Taxonomic significance of morphology and palynology of two species of Dracaena found in south eastern Nigeria

Morphology and Palynology were carried out on Dracaena mannii Bak and Dracaena arborea Link various parts. T test was used for the analyses. Morphologically, the leaves of D. arborea are longer and wider than those of D. mannii, and the length of the internode of D. arborea is more than those of the D. mannii. Both species are trees and have the same inflorescence, floral, fruit and seed Morphology. Palynologically, the pollen grains of the two species are monocolpate with scabrate surfaces. The colpus in D. mannii is longer than that of D. arborea. The result also showed that there was no significant difference in the sizes of the pollen grains, the pollen form indices as well as length and diameter of the colpi of both D. arborea and D. mannii. The overall analyses showed no significant statistical difference in the morpology and palynology of the two Dracaena species. The implication is that the two species are closely related and this justified their placement under the same genus Draceana while the slight differences between them suggest the reason for their separation into different species.


Introduction
The genus Dracaena L. belongs to family Agavaceae. The genus Dracaena consists of about 50 -80 species native primarily to the old world tropics (Judd et al, 1998). According to Waterhouse (1987), Venter (1996), Dracaena consists of about 40 species, and to Huxley (1992), it consists of 50 species. Sharma (1993) and Dutta (2003) (Waterhouse, 1987). Dracaenas are used as ornamentals, medicinal plants, in photo engraving, in research, as hedge plants, colourants, etc. In Europe and Canada, they are cultivated and sold as ornamentals, (Huxley, 1992). Dracaena arborea Link. and Dracaena mannii Bak. which are commonly found in south eastern Nigeria are the points of interest in this research. A system of classification of biological organisms which best reflects the totality of their similarities and differences is termed taxonomy, (Gill, 1988). To classify plants, taxonomists make use of morphology, phylogeny, physiology, phytochemistry, anatomy, cytology, palynology etc. as taxonomic lines of evidence to determine their similarities and differences in order to group them into various taxa. Dracaena mannii and Draceana arborea are grouped into the genus Draceana based on their similarities and into different species based on differences. According to Radford, (1986), the morphological data of plants are easily observable and obtainable, and are "thus used most frequently in taxonomic studies". The evidence from external morphology provides the "basic language for plant characterization, identification, classification and relationships" (Radford, 1986). Morphological features of plants are those external diagnostic features of plants (Philipson, 1971). These morphological features of plants according to Sharma (1993) include: vegetative characters, phonological characters, floral characters, seeds and fruits morphology etc. Palynology is the "study of pollen and spores'' (Judd et al, 1999 andSharma, 1993). Palynological characters have been used in solving several taxonomic problems, including the repositioning of several disputed taxa, and interpretation of problems relating to the origin and evolution of different groups. Determination of these differences and similarities with regards to morphology and palynology of the two species based on the outcome of the study were the objectives of this research.

Sources of materials
Leaves, stems, roots, flowers, seeds and fruits of Dracaena species were collected from Nsukka town (N06 0 .86. 43.5 and E07 0 .42.56.0) in Nsukka, Nsukka Local Government Area Enugu State, Nigeria. The Dracaena Species were authenticated at Bio-diversity Development and conservation, Nsukka, where the voucher specimens were deposited.

Morphological studies
The vegetative studies on the two Dracaena species collected was done by visual observation and use of hand lens. Leaf morphology (leaf length and leaf width) stem morphology (internode length and stem colour), floral, fruit and seed morphology were also analyzed.

Palynological studies
Pollen extraction and concentration Polleniferous materials were extracted from the flowers of D. mannii and D. arborea with the aid of a forcep and alcohol. Each sample was sieved through 200u-mesh copper wire guaze to filter off other large floral particles. The samples were then centrifuged at 2000 revolutions per minute (rpm) for three minutes. The supernatant was decanted and residue retained. The residue (ppt.) was washed three times with water and centrifuged in order to recover the polleniferous residue.

Acetolysis
This is a process employed in the preparation of the palynomorphs for microscopic analysis. The acetolysing agent is a mixture of concentrated tetraoxosulphate (VI) acid and acetic anhydride (1: 9) (Erdtman, 1934). The steps involved in the treatment of the residue are as follows: 2-3ml of the acetolysis mixture was added into each of the residues. This was followed by heating in a water bath for five minutes at 100 0 C. The samples were each washed with glacial acetic acid and three times with distilled water. Each wash was followed by centrifugation and decanting in order to recover the polleniferfous residue. The acetolysed samples were each transferred into specimen (vial) tubes and 5ml glycerol/alcohol was added and stored for analysis.

Microscopic analysis of temporary slide
2ml of each sample were placed on a microscopic slide and covered carefully with a 22mm x 22mm cover slip. The mount was then sealed off around the edges with nail varnish to avoid dessication and movement of the cover slip. The slides were each examined microscopically and the palynomorphs identified with the use of "Leica Gallen" 11 1500008KX Microscope at x 400 magnification. The x100 magnification (oil immersion) was used for the detailed morphological study of pollen grains. Measurements were taken with the aid of ocular micrometer and photomicrographs were made.

Statistical procedure
The results were analyzed using t-test and results were presented in mean ± SD.

Results
The results of the study were presented below.

Discussion
From the research carried out, there is significant difference in the external morphology of Dracaena arborea and Dracaena mannii. Both species are trees, however, D. arborea has smooth bark with white patches, with branches of older trees having prop roots while D. mannii tree bark is deeply fissured longitudinally and lacks prop roots( fig.2 a and b respectively). Significant statistical difference exists between the lengths and widths of the leaves as well as the distance between the nodes of the two species at P= 0.05. D. arborea had the longest leaf of 156cm ±13.57 in length, broadest leaf of 12cm ±5.35 and an internode length of 4.60cm±2.19. The inflorescence, flower, fruit and seed morphology of both plants are the same as shown by the result of the study.
With the exception of the fruits of both species being drupes, the result of the inflorescence, flower, fruit and seed morphology of both species tally with an earlier study by Watson and Dallwitz (1992) in which they outlined the external morphology of Dracaenas as follows: Habit and leaf form: shrubs, or arborescent; leaves small to very large, alternate, spiral, herbac eous or leathery or fleshy, with free margins. Leaves simple; lamina entire, linear or lanceolate; parallel-veined, without cross venules, internode present (short). Inflorescence, floral, fruit and seed morphology: flower aggregated in inflorescences, in racemes, in umbels or in panicles. The terminal inflorescence unit is racemose. Flower s minute to large, regular, 3 merous, cyclic or pentacyclic; perianth of tepals 6 and joined, 2 whorled, isomerous petaloid (similar in the two whorls); unisexual flowers absent; plant hermaphrodite; androecium 6; androecial members adnate (at the bases of the tepal lobes), all equal, free from one another, 1 whorled or 2 whorled; androecium exclusively of fertile stamens; stamens 6, dipostemonous, anthers dorsifixed, dehiscing via longitudinal slit; gynoecium 3 carpelled; carpels isomerous with the perianth; the pistol 3 celled; gynoecium syucarpous, superior; ovary 3 locular; gynoecium stylate; styles 1, apical; stigmas 1, trilobite or capitates; placentation axile; ovules 1 per locule; fruit usually fleshy or non-fleshy, indehiscent, usually a berry(typically red or orange), or a capsule or capsular-indehiscent(usually woody); seeds endospermic; endosperm oily; cotyledon 1; embryo straight. The result of the pollen analysis of the two species showed that there was no significant difference in the: sizes of the pollen grains of species, their pollen form indices as well as the length and diameter of their colpi. Both of them are monocolpate. The surfaces of their pollen grains are scabrate. Sizes of their colpi differ in length and diameter. The colpus in D. mannii is longer than that of D. arborea.

Conclusion
The overall result of the study showed that there is no significant difference in the morphology and palynology of D. mannii and D. arborea. In other words, the two species are closely related and this justifies their placement in the same genus Dracaena. The slight differences existing between them also justify their separation into different species.