Ultrastructure and movements of cell structures in normal pea and an ageotropic mutant

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Abstract
The pea mutant (Pisum sativum ageotropum) and the normal pea (P. sativum cv. Sabel) were compared in order to see if there were any differences in root anatomy or submorphology which could explain the presumed ageotropic behaviour of the mutant. In both types the root cap consists of a central core (columella) distinct from the peripheral part. The core contains five to six rows of columella cells, each consisting of 10 to 16 storeys of statocytes. The ultrastructure of the columella cells in the two types is very similar; the main difference is confined to the distribution of rough endoplasmic reticulum (ER), which in the mutant statocytes is evenly distributed throughout the cell, while in the normal pea statocytes it is mainly concentrated in the distal part at the “floor” of the cell.Using light micrographs, the movement of amyloplasts and nuclei have been followed in detail during a 40 min inversion period. The pattern of movement of the amyloplasts is apparently identical in the two types and the distances moved during the inversion period are 39 μm and 44 μm in the normal and mutant statocytes, respectively. The nucleus has not been observed to move in normal pea; a slight rearrangement of the nucleus position can be observed during the period 30 to 40 min after the start of inversion of the mutant.Based on magnified electron micrographs of the statocytes a morphometrical analysis was made of five cell structures – amyloplasts, nuclei, mitochondria, vacuoles and ER – which appeared to be freely movable or redistributable under the influence of the gravitational force.