STEREOLOGICAL ANALYSIS OF GRAVITROPISM IN PROTONEMATA OF THE MOSS CERATODON

'Apical cells of dark-grown protonemata of the moss Ceratodon purpureu s are negatively gravitropic. Previous light microscopy has shown that reorientation to the horizontal induces amyloplast sedimentation and redistribution of microtubules. To determine whether other components become redistributed laterally or axially, the apical 35 mu m of both vertical and horizontal apical cells were compared stereologically usi ng transmission electron microscopy. Reorientation to the horizontal c hanged the longitudinal distributions of tubular ER, Golgi stacks, and vesicles but not cisternal ER, mitochondria, and plastids. Only plast ids showed a statistically significant lateral redistribution after ho rizontal placement. Qualitative examination of the sedimentation zone showed plastids sedimented close to peripherally located ER with vacuo les displaced above plastids. These results argue against a model wher e differential tip growth results from a redistribution of Golgi stack s or exocytic vesicles.