Ultrastructural Changes in Statocytes in Grass Nodes after Short Periods of Gravistimulation: Do Statoliths Fall Through the Vacuole? Evidence from Thick Section High Voltage EM

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Abstract
Leaf sheath bases excised at the nodes from several different grasses were placed horizontally for periods of 1 to 10 mm and then flooded with fixative in situ. Ultrathin sections were viewed with conventional TEM at 60, 80 and 100kv and, after Z10 treatment coupled with thick sectioning, with the high voltage TEM. In the outer, parenchymatous bundle sheath cells, which behave as statocytes, the large starch-filled a.myloplast.s, the statoliths, fall, under the influence of gravity, through the large central vacuole. Each amyloplast is surrounded by a layer of membrane-bounded cytoplasm. This membrane is apparently partly of tonoplast and partly of new ER origin. The statoliths sediment onto cushions of tubular ER which the HVEM shows to be abundant in these cells. An hypothesis of gravistimulation is suggested which depends on the initial movement of the statoliths in the cytosol and is reinforced by completion of the fall when the statoliths settle onto the tubular ER.