The Physiology of Flowering Plants - KHAM PHA MOI

TROPISMS 333The role of statolithsThe primary perception of gravity must involve the movement ofsome entity in the sensitive cells – this is the only way in which thephysical force can interact with the cell. In 1900, Haberlandt andNeměc independently put forward the statolith theory of gravitropism.This supposes that in the perceptive cells (the statocytes),mobile starch grains (the statoliths) move under gravity to lie inthe lower parts of the cell. The cells have therefore acquired an‘up-and-downness’, i.e. a polarization, which acts as a signal to triggerthe response.Statocytes are found within the gravity-sensing regions of theplant – the columella of the root cap and the starch sheath parenchymacells that surround the vascular tissue of shoots and grasspulvini (Fig. 12.10) – and are absent from non-gravitropic organs. Thestarch which forms the statoliths differs from starch found withinother tissues as it persists even under extreme starvation. There issome evidence that crystals of calcium oxalate may function asstatoliths in some plants. Also the rhizoids of characean algae containstatoliths enriched in barium sulphate.As well as their location in gravity-sensing regions, other evidencesupports the role of statoliths in gravity perception. There is a goodcorrelation between the density of the starch grains and the gravitropicresponse of the organ. Physiological treatments, such asA BFig: 12:10 Statoliths andstatocytes in root caps. (A) Electronmicrograph of a central columellacell of Arabidopsis. The statoliths arewhite and have sedimented at thepcbottom of the cell. The endoplasmicreticulum has stained darkly and islocated at the periphery of the cell.ccA point of contact between anaamyloplast and the ER has beenindicated with an arrowhead. (B)and (C) are brightfield images of the50 µm root cap of Medicago truncatula andLinum usitatissimum. (B) The centralCcolumella, cc, cells of the root capcontain many amyloplasts, a, whilstfar fewer are found in cells of thenperipheral root cap, pc. (C) Atahigher magnification the amyloplastsaof the central columella cells are5 µm10 µmclearly visible and located at thebase of the cell. The position of thenucleus, n, is indicated and istypically at the apex of these cells.(A) is from Sack (1997). (B) and (C)are from Collings et al. (2001).# Springer-Verlag.