Comparative Morphology, Anatomy and Palynology of Three Genera of the Apocynaceae in Nigeria

Abstract

A study of the morphological, anatomical and palynological characters of Allamanda cathartica L., Thevetia neriifolia Juss. ex Steud and Nerium oleander L. from three ecological zones was undertaken. The study areas were Otukpa (Guinea Savannah vegetation), Nsukka (Derived Savannah vegetation) and Warri (Tropical Rainforest vegetation). Fifteen accessions of three specimens from the different parts of the three genera studied were collected randomly from each of the vegetation zones; examined in details and compared. Observed differences among them at P ˂ 0.01 included the vegetative and reproductive parts, pollen morphology and anatomical characters. Their reproductive parts were very much similar with the exception of few variations. They were all monoecious, having large flowers with trumpet-shaped, pentamerous corolla tubes with imbricate aestivation; pentamerous and aposepalous calyx; numerous fluffy filaments with lancelike stamens; single style; perigynous floral leaves; parietal placentation and albuminous seeds. The differences in their reproductive parts were observed in the carpels, fruits, flower size and fragrance. Wider and longer leaves (mean value of 4.46 ± 0 .179 and 15.05 ± 1.085 respectively), longer petioles (mean value of 0.87 ± 0.034), longer internodes (mean value of 15.87 ± 0.616), larger flowers (mean value of 8.87 ± 0.133) were recorded in A. cathartica (Table 11). On the other hand, T. neriifolia had narrower and shorter leaves (mean value of 1.06 ± 0.014 and 9.55 ± 0.582), shorter petioles (mean value of 0.31± 0.021), shorter internodes (mean value of 1.92± 0 .047) and shorter flowers (mean value of 5.88 ± 0.124) (Table 11). The three genera were also compared with respect to their degree of apomorphy or plesiomorphy. A. cathartica was found to be slightly more plesiomorphous than N. oleander which are a little more pleisiomorphous than T. neriifolia (Tables 8, 9 and 10). In other words, T. neriifolia was more advanced among the three genera. Secondly, the anatomical features of A. cathartica, T. neriifolia and N. oleander as observed in TS differed greatly with respect to vessel density, vessel length and vessel tangential diameter as well as number of vessels per multiple. The highest number of vessels per field of view at x400 (65.33±0.662) was recorded in the root of N. oleander collected from the Tropical Rainforest vegetation (Table 14). A. cathartica which showed vessel dimorphism had fewer, longer and wider vessels across the three zones with the length and vessel tangential diameter of the vessels increasing with wetter conditions while the density of the vessels decreased (Table 14). The highest in length of vessel (8.55 ± .565) and tangential diameter (6.76 ± .699) was recorded in the roots collected from the Tropical Rainforest vegetation while the least in vessel density (17.12 ± 0. 210) was recorded in the leaf of A. cathartica from same Tropical Rainforest vegetation (Tables 15 and 16). There was no significant difference in the vessel densities of mature and young stems of the three genera at P ˃ 0.05 (Table 14). The parenchyma type was paratracheal while the ray type was heterogenous as the three had both uniseriate fusiform and multiseriate ray initials in their TLS (Tables 11-14, 20-24, 30-33). They have both procumbent and upright rays in their RLS (Tables 12, 20-21, 31). The three genera also, had paracytic stomata on their abaxial surfaces, and the stomatal density decreased with drier climatic conditions (Plates 10, 19, 29 and Table 19). The fibres from the stems and roots of the three genera were non-septate, libriform and thin-walled with the exception of T. neriifolia roots which had thick-walled fibres (Plates 17, 27, 36). The stem of T. neriifolia had the longest fibres with the highest (0.2557 ± 0.9152) recorded in the specimen from Guinea Savannah while the highest value (0.1766 ± 0.5412) of fibre length in the roots was recorded in A. cathartica equally from Guinea Savannah zone (Tables 17 and 18). The study also revealed the potential of the three genera for paper making due to the high pulpabilty of their woods as indicated by their fibre characteristics. Although, the biomass of the three genera is low compared to other wood fibre species, their fast growth rate is an added advantage. The data from the pollen analyses showed significant differences at P ˂ 0.01 in the lengths of equatorial axes and colpi, breadth of colpi and diameter of pores and exine thickness while the pollen form indexes and lengths of polar axes did not show any significant difference in the three genera (Table 20). The pollen grains of A. cathartica and T. neriifolia had tricolporate aperture with striate exine sculpturing (Plates 18, 28, 37). A. cathartica pollen was found to be prolate spheroidal in shape whereas that of T. neriifolia was oblate spheroidal. N. oleander pollen showed triporate apertures in Derived Savannah and Tropical Rainforest vegetations and tetraporate aperture in Guinea Savannah vegetation (Plates 18, 28, 37). The shape of its pollen was spherical with indistinct exine sculpturing.