Observations of Chromosomes in Ludwigia (Onagraceae)

romosome numbers are reported for 38 of the 45 species of Ludwigia (Onagraceae) exclusive of sect.Myrtocarpus s. lat., based on a review of the literature and the study of 302 гө a from 283 naturally occurring populations from throughout the range of the genus.The basic chromosome number of the genus = 8, with no aneuploidy but extensive polyploidy.Among the 38 species f Ludwigia exclusive ye sect.оин s. lat.that have been examined сугу, {һеге аге 14 diploids (л = 8), 8 tetraploids п.т = 16), 6 hexaploids (п = 24), 1 octoploid (п = 32), and 1 dodecaploid (п = 48), together with 4 species that include both tetraploids and hexaploids, 1 that includes both diploids and tetraploids, 1 that includes hexaploids and octoploids, and 1 that includes octoploids and decaploids.In the first two groups of species with intraspecific polyploidy, we believe that the tetraploids have given rise directly by the functioning of unreduced gametes to the populations with higher chromosome numbers.Multivalents have been observed in the genus only in wo plants of tetraploid individuals of L. octovalvis and L. pilosa, and in L. uruguayensis (n -24, 40, 48).There e comm 0 has been very frequent (at least two-thirds of the species are polyploid), and the genus has attained a very widespread distribut tion, pepe in the tropics and temperate North America, in the moist habitats where it occurs.Ludwigia, a pantropical genus that is also well represented in temperate North America and has three endemic taxa in temperate Asia, includes some 80 species and is one of the largest genera of Onagraceae.It is very distinctive within this family and is the only genus of the tribe Jussiaeeae.Until the 1960s, the genera Jussiaea and Oocarpon, and sometimes also the genera Isnardia and Ludwigiantha, were conventionally segregated from Ludwigia, but more recent research and a better understanding of the phylogenetic relationships within the group has led to their grouping together within the genus Ludwigia (summary in Raven, 1963a), an alignment that now seems to have been adopted universally.The chromosomes of Ludwigia sect.Myrtocarpus s. lat.will be treated by T. P. Ramamoorthy in his forthcoming revision of that group (Ramamoorthy, in ргер.).Chromosome counts have been reported earlier for 6 species of sect.Myrtocarpus s. lat.out of a total of 32 (Gregory & Klein, 1960;Kurabayashi et al., 1962).In the ' Supported by grants to the senior author