<p> The mode of chromosome duplication during meiosis and mitosis in Haplopappus gracilis was investigated. Tritiated thymidine was incorporated into the pollen mother cells during premeiotic interphase, and the cells were allowed to reach the tetrad stage. The autoradiographs prepared from the tetrads showed an unequal distribution of grains over their nuclei, suggesting a conservative mode of chromosome duplication during meiosis. Seedlings were fed with tritiated thymidine for the duration of one cell cycle and also for the duration of several cell cycles. The autoradiographs prepared from the root tip cells, thus treated, showed both labelled and unlabeled chromatids in the anaphases of all the experiments, thus again suggesting a conservative mode of chromosome duplication. A chromosome model to explain the results is discussed. </p> / Thesis / Doctor of Philosophy (PhD)