelial cell layer, Gl H – glandular hair, LE – Lower
epidermis, M – midrib of leaf, SC – secretory cavity, SGC – stomatal guard cell, Su C – subsidiary cell, St C – stalk cell,
TC – terminal cell, UE – upper epidermis).
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Feddes Repertorium 2013, 124, 50–60
Figure 5. TEM images of glandular hairs (transverse section of leaf) (A) young stage of glandular hair with single terminal cell, single stalk cell and two basal cells; (B) glandular hair with premature terminal cells; (C) mature view with distinct
four terminal cells, two stalk cells and two basal cells; (D) magnifies terminal cells showing cellular organelles.
(BC – basal cell, CU/Cu – cuticle, CC – collecting cavity, El – electron-dense layer, ER – endoplasmic reticulum,
G – golgi bodies, M – mitochondria, N – nucleus, ST C – stalk cells, TC – terminal cell, V – vacuole).
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Leaf anatomy and leaf gland ontogeny of Myoporum bontioides
their extensive leaf anatomical study, commented that
the structural differences of glandular and non-glandular
leaf are rather in function and not in early development.
Waisel (1972) described that the glandular hairs in halophytes lack chloroplasts and that central vacuoles are
rich in cellular organelles. The present observation is
conform with this. A common feature in leaf glandular
hairs of some monocots and some dicots is the large
space between the outer wall of the cells and the cuticle
(Oross & Thomson 1982). Campbell & Thomson (1976)
opined that this cavity apparently represents a collecting
compartment where salts, after being sequestrated by
the gland cells, accumulate prior to emission through the
cuticle. The glandular hairs of dicotyledonous plants are
multicellular and more complex than those of monocots
like some grasses (Liphschitz & Waisel 1974). The present observation is in conformity with the earlier views.
The glandular hairs are present on both surfaces of
young leaves, but disappear at leaf maturity. Karrfalt
& Tomb (1983) worked on the leaf hairs of Bontia
daphnoides (Myoporaceae) and commented that the
leaf hairs were found on the leaf bud and were absent
on unfolded mature leaves. The ultrastructure shows
that the two basal cells are embedded in the leaf mesophyll and that they are assumed to be the collecting
cells. The capitate terminal part formed by four cells is
responsible for the excretion of ions, primarily in the
space (collecting chamber) between the cuticle and
the outer wall of the terminal cells. Finally it passes outside the cuticle. Barhoumi et al. (2008) explained that
such a periplasmic space in the terminal cells signifies
that secreted ions (mineral salts) can be preferentially
transported from the basal cells, through stalk cells
and ultimately to the terminal cells (following the
apoplastic pathway). Osmand et al. (1969) opined that
the superfluous salt is collected from the mesophyll by
the basal or collecting cells and secreted by the terminal
cells of the glandular hairs into the subcuticular space
around them, and ultimately the terminal cells dry out
and salt is left on the leaf surface as a white powdery
layer.
Considering the comparatively weaker leaf anatomical features (such as a reduced amount of packed
chlorophyllous cells within the mesophyll region, devoid
of any water storage tissues, sclerotic bundle sheaths
and sclereids) in comparison to typical mangrove taxa,
the taxon diverges much from the latter. Tomlinson
(1986) opined that, in spite of assemblage of different
families, mangroves have some uniqueness and are
readily identified by their leaf anatomical features. The
total disappearance of glandular hairs on mature leaves
of M. bontioides, conceivably can be attributed to mangrove-associated plants rather than to a true mangrove
and is thus a considerably weaker adaptation for their
natural saline habitat.
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
5
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