Abstract
The paper deals with cytological investigations on twenty-four species and varieties of Bauhinia, Phanera and Piliostigma. Both meiotic and detailed karyotype studies were done using isopsoralene, as a pre-treatment agent. In majority of the species the chromosome number is 2n=28 excepting P. malabaricum. In this species both n=14 and 21 chromosomes have been observed. On this basis the allopolyploid nature of the polyploid forms has been suggested.
Whether 14 or 7 chromosomes should be the basic set of these genera is debatable. All chromosome numbers as yet recorded are multiples of 7. The occasional occurrence of secondary association of bivalents in certain species, namely Bauhinia diphylla, shows that the number 14 may be a derived one.
All the species of Bauhinia, Phanera and Piliostigma show a gross similarity in the nature of karyotype, having rather short chromosomes with gradation in size. Due to the marked resemblance in karyotype, these three genera apparently represent a homogeneous assemblage and their inclusion within the same genus Bauhinia under Bentham and Hooker's system appears to be justified. Minute karyotype differences exist between different species and varieties suggesting the role of structural alteration of chromosomes in the evolution of species. Variations in chromosome number and chromosome morphology have been recorded in many species. These variations suggest that speciation and evolution in these species have been made possible through them being brought into effect through vegetative propagation. Drastic structural alterations have not been observed possibly due to the small size of the chromosomes.
The histogram reveals a similarity in the chromatin content in all the species, indicating the homogeneity of the taxa and suggesting that structural alterations have principally involved rearrangement of parts and not deletion or duplication of segments. These genera, being horticultural ones, have been continually subjected to judicious selection and homozygosity has also been attained for structural alterations.
P. M. C.'s also show a large number of extra-chromosomal droplets of stainable bodies in the cell. Being carmine-positive they may have a nucleic acid constitution. The reason for their formation through disturbance of DNA:RNA ratio has been discussed.
Definite evidences of heterozygosity namely, inversion bridge, have been observed in Bauhinia diphylla. Such inversion heterozygosity as noted in Bauhinia is principally maintained through extensive propagation by cuttings in horticulture. The possible reasons for the occurrence of polyspory and its fate have been discussed.
