A morphometric analysis of the ultrastructure of columella statocytes in primary roots of Zea mays L.

A morphometric analysis of the ultrastructure of columella statocytes in primary roots of Zea mays was performed to determine the precise location of cellular organelles in graviperceptive cells. Vacuoles occupy the largest volume in the cell (11.4 per cent of the protoplasm). The nucleus (9.51 per cent), amyloplasts (7.57 per cent), mitochondria (3.42 per cent), spherosomes (2.13 per cent) and dictyosomes (0.55 per cent) occupy progressively smaller volumes of the statocytes. All organelles are distributed asymmetrically within the cell. Amyloplasts, spherosomes and dictyosomes are found in greatest numbers (and relative volumes) in the lower (i.e. 'bottom') third of the cell. The largest numbers and relative volumes of mitochondria are in the lower and middle thirds of the cell. Nuclei tend to be found in the middle third of the statocytes. Only the hyaloplasm is concentrated in the upper (i.e. 'top') third of Z. mays statocytes. When the sedimentation of amyloplasts (and the resulting exclusion of other organelles from the lower third of the cell) is corrected for, all cellular constituents remain asymmetrically distributed within the cell. Therefore, the sedimentation of amyloplasts alone is not responsible for the differential distribution of other cellular organelles in Z. mays statocytes. The quantitative ultrastructure of Z. mays statocytes is discussed relative to the graviperceptive function of these cells.