Abstract
Cytological studies were carried out on 18 species of the genus Euphorbia, of which chromosome counts for five species are reported for the first time. The chromosome numbers in four other species of this genus are found to be at variance with the earlier reports. Of the 18 investigated species, 12 were found to be at different levels of ploidy, while 6 are at the diploid level. Abnormal meiotic behaviour (multivalents and univalents at diakinesis, laggards at A-I, sterility of pollen grains and formation of micropollen) is recorded in the two morphotypes of E. neriifolia collected from Chandigarh and Ahmedabad. Both of these exhibited the chromosome number 2n=90 but in the Chandigarh population 90 bivalents were observed in an occasional spore mother cell too at M-I. It is suggested that this taxon has probably arisen through hybridisation between E. royleana (n=60) and E. nivulia (n=30). On the basis of available data x=10 is suggested as the primary base number for the genus. It has been observed that sections Anisophyllum and Tithymallus are greatly diversified in regard to the basic chromosome number, while the section Euphorbium seems to be uniform in this respect. The role of polyploidy and aneuploidy in speciation and evolution of the genus has been stressed.
