Structure of columella cells in primary and lateral roots of Ricinus communis (Euphorbiaceae)

Horizontally oriented primary roots of Ricinus communis are more graviresponsive than similarly oriented lateral roots. The more pronounced graviresponsiveness of primary roots is positively correlated with their caps having a more extensive columella tissue than caps of lateral roots. Individual columella cells of primary roots contain 2.6 times more protoplasm than do columella cells of lateral roots. Similarly, the absolute volumes of all cellular components in columella cells of primary roots are larger than those of lateral roots. However, there are no statistically significant differences in the relative volumes of any cellular component in columella cells of primary vs lateral roots. Endoplasmic reticulum is distributed randomly in columella cells of both types of roots. Columella cells of primary and lateral roots contain numerous sedimented amyloplasts which do not consistently contact any cellular structure. Nuclei tend to be located in the middle thirds of the columella cells, and the vacuole is found in largest concentrations in the middle and upper thirds of columella cells of both types of roots. The largest protoplasmic volumes of mitochondria occur in the lower thirds of columella cells, and dictyosomes are found in similar concentrations throughout the cells. There is no significant difference in the intracellular distributions of organelles in columella cells of primary vs lateral roots. We believe that the differing graviresponsiveness of primary vs lateral roots of R. communis is probably due to factors other than the structures of their individual columella cells.