Structure of Columella Cells in Primary and Lateral Roots of Ricinus communis (Euphorbiaceae)

Horizontally oriented primary roots of Ricinus communis aremore graviresponsive than similarly oriented lateral roots.The more pronounced graviresponsiveness of primary roots ispositively correlated with their caps having a more extensivecolumella tissue than caps of lateral roots. Individual columellacells of primary roots contain 2.6 times more protoplasm thando columella cells of lateral roots. Similarly, the absolutevolumes of all cellular components in columella cells of primaryroots are larger than those of lateral roots. However, thereare no statistically significant differences in the relativevolumes of any cellular component in columella cells of primaryvs lateral roots. Endoplasmic reticulum is distributed randomlyin columella cells of both types of roots. Columella cells ofprimary and lateral roots contain numerous sedimented amyloplastswhich do not consistently contact any cellular structure. Nucleitend to be located in the middle thirds of the columella cells,and the vacuole is found in largest concentrations in the middleand upper thirds of columella cells of both types of roots.The largest protoplasmic volumes of mitochondria occur in thelower thirds of columella cells, and dictyosomes are found insimilar concentrations throughout the cells. There is no significantdifference in the intracellular distributions of organellesin columella cells of primary vs lateral roots. We believe thatthe differing graviresponsiveness of primary vs lateral rootsof R. communis is probably due to factors other than the structuresof their individual columella cells. Ricinus communis, columella, graviperception, graviresponsiveness, roots, root cap