Taxonomic significance of foliar epidermal characters in the Caesalpinoideae Aworinde

A detailed morphological study of the leaf epidermis of some species in the genera Bauhinia Linn., Caesalpinia Linn. Daniellia Hutch. & Dalz. and Senna Linn in Nigeria was undertaken in search of useful and stable taxonomic characters. The study reveals several interesting epidermal features some of which are novel in the genera. Leaf epidermal characters such as epidermal cell types, stomata types and the presence of trichomes were constant in some species and variable in others, making them to be of great significance in determining the relationships among and within species. Stomata were amphistomatic in all the species except in Senna alata, Senna siamea and Senna siberiana which are epistomatic. The species showed variability in their stomata length, width, density and index, which was reflected in their taxonomic delimitations.


INTRODUCTION
Caesalpinoideae is a large sub-family of about 150 genera with 2200 to 3000 species of flowering plants in the order Fabales (Eddy, 1997).The genus Senna is native throughout the tropics, with a few species extending into the temperate region.Some Senna species, especially Senna alata, have been reported to be used in the treatment of skin infections.Species of Senna were formerly included among the approximately 600 species of Cassia (Irwin and Turner, 1960).Shifting taxonomic boundaries mark the history of traditional systematic treatments of Senna.These shifts are best explained by the difficult taxonomic interpretation of morphological variations in Senna.Bauhinia species are commonly found at lowland and medium altitudes in most woodland types and on anthills 150 to 1800 m high; their seeds serve as food and their shoot as vegetables.The roots and leaves of Bauhinia species are used for medicine, fodder, shade, firewood, tannin, fibres and rope; they also serve aesthetic purposes (FAO, 1983).The members of the genus Caesalpinia are planted as an ornamental as is common in gardens on the coast of East and West Africa.Some species of Caesalpinia also have extra floral nectars on their leaves or flower stalks, which attract ants (Marazzi, 2006).The species in the genus Daniellia have been reported to have medicinal properties.A decoction of leaves and bark of Daniellia oliveri is used as a refreshing lotion of bath, internally against colic and as a mouth-wash against toothache   (Irvine, 1961).
According to Stace (1965), the leaf is perhaps the most anatomically varied organ in angiosperms and it provides a variety of anatomical features that can be employed as useful taxonomic characters.Many research works have been done on the physiology, chemotaxonomy and medicinal use of the species of these genera and other genera in the family but information on the anatomy and taxonomy is still fragmentary.Therefore the aim of this work was to determine the taxonomic significance of leaf epidermis and other anatomical features with a view to further establishing a stable taxonomic character among the selected genera.

MATERIALS AND METHODS
Plant specimens used were collected in different ecological areas in Abeokuta (Ogun State), Ibadan, Sepeteri and Iseyin (Oyo State) South-western Nigeria.The collected specimens are: Bauhinia rufescens, B. tomentosa, Caesalpinia bonduc, C. pulcherima, Danielia ogea, D. oliveri, Senna acutifolia, S. alata, S. fistula, S. hirsuta, S. obtusifolia, S. occidentalis, S. podocarpa, S. siamea and S. siberiana.Identification was done at Forestry Herbarium Ibadan (FHI) and University of Ibadan Herbarium (UIH).Voucher specimens were deposited in the two herbaria.Quantitative characters assessed include leaf length and width (taken at the widest point) while qualitative characters such as leaf shape, margin, base and apex were also assessed in situ.Microcharacters such as cell wall thickness, size of epidermal cell, stomata size and index were also measured.

Preparation of leaf epidermal surfaces
Leaf epidermal morphology was studied using fresh specimens.About 5 mm 2 to 1 cm 2 leaf portions were obtained from the standard median portion of the leaves.Three to five specimens of each species were used depending on geographical spread of the species except for those known from only one or two localities.Epidermal peeling was carried out according to the procedure of Johansen (1940) as modified by Jayeola and Thorpe (2000).The peeled specimens were later stained with Safranin and counter stained with Fast green and then cleared in clove oil and mounted with DPX ® .The slides were appropriately labeled and examined under a light microscope Olympus BX51 while photomicrographs of the micro-morphological features were taken at a magnification of X400 using a photomicrograph Olympus BX 51with installed digital camera optics.
For statistical analysis, 10 epidermal cells and 10 stomata were chosen randomly for each species and measured using a micrometer eyepiece.For each quantitative character, the mean and standard error were determined for all the taxa.The stomata index (SI) was calculated based on the formula derived by Metcalfe and Chalk (1979).

S SI =
x 100 S+E Where, SI = Stomata index, S = Number of stomata per unit area, E = Number of epidermal cells in the same unit area.

RESULTS
Tables 1 and 2 and Figures 1 to 4 show the results obtained from this study while Plates 1-30 show photomicrographs of the abaxial and adaxial surfaces of the plant specimens.Generally, the leaves of the genera (Bauhinia, Ceasalpinia, Daniellia, Senna) were alternate, rarely opposite and mostly entire (Table 1).Leaves were mostly glabrous in all species except in S. hirsuta and S. alata which were pubescent.The apices were emarginate,   acute, acuminate, retuse, or apiculate.The leaflet sizes showed considerable variations within and among the genera with the largest recorded in S. fistula (14.8 cm) and the smallest in B. rufescens (0.8 cm) (Figure 1).The lowest leaf length/width ratio 1:1 was recorded in B. rufescens, B. tomentosa and S. obtusifolia (Table 1), while the highest was recorded in S. acutifolia.Trichome bases were, however, noticed in some species occurring on their adaxial or abaxial surface.The leaf epidermal cells were more often polygonal (Table 2) but sometimes irregular.Polygonal cells occurred most often on the abaxial surfaces but were also noticed on the adaxial surfaces of C. bonduc, D. ogea, S. alata, S. fistula, S. obtusifolia, S. hirsuta and S. siamea (Plates 1-30).Sometimes, polygonal cells were seen interspersed with irregular cells as seen in C. pulcherima, D. ogea, D. oliveri, S. alata and S. podocarpa.Stomata were amphistomatic in all the species and have commonly anomocytic, anisocytic and paracytic types apart from the rare to occasional occurrence of other stomata types and  abnormalities found on the adaxial surface of D. oliveri.Trichomes were absent in most of the species except for the presence of whip-like and non-glandular trichomes found on both surfaces of S. hirsuta and S. alata.

DISCUSSION
There was a wide variation in the number and distribution of stomata found in all the species.Classification of different types of stomata complexes was based on the number and position of the subsidiary cells and the ontogeny of the cell types.According to Richard et al. (2007), stomata comprise two elongated guard cells bracketing a stomatal pore, and often but not always, surrounded by one to many subsidiary cells.The presence of desmocytic stomata in D. oliveri distinguished it from other species.This was corroborated Irr = Irregular; P = polygonal; C = curved; S = straight; An = Anomocytic; Ani = anisocytic; Par = paracytic; De = Desmocytic; S/S= straight/slightly curved; -= Absent/not present.

Figure 1 .
Figure 1.Mean leaf length of some species of sub-family Caesalpinoideae.

Figure 2 .
Figure 2. Mean leaf width of some species of sub-family Caesalpinoideae.

Figure 3 .
Figure 3. Mean stomata density of some species of sub-family Caesalpinoideae.

Table 1 .
Leaf morphological features of some species of sub-family Caesalpinoideae.

Table 2 .
Epidermal characters of some species of sub-family Caesalpinoideae.