Plant classification has been a subject of discussion among plant taxonomists
and systematists over the years. Plants are classified and reclassified as soon
as new evidences arise and this is going to be a continuous exercise over some
years to come. Most plants are classified based on external morphological structures
such as flowers and fruits. These structures are not always available on plants
because they are seasonal in production. Due to this reason, other means of
classification need to be involved; one of which is anatomical studies especially
of leaves (Davis and Heywood, 1963).
The epidermis is the outer layer of cells covering the leaf. It has several
functions: protecting the plants against water loss by transpiration, regulation
of gaseous exchange, secretion of metabolic compounds and absorption of water.
Most leaves show dorsiventral anatomy: the upper (adaxial) and lower (abaxial)
surfaces have somewhat different construction and may serve different functions
(Hardie, 2009). It can also be used as taxonomic character
delimiting plants. Some characters or features on the epidermis which are useful
taxonomically include several differentiated cell types: epidermal cells, subsidiary
cells, guard cells and epidermal hairs (i.e., trichomes). These features have
been used previously to resolve some taxonomic problems or to contribute to
ever increasing taxonomic data base in some genera and even families of plants
(Watson, 1964; Van Wyk et al.,
1982; Ren et al., 2007; Saheed
and Illoh, 2010). Continuity and/or discontinuity of the characters can
be used to show relationships and differences between plant taxa within a genus
or between families.
Morphologically the Cucurbitaceae are mostly prostrate or climbing herbaceous
annuals comprising about 90 genera and 700 species that are further characterized
by commonly having 5 angled stems and coiled tendrils. The leaves are alternate
and usually palmately 5-lobed or divided; stipules are absent. The flowers are
actinomorphic and nearly always unisexual. The perianth has a short to prolonged
epigynous zone that bears a calyx of 3-6 segments or lobes and 3-6 petals or
more frequently a 3-6-lobed sympetalous corolla. The androecium is highly variable,
consisting of basically 5 distinct to completely connate stamens that frequently
are twisted, folded or reduced in number. The gynoecium consists of a single
compound pistil of 2-5 carpels, generally with one style and as many style branches
or major stigma lobes as carpels and an inferior ovary with one locule and usually
numerous ovules on 2-5 parietal placentae or 3 locules with numerous ovules
on axile placentae. The fruit is a type of berry called a pepo (Jeffrey,
To complement the above morphological description, leaves of fourteen species in three plant families namely Cucurbitaceae were studied anatomically to elucidate their taxonomic potentials with aim of using features such as stomata, epidermal cells and trichomes to diagnose and classify various taxa in the family.
MATERIALS AND METHODS
Leaves of 14 species of Cucurbitaceae family were harvested for anatomical
study. Leaf cuticles were macerated in concentrated nitric acid or trioxonitrate
(v) acid, rinsed in distilled water, stained in 1% aqueous safranin solution
and mounted in dilute glycerine. Parameters on stomata determined were stomatal
density, stomatal index and stomatal size (Dilcher, 1974)
and frequency of each complex type was expressed as percentage occurrence of
such complex type based on all occurrences (Obiremi and
Oladele, 2001). Epidermal cell size was determined as product of length
and breadth of a cell based on a sample size of 35. Index of similarity of leaf
surface followed the formula of Philips (1959). Statistical
analysis of data consisted of Analysis of Variance and Duncan Multiple Range
Test (Duncan, 1955).
The 14 species studied can be categorized into 2 groups namely (1) those
in which stomata are absent on the adaxial epidermis or occur on abaxial surface
only. These include Cocccinia barteri, Coccinia grandis, Cucumis sativus,
Momordica charantia, Telfaria occidentalis and Trichosanthes cucumerina
and (2) those in which stomata occur on both adaxial and abaxial leaf surfaces.
These include Citrullus lanatus, Citrullus colocynthis, Cucumis
melo, Lagenaria breviflorus, Lagenaria siceraria, Luffa
acutangula, Luffa cylindrica and Momordica foetida. Six types
of stomatal complex were observed in the family. These are namely paracytic,
diacytic, anisocytic, actinocytic, cyclocytic, staurocytic (Fig.
1, Table 1). All of them occur with a frequency of 100.
The range of stomatal density was from 17.57 mm-2 in Momordica
foetida to 870 mm-2 in Cucumis melo. Stomatal size also
varied from 10.00 μm2 in Cucumis melo to 64.00 μm2
in Trichosanthes cucumerina (Table 1). The pattern
of anticlinal wall, epidermal cell shape, epidermal cell size and distribution
of trichomes are as indicated in Table 2 and 3.
Cucumis melo stands out as a species with high stomatal density of 800
mm-2 and above on both leaf surfaces possibly due to the small size
of the stomata while Cucumis sativus has a low stomatal density of 45
mm-2 and stomata occurring on the abaxial leaf surface only.
Momordica foetida has stomata on both leaf surfaces while Momordica charantia
has stomata on abaxial surface only.
| Table 1: List of the plant species studied
| Table 2: Leaf epidermal features in some selected members
|Means with same letters are not significantly different at
| Table 3: Stomatal and trichome features in some selected
members of Cucurbitaceae
|Means with same letters are not significantly different at
Lagenaria breviflorus is characterized by paracytic and diacytic stomata
and Lagenaria siceraria by paracytic and staurocytic stomata. Telfairia
occidentalis is characterized by paracytic stomata and Trichosanthes
cucumerina by diacytic stomata.
Indented dichotomous taxonomic key is hereby constructed to assist in the process of plant identification using anatomicalfeatures:
|Fig. 1: Leaf surface view of (a) Coccinia barteri showing
actinocytic stoma, (b) Coccinia grandis showing actinocytic stoma,
(c) Citrullus lanatus showing anisocytic stoma, (d) Citrullus
colocynthis showing diacytic and cyclocytic stomata, (e) adaxial, (f)
abaxial, Cucumis melo anisocytic stoma, (g) Cucumis sativus showing
paracytic stoma, (h) Lagenaria breviflorus showing diacytic and paracytic
stomata, (i) adaxial, (j ) abaxial), Lagenaria siceraria showing
staurocytic and paracytic stomata, (k) adaxial, (l) abaxial), Luffa acutangula
actinocytic stoma, (m) Luffa cylindrica showing actinocytic
and staurocytic stomatam, (n) adaxial, (o) abaxial), Momordica charantia
showing diacytic stoma, (p) Momordica foetida showing actinocytic
stoma, (q) Telfairia occidentalis paracytic stoma and (r) Trichosanthes
cucumerina showing diacytic stoma
Anatomical evidences observed in the 14 species of the family Cucurbitaceae
(Table 1) seems to be enough diagnostic values for delimiting
species and genera studied. Plant anatomy has been found to be very essential
in plant taxonomy; the purpose is to develop a system of classifying plants
in a way that all the differences and similarities are set out in ordered manner
(Olorode, 1984). In spite of the fact that vegetative
and floral characters are markedly modified in relation to the habitat and pollination
mechanisms, the preceding observation and the summaries of character variation
in Table 2 indicated that the taxonomic application of the
diversity of epidermal morphology in Cucurbitaceae cannot be over emphasized.
The decision to choice epidermal characters for this study was informed by earlier
declaration that these characters represented genetic variations and have been
used to solve taxonomic problems in certain other plant groups by taxonomists
(Srivastava, 1978; Oladele, 1990;
Adegbite, 1995; Nwokeocha, 1996;
Ogunkunle and Oladele, 1997; Ogunkunle
and Oladele, 2008; Abdulrahaman and Oladele, 2010a;
Abdulrahaman and Oladele, 2010b).
It is worth note the presence of curved ACWPs on both leaf surfaces of L. acutangula and L. cylindrica. The ECWSs are polygonal on both leaf surfaces in all 14 species of Cucurbitaceae except in C. barteri, C. sativus, L. breviforus and M. charantia and in C. melo, L. siceraria and T. cucumerina where, they are irregular on one surface and on both surfaces respectively (Table 3). The SD in Cucurbitaceae was very large in C. melo (821 and 870) on both leaf surfacesand smaller in M. foetida (17.57 μm) in adaxial surface (Table 3).
Trichome is another taxonomic significance feature in the species of the three
families. Occurrences of trichomes on the abaxial surface of all the species
may support the claim that trichomes themselves transpire. According to Shields
(1950), living trichomes which themselves lose water do not protect the
plants from excessive transpiration as do dead trichomes which form protective
layers. Similar observation was made in two species of Jatropha by Abdulrahaman
(2009) where, J. gossypifolia with high trichome density and index
transpired faster than J. curcas which possessed lower trichome density
Stomatal frequency varies from one leaf to the other and these were observed
among the species in the family studied. Homogeneous nature of stomatal complex
type on leaves of the species of Cucurbitaceae is responsible for 100% occurrence
of Stomatal Complex Types (SCTs) where, a single SCT present on a leaf surface.
Stomatal Density (SD) according to Esau (1977) was found
to vary from one plant to another. All species of Cucurbitaceae studied possessed
more SD in the abaxial surface than in the adaxial surface. This pattern is
in corroboration with Oyeleke et al. (2004),
Abdulrahaman (2009), Abdulrahaman
and Oladele (2009) and Saadu et al. (2009).
SI and SS are higher (ranging from 22.73-26.91%) and larger in the abaxial surface
than in the adaxial surface in most species of Cucurbitaceae where, stomata
occurred on both surfaces of leaves (Table 3). According to
Esau (1965), stomata index varied on different part of
the leaves or on different leaves of same plants. While Davis
and Heywood (1963) considered stomatal size to be too variable as diagnostic
features other researchers like Wilkinson (1971) had
contrary view. He indicated that stomatal size shows a much wider range in some
taxa than in others, as it may be sometimes be useful as diagnostic character
when dealing with taxa in which size ranges are restricted. This present study
conforms to Wilkinsons observation.
The above leaf epidermal features could be said to be taxonomically significant because of discontinuities that occurred within and between genus, genera within the family as shown on Table 2 and 3 and Fig. 1. The leaf epidermal features observed in all the fourteen species of Cucurbitaceae are enough taxonomic characters which could be implored to support hitherto external morphological characters used to classify plants in this family.
Based on epidermal features, some members of the family Cucurbitaceae can
readily be distinguished from one another. Such epidermal features as on stomata
and trichomes are thus useful tools for diagnostic and taxonomic works. The
approach can be put to use in herbal medicine and forensic science where, identification
and authentication of plant specimens are essential.
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