STUDIES ON THE MORPHOLOGY OF POLLEN GRAINS OF THE LEGUMINOSAE-THE MIMOSOIDEAE

The morphology of pollen grains of 23 species of the Mimosoideae in Ghana as seen under the light microscope is described. Six ofthe species had solitary grains (monads) while the remaining 17 species had compound grains (polyads) with 4, 8, 16, 20, 24, 28 or 32 grains, depending on the species. Features such as differences in pollen grain size, arrangement of individual grains in the polyads, apertural conditions, wall thickness and wall sculpturing varied sufficiently for most ofthe species to be recognized by their pollen morphology.


Introduction
No research on pollen has been reported in Ghana despite the importance of pollen characters in plant taxonomic and phylogenetic studies.There are some recordS"of relevant studies in other West African countries.For example, before 1973, there existed only short descriptions of pollen grains of Nigerian species scattered over several journals (Thanikaimoni. 1972).A major work was published almost 20 years ago by Sowunmi (1973) on pollen grains of 150 woody plants of Nigeria belonging to 47 families.Studies in other countries in the West African sub-region will complement the work in progress on the Nigerian flora.

Treatment of pollen grains
Studies were based on both acetolysed and nonacetolysed materials.Acetolysis was according to Erdtman (1952).The acetolysed material was chlorinated (Erdtman, 1952), mounted in glycerine jelly and the slides sealed with nail varnish.Fresh grains (non-acetolysed) were mounted in aceto-carmine glycerol jelly (Mark, 1954).Pollen measurements were made under x 40 and x 100 objective lenses using a calibrated graticule.Measurements of pollen grains and polyad size, pore size and wall thickness were based on 40 grains selected randomly GHANA JOURNAL OF SCIENCE from five slides from each species.Pollen terminology is after Erdtman (1969).

Results and discussion
The pollen grains of the 23 species showed a wide range offorms: monads (single grains) and polyads (compound grains) of 4,8,12,16,20,24,25 and 32 grains.While the individual grains of the polyads in some species were easily separable, those of other species adhered so firmly that they were not separated by acetolysis.In the case of the latter, accurate measurements' could only be made on the entire polyads.

Acacia angustissina (Mill.) Kuntze (Fig.] c). Fresh material, University Research Station, !-egon, Accra
Pollen shed in octads, circular to ovoid in shape; mean widest diameter 25.92 ± 2.05 !lm (range 22.4 -28.8 !lm).Grains in a regular arrangement of 4 and 4 in two planes.One directly above the other.Grains failed to separate and, therefore, measurements could not be taken on individual grains.
Exine of exposed walls about 2.0 /lm thick, that of inner walls thinner; sexine slightly thicker than nexine; sexine reticulate.The observations made in this study agree generally with those of Sowunmi (1973).
Exine about 3.2 /lm thick on distal side, proximal side thinner; sexine and nexine of equal thickness; sexine psilate.Apertures were not discernible in this material.

Calliandra surinamensis Benth. (Fig. I d). Fresh material. Legon, Accra
Pollen shed in club-shaped octads, mean widest diameter 166.4 ± 6.56 JlID(range 156.0 -176.0JlID), grains in a regular arrangement: 2 grains in the centre, surrounded by 6 peripheral grains.Octads provided with a stalk with a basal disc which probably attaches to insect pollinators.Grains failed to separate and, therefore, measurements could not be taken on individual grains.
The stalk with basal disc of C. surinamensis is probably similar to that of lnga anamala described by Mohl (1835).Woodhouse (1935) suggested that the octad with stalk and disc is the highest development attained among the compound grains of dicotyledons.
Exine about 3.5 11m thick, sexine thicker than nexine except at the pores where nexine is distinctly thicker (crassinexinous); sexine coarsely granular.Ora circular in shape.The observations recorded here generally agree with the short description provided by Erdtman (1952) for the species.
Observations made in this study agree largely with those of Woodhouse (1935) and Erdtman (1952) for P. glandulosa except that the 2-colporate grains observed by Erdtman were not seen in this material.

Conclusion
The species investigated here illustrate the pollen morphological variation in the Mimosoideae.The greatest variation occurs in pollen unit types which ranged from monads to polyads of32 grains.
Interspecific variation in the number and arrangement of individual grains in the polyads was observed in Calliandra and Acacia.In Calliandra, spherical polyads of 16 grains were observed in C. portoricensis whereas in C. surinamensis, the grains were arranged in club-shaped octads.Similarly, spherical polyads of 16 grains were observed in Acacia karroo; A. nilotica var.adansoni, A. nil{)tica var.tomentosa and A. polyacantha subtp.campylacantha.The pollen A. angustissima, on the other hand, occur in spherical octads.The findings underscore the taxonomic importance of pollen morphological variation.
The pollen morphological survey of the Mimosoideae by Sorea (1969) in which 202 species representing 50 genera were studied is a major contribution to the systematic and evolutionary arrangement in the subfamily.The pollen grains were classified into five groups on the basis of established palynological trends and these in turn into 25 pollen types.He regarded the five pollen groups as five different evolutionary branches.He concluded that the evolutionary trend in pollen grains has been from colpate types through colporate to porate types; from relatively thin and simple exine to thicker exines with distinctly differentiated layers and sculpturing; from small-sized grains to larger ones; and from monads through tetrads to polyads with increasing number of grains.
By this classification, all the five groups were represented by species studied in this work.The gen-' era Aubrevillea, Entada, Leucaena, Neptunia and Prosopis fall in Group I, characterized by solitary, corporate grains with granular exine.Group 2 is made up of some species of only one genus-Calliandra -which produces club-shaped octads.The group is represented by C. surinamensis.Group 3 is made up of both monads and polyads; the monads of this group are supposed to be more advanced than those of Group I.They have thicker and more elaborately sculptured exine and are porate.The member studied was Dichrostacys cinerea.Grains of Groups 4 and 5 are polyads.Members of Group 4 generally have small-sized polyads as exemplified by the genera Calpocalyx, Mimosa, Schrankia and Xylia.Group 5, characterized by larger polyads, was represented by the genera Acacia, Albizia,, Pithecellobium and Samanea.
A certain degree of intraspecific variation in pollen morphology was noted in some cases.This in-volved polyad types, shapes and sizes e.g.Samanea sam an and Tetrapleura tetraptera, as well as shape and apertural condition of individual grains as in Dichrostachys cinerea.
Such intraspecific variation was also noted by Sowunmi (1973).This observation supports the remark by Guinet (1962) based on his study of the pollen of tropical plants in Asia that such variability seems widespread in the tropics.It is, therefore, important that in pollen studies, attention is paid to probable intraspecific variation so that variants of one species are not inadvertently assigned to two or more different species.