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Title: Physiological and cytological studies of the statolith apparatus in plants
Author: Griffiths, Hilary J.
Awarding Body: University of London
Current Institution: Royal Holloway, University of London
Date of Award: 1963
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Sedimentation of starch in geoperceptive cells of plants was observed more than sixty years ago and proposed as the mechanism whereby changes in gravitational stimulus are perceived. The idea was that there is an interaction between the sedimented starch grains and the protoplasm along the lower walls of the sensitive cells, which might lead to the synthesis or release of an enzyme. It has been suggested that the highest sensitivity resides in the cytoplasm along the outer tangential walls of the horizontal root. The lower part of a horizontally placed root would thus acquire a higher concentration of the enzyme than the upper and thereby perhaps a higher concentration of a specific growth inhibitor. Experiments have shown that the sedimentation behaviour of statolith starch under different conditions fits in well with the geotropic responses of roots under those conditions. However, the statolith starch theory of geoperception, although substantiated by much supporting evidence, still remains unproven. Since the statolith starch theory was proposed, other cell organelles have been suggested as the perceivers of gravitational stimulus. Perhaps the most acceptable are the mitochondria. Ziegler (1953) believes that the reduction of tetrazolium salts, which he observed at the lowermost side walls of horizontal shoots, was located in mitochondria which sediment under the influence of gravity. Hertz and Grahm (1958, 1962) incorporate Ziegler's observation and interpretation in a theory to explain the oxygen-dependent potential difference which develops between the lowermost and uppermost sides of roots and shoots during fifteen minutes of horizontal stimulation. They propose a mechanism whereby this can cause geotropic curvature. Electron microscope observations of root-cap cells do not support the contention that the mitochondria sediment rapidly under the influence of gravity. The roots which do show significant differences between lower and upper halves of the cap-cells, have more mitochondria in the upper halves, not the lower. This difference probably results from displacement by sedimented amyloplasts. Dictyosomes show the same general pattern of distribution as mitochondria and are probably also displaced by amyloplasts as they sediment. Nemec (1901) observed densely staining lamellate and granular bodies in geoperceptive cells, which moved in relation to the direction of gravitational stimulus. He suggested that the interaction of starch movement with these bodies leads to geoperception. With the electron microscope high concentrations of endoplasmic reticulum membranes have frequently been observed in root-cap cells in a similar position to the "lamellate bodies" of Nemec, and it is considered possible that these structures are analagous. It seems probable that the endoplasmic reticulum membranes are displaced by amyloplasts as the latter sediment. As a result of electron microscope observations of geoperceptive root-cap cells, it is considered that the amyloplasts are still the most likely organelles to trigger off the sequence of reactions leading to geotropic curvature.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physiology