Abstract: In this study, we investigated the comparative anatomy of 17 species from 14 genera of Asteraceae occurring in Nigeria. Dried materials obtained from University of Port Harcourt Herbarium (UPH) were used for this study. The midrib of specimens were hand sectioned, stained with 1% safranin or alcian blue, mounted on slide, observed under microscope and micro-photographed using Leica WILD MPS 52 microscope camera on Leitz Diaplan microscope. The results showed that twelve species Aspilia africana, Chromolaena odorata, Conyza sumatrensis, Emilia preatamissa, Eleutheranthera ruderalis, Metanthera scandens, Synedrella nodiflora, Tithonia diversifolia, Tridax procumbens, Vernonia cinarae, Vernonia biafrae and Emilia coccinea have secretory duct while Adenostemma sp., Ageratum conyzoides, Bidens pilosa, Eclipta alba and Vernonia amygdalina not have secretory ducts. The number of abaxial and adaxial parenchymatous cells, shape of the midrib, number and arrangement of the vascular bundle varied form one species or genera to other. These characters are dependable in delimitating the family.
INTRODUCTION
The Asteraceae family consists of predominantly herbaceous but includes some woody and arboreal types (Metcalfe and Chalk, 1950; Cronquist, 1981) and has approximately 25000 species and over 1100 genera (Barroso, 1986). In West Africa, the family is represented by 84 genera and 273 species (Hutchinson and Dalziel, 1954). Members of Asteraceae exhibit considerable anatomical diversity, which is attributed to their ecological specialization and could account for their occurrence in different habitats (Metcalfe and Chalk, 1950). Among the members of this family, secretory structures are of great taxonomical interest and their restricted distribution has an important diagnostic value (Metcalfe and Chalk, 1950; Fahn, 1979; Castro et al., 1997; Solereder, 1908). Also, difference in vascular bundle types and presence of secretory cell in the phloem varied from species to species (Makbul et al., 2011). In addition the differences in the midrib shape could be used to classify some members of this family (Noorbakhsh et al., 2008).
Members of this family have a remarkable ecological and economical importance. These include ethno-botanical, phytochemical, antimicrobial and medicinal purposes (Kamboj and Saluja, 2008; Abii and Onuoha, 2011; Chono et al., 2014; Bartolome et al., 2013; Adebayo et al., 2010; Ajiboye et al., 2014; Teke et al., 2007; Toyang and Verpoorte, 2013). In Nigeria, most of these specimens used for treatment of illnesses by the Traditional Medicine Practitioners (TMP) are sort from the local markets and are collected from the wild by the locals who may not be able to identify the plants scientifically. Also, the traditional medicine practitioners in the urban areas rely on the dried specimens from the rural communities for the preparation of concoctions and decoctions before they administer same to their patients (unpublished). The need for an alternative way of identifying these specimens becomes inevitable. Therefore, this study seeks to explore the information from the midrib anatomy to complement the existing data in delimitating the species of this family.
MATERIALS AND METHODS
Seventeen species from 14 genera of Asteraceae occurring in Nigeria were investigated. Dried materials were obtained from specimens in University of Port Harcourt Herbarium (UPH) and the information about the voucher specimens are listed in Table 1.
The specimens were boiled in hot water for 30 min and the cut sections of the midrib fixed in in 70% ethanol. The specimen is trimmed, insert in wax block and hand-sectioned. Thereafter, thin sections selected, stained with 1% safranin or alcian blue, mounted on slide, observed under microscope and micro-photographed using Leica WILD MPS 52 microscope camera on Leitz Diaplan microscope. This method is modified by Metcalfe and Chalk (1979) and Cutler (1978).
RESULTS AND DISCUSSION
The results of this study are presented in Table 2 and Fig. 1. Seventeen species belonging to 14 genera were studied. Among these species, twelve have secretory Adenostemma sp., Ageratum conyzoides, Bidens pilosa, Eclipta alba and Vernonia amygdalina do not secretory ducts. The species with secretory ducts include; Aspilia africana (Fig. 1a-b), Chromolaena odorata (Fig. 1e), Conyza sumatrensis (Fig. 1f), Emilia preatamissa (Fig. 1h), Eleutheranthera ruderalis (Fig. 1i), Metanthera scandens (Fig. 1j), Synedrella nodiflora (Fig. 1k-l), Tithonia diversifolia (Fig. 1m-n), Tridax procumbens (Fig. 1o), Vernonia cinarae (Fig. 1q), Vernonia biafrae (Fig. 1r) and Emilia coccinea (Fig. 1s). This character has been used to delimit the members of this family (Metcalfe and Chalk, 1950; Makbul et al., 2011; Noorbakhsh et al., 2008; Castro et al., 1997; Fahn, 1979; Solereder, 1908). In their studies, Metcalfe and Chalk (1950) noted the presence of secretory ducts in Chromolaena, Ageratum, Vernonia and Tridax.
| Table 1: | Description of voucher specimens studied form University of Port Harcourt Herbarium |
| Table 2: | Anatomical characteristics of the midrib of the Asteraceae species studied |
Similarly, this character was observed in Chromolaena, Ageratum, Tridax and Vernonia. In addition, secretory ducts were found in Aspilia, Eleutheranthera, Synedrella, Emilia, Conyza metanthera and Tithonia. Makbul et al. (2011) used vascular bundle types, presence/absence of cavity on midrib, presence or absence of secretory cells in phloem to distinguish the members of the genus Scorzonera L. (Asteraceae) from North East Anatolia. They noted that some of the species have cavity in the midrib while others do not and that the occurrence of different vascular bundle types and presence of secretory cell in the phloem varied from species to species. Noorbakhsh et al. (2008) noted the differences in the midrib shape of Artamisia as a diagnostic character while the presence of secretory ducts and cavities were used by Castro et al. (1997), Fahn (1979) and Solereder (1908) to delimit other members of Asteraceae.
Despite the occurrence of the secretory ducts, the nature of abaxial and adaxial parenchymatous cells, the shape, number and arrangement of vascular bundles in the midrib vary among the species studied and could be used to distinguish them. For instance, the midrib of A. africana has arced vascular bundle with more than 13 traces including medullary ones. The parenchymatous cells in the abaxial and adaxial epidermises are 6-8 and 8-14 layers, respectively. Also, the midrib is elevated at the middle to form more or less acute angle with the leaf blades (Fig. 1a-b). Chromolaena odorata midrib has 5 vascular bundles (3 separate traces and 2 medullary ones) with the abaxial and adaxial parenchymatous cells 4-6 and 5-7 layers, respectively and the middle portion of the midrib elevated but flattened at the apex (Fig. 1e). Furthermore, C. sumatrensis has single trace of vascular bundle with oval shape. The middle portion of the midrib is relatively flat with the abaxial parenchymatous cells 6-7 layers and the adaxial cells 4-7 layers (Fig. 1f). On the other hand, the midrib of E. ruderalis has 3 separate traces of vascular bundles, 9-13 layers of cells at the abaxial surface and 5-8 layers on the adaxial surface. It is also elevated forming obtuse angle or convex surface (Fig. 1i). In T. diversifolia the midrib is elevated with curved apex and has 10 vascular bundles including 3 medullary ones with arced shape.
| Fig. 1(a-t): | Anatomical sections showing the transverse section of the midribs of (a and b) Aspilia africana, (c) Ageratum conyzoides, (d) Bidens pilosa, (e) C. ordorata, (f) Conyza sumatrensis, (g) Eclipta alba, (h) E. preatamissa, (i) Eleutheranthera ruderalis, (j) M. scandens, (k and l) Synedrella nodiflora, (m and n) T. diversifolia, (o) Tridax procunbense, (p) Vernonia amygdalina, (q) Vernonia cinarae, (r) Vernonia biafrae, (s) Emilia coccinea and (t) Adenostemma sp., Ep: Epidermis, Vb: Vascular bundle |
The abaxial and adaxial parenchymatous cells have 9-11 and 12-14 layers, respectively (Table 2 and Fig. 1m-n). In T. procumbens, the vascular bundle has single trace with oval shape. The abaxial and adaxial parenchymatous cells are 5-6 and 6-7 layers, respectively and the midrib relatively flat or concave (Fig. 1o). This same differences was also observed in M. scandens and S. nodiflora. The vascular bundles in M. scandens are 6 (4 close traces and 2 medullary ones) (Fig. 1j) while in S. nodiflora it is 3 separate traces (Fig. 1k-l).
The shapes of the midribs in the two Emilia species studied are relatively same however; the number of vascular bundles in the midrib could be used to distinguish them. For example, the midrib of E. preatamissa has one-trace of vascular bundle (Fig. 1h) while E. coccinea has 3-traces of vascular bundles (Fig. 1s). Among the genus Vernonia studied, the shape of the midrib, number of vascular bundles and parenchymatous layers in the abaxial and adaxial surfaces of the midrib varied from species to species. For instance, V. amygdalina has 7 separate bundles (5 large and 2 small ones) V or U-shaped, the abaxial parenchyma 10-13 layers and the adaxial parenchyma 15-20 layers while the midrib is elevated but flattened at the apex (Fig. 1p). In V. biafrae the vascular bundle has single trace and U-shaped, abaxial parenchymatous cell more than 25 layers while the adaxial surface is 14-16 layers and the midrib is elevated forming obtuse angle or curved surface (Fig. 1q) and in V. cinerea the vascular bundle has single trace, oval in shape, the abaxial and adaxial parenchymatous cells 7-10 and 6-9 layers, respectively while the midrib elevated forming acute angle or curved surface.
Considering the species without the secretory ducts, the midrib of A. conyzoides has single trace of vascular bundle, 4-5 layers of abaxial and 6-7 layers adaxial parenchymatous cells and the midrib elevated to form an obtuse angle at the apex (Fig. 1c). Similarly, in B. pilosa, the vascular bundle has one-trace, the middle of the midrib relatively flat or concave with 3-5 layers of abaxial and 3-4 layers of adaxial parenchymatous cells. In contrast, E. alba has 3 separate traces of vascular bundles and 5-7 layers of parechymatous cells in both abaxial and adaxial surfaces while the midrib is elevated but relatively flat at the apex (Fig. 1g). Also Adenostemma sp. has arced vascular bundles with 3 separate traces, 6-7 layers of parenchymatous cells abaxial and adaxial each. The middle portion of the midrib elevated forming acute angle or convex surface (Fig. 1t).
This present findings indicates that the dried specimens from these species could be identified based on the presence or absence of secretory duct, the number of abaxial and adaxial parenchymatous cells, shape of the midrib, number and arrangement of the vascular bundle varied form one species or genera to other. These characters are dependable in delimitating the family and support the previous works by Metcalfe and Chalk (1950), Makbul et al. (2011), Noorbakhsh et al. (2008), Castro et al. (1997), Fahn (1979) and Solereder (1908) on Asteraceae.
ACKNOWLEDGMENT
The authors wish to acknowledge the contribution and assistance of all the members of the University of Port Harcourt Herbarium.