MORPHOLOGY IN RELATION TO TAXONOMY

            A. INTRODUCTION

            Morphological characters of the plant have proved fundamental and framework of taxonomy. Most of the taxonomy evidence is generally drawn from the external morphological characters. According to Radford (1986), the morphological data of plants are easily observable to obtainable, and are thus used most frequently in taxonomic studies. The evidences from the external morphology, provides the basic language for plant characterization, identification, classification and affinities.

            The basic similarity in the morphology of leaves, flowers, fruits and seeds within different species, genera, families and orders provides majority of the characters used in the construction of systems of taxonomy. Therefore, morphology is closely related to taxonomy, and taxonomy still rely, to a very large extent on morphological characters mainly because they are easily visible and can conveniently be employed in the classification.

            B. BASIC MORPHOLOGICAL CHARACTERS USEFUL IN TAXONOMY

The specific examples showing the relationship closeness on the basis of morphological point of view are as follows –

1. Growth habit like herbs, shrubs, trees, etc. may be variable or constant within a genus or a family. Example – All the members of Brassicaceae family are herbaceous as well as woody.

2. Growth patterns have been used in defining taxonomic groups above the generic level in some species such as Zingiberaceae.

3. Seedling characters such as germination, cotyledon characters, leaf dimorphism, etc. have been of taxonomic value in the family like Convolvulaceae, Brassicaceae, etc. Tubers are helpful in the taxonomy of Dioscorea spp. and Cyperaceae.

4. Leaf characters such as arrangement, type, form, and venation characters are extensively used in both the classification and identification. The specimens in many genera such as Ulmus and Betula are delimited only on the basis of leaf characters. Species of Dalbergia are distinguished on the basis of their leaflet size, shape and arrangement on the rachis. In Trifolium, the species are separated on the basis of stipules morphology. Leaf venation is more carefully studied by paleobotanists and used in the description of fossils. Numerous venation patterns have been described in the leaves of Glossopteris by Pant (1958). Cinnamomum, a highly polymorphic genus, has been divided into three groups based on the venation patterns (Baruah and Nath, 2002).

5. Inflorescence and flowers are the two main organs based on which the classification system and other principles of taxonomy are established. Species are distinguished on the basis of calyx types in Anthyllis and shape and degree of inflation of bracts in Calystegia. Floral discs are of great diagnostic value in Brassicaceae. Species are distinguished on the basis of staminodes in Scrophularia, anthers in Eucalyptus and degree of branching of inflorescence in Nepetia.

6. Fruit characters are used in distinguishing different families of Rhoeadales (Papaveraceae, Capparidaceae, Brassicaceae, Moringaceae, etc.). Genera can be distinguished on the basis of fruit characters.

            7. Seed characters have been widely used in the construction of diagnostic keys and in the distinguishing species. Features of seeds, such as colour, shape and sculpturing prove helpful in distinguishing species of Drymaria of Caryophyllaceae. The number and shape of the seed is the only reliable distinction between Anthericium and Chlorophytum of Liliaceae. Two genera of Brassicaceae like Sisymbrella and Nasturtium are distinguished on the basis of presence or absence of mucilage on the testa of seeds. Presence or absence of mucilage on testa of seed distinguishes species of Ocimum (Lamiaceae) to some extent. Several species and genera of Acanthaceae, Asclepiadaceae, Convolvulaceae and Malvaceae are distinguished on the basis of their seed characters.

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