THE EVOLUTION OF KARYOTYPE AND POLYPLOIDY IN ARBOREAL PLANTS

Summary Although relatively few biochemical studies have been carried out on woody plants, interesting correlations (and discrepancies) have been noted between nuclear DNA content and several parameters such as geographic range (latitude), ecological adaptation, nuclear volume, and karyotypic differences such as chromosome length. Hybridization between genomes with complements possessing chromosomes of different relative sizes, B chromosomes, and repetitious DNA and heterochromatin, have been reasons advanced for changes in karyotypes and nuclear DNA response. B chromosomes may not be as rare in tree species as previously considered. It may be expected that DNA‐RNA hybridization and Giemsa staining procedures will reveal differences in karyotypes between species in genera where genomes at present exhibit only moderate differences between species. While successful callus production has been induced for a number of woody species, so far no haploid trees have been produced. In Betula there appears to be little barrier to cross fertilization and plants with different euploid chromosome numbers have been obtained from seed from the same parental tree.