Biosystematic Studies In The Genus Euphorbia L. In Nigeria

ABSTRACT

Euphorbia L. is a genus of the family Euphorbiaceae. Some of the species are culturally, economically and medicinally valuable. The taxonomy of the genus was re-evaluated using classical and experimental approaches to unravel the complex inter-relationships and problems of identification existing within the genus. Herbarium studies, natural population sampling, ecological observation, experimental cultivation, morphological and anatomical characterization were some of the study approaches. A total of thirty species, four of which are new to literature in Nigeria were circumscribed. Although there were various growth habits observed, species were found as herbs, trees and shrubs. Some species possess distinct leaves while others had ephemeral leaves. Cyathium was found to be a unique feature of the genus. It was also observed that the leaves, inflorescences, fruits and stems of most herbaceous species were covered by dense, woolly, soft- matted tomentose hairs which occur singly as multicellular hairs. The inflorescences occurred either singly, in a group of three or four or as a bunch of flowers (sessile or stalked). Each flower has one or more glands attached to it. The over-all appearances of the stem provided useful taxonomic data. All taxa studied have simple, monad, tri-colporate pollen grains with radial symmetry. The anti-clinal wall patterns of the leaf epidermis were wavy, undulate or straight. Stomata was either epistomatic or hypo-stomatic.Scanning Electron Microscopy revealed surface ornamentations on the abaxial surface of Euphorbia hirta. Epicuticular wax was present and granular in all taxa except the abaxial surface of Euphorbia glomerifera. The cuticle was either smooth, ridge or striated. Also, trichomes were either present or absent; where present, they were of acicular or fili-form types which were non-glandular (either uni-serrated or multi-serrated). All the species studied have uniform venation patterns that were pinnate with a single primary vein serving as the origin for the higher order venation and craspedodromous with secondary veins terminating at the margin. These were either semicraspedodromous or mixed-craspedodromous. The cross section of the stems revealed that they were circular in outline. Variable chromosome numbers were recorded. Therefore, based on the results obtained, the Nigerian Euphorbia were separated into eight groups i. e: (a) Woody shrubs with spines (b) Succulent shrubs without spines and with deciduous leaves (c) Succulent or cacti form shrub or tree with spines and deciduous leaves (d) poorly understood un-branched geophyte (with tuberous root) (e) Dwarf perennial herbs (f) Erect annual herbs (g) Semi erect herbs or shrubs with stems and leaves that have indumenta and (h) Woody shrubs without spines. Systematics descriptions of all the taxa have been provided while dichotomous key for identifying all species has also been produced. This work has helped to solve some of the problems of misidentifications and synonymy in the genus Euphorbia.