By means of the electron microscope a study has been made of the changes in organelle distribution in the cells of the root cap of Vicia faba seedlings resulting from a 20 minute geotropic stimulation in the horizontal position. From photographs of whole cells at constant magnification, counts were made of the numbers of amyloplasts, mitochondria and dictyosomes in each of four equal quarters of the cell divided along, and transverse to, the main root axis. The volumes of the characteristic endoplasmic reticulum masses per cell quarter were also measured.
The only bodies showing marked sedimentation to the lower side of the cell were the amyloplasts. The dictyosomes showed a small but statistically significant displacement into the quarter not occupied by the amyloplasts but since their concentration per unit of cytoplasm did not change, it was inferred that they moved passively in the cytoplasm displaced by the sedi-menting amyloplasts. Similar trends were shown by the mitochondria but these did not reach 5% significance levels. There also appeared to be significant displacement of the endoplasmic reticulum masses into the upper halves of the cells.
The implications of these findings are discussed in the context of current theories of gravi-perception in plants.