Text-book of botany, morphological and physiological . and considerationsthere stated may be repeated here mutatis mutandis. Particular stress must be laidon the fact that movements are induced in protoplasm by the action of gravitationjust as by the action of light. Thus Rosanoff showed^ that the plasmodia oi ^tha-lium sepiicum are negatively geotropic, creeping, under the influence of gravitation,over steep moist walls, and turning, under the action of centrifugal force, towardsthe centre of rotation ; they take therefore those directions which would beleast expected from their apparently fl
Text-book of botany, morphological and physiological . and considerationsthere stated may be repeated here mutatis mutandis. Particular stress must be laidon the fact that movements are induced in protoplasm by the action of gravitationjust as by the action of light. Thus Rosanoff showed^ that the plasmodia oi ^tha-lium sepiicum are negatively geotropic, creeping, under the influence of gravitation,over steep moist walls, and turning, under the action of centrifugal force, towardsthe centre of rotation ; they take therefore those directions which would beleast expected from their apparently fluid condition. The question suggests itselfwhether there is not also protoplasm which behaves in this respect in an exactlyopposite manner; and from the dependence of the growth of the cell-wall on theactivity and probably also on the disposition of the protoplasm in the cell, the hypo-thesis must not be altogether set aside that all geotropic phenomena are in the firstplace caused by the protoplasm taking up definite positions in the cells under the. Fig. 452.—Diagram for illustrating geotropic upward and downward curvature. influence of gravitation, and thus accelerating or retarding the growth of the cell-wallson the under sides. Since nothing is known on this subject, we must direct ourattention solely to the growth of the cell-walls, leaving it undecided whether the efTectof gravitation be direct or indirect. In order to state clearly the problem how gravitation acts on the growth of thecell-wall^, we may consider as the simplest example a unicellular tube, such as wefind in Vaucheria, the posterior end of which developes as a positively geotropicroot, the anterior end as a negatively geotropic stem. Fig. 452 ^ may represent this,assuming that the whole tube grew at first in a vertical direction either upwards ordownwards, but was then placed in a horizontal position, as shown by the light out-lines -S* and W. After some time the radical end would show a downward curvatu
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectbotany, bookyear1875
