. A textbook of botany for colleges and universities ... Botany. GROWTH AND MOVEMENT 453 Mechanism. — It is evident that the central position of the vascular bundles permits flexure more readily than if they were scattered and more peripheral; while the peripheral position of the thin-walled cells of the cortex is such that any variation in their turgor will produce a cur- vature, the side with less turgor becoming concave, since its cells no longer oppose fully the turgid cells of the opposite side. Correspond- ingly, the parts beyond the curving motor organ will be displaced by it. These tur
. A textbook of botany for colleges and universities ... Botany. GROWTH AND MOVEMENT 453 Mechanism. — It is evident that the central position of the vascular bundles permits flexure more readily than if they were scattered and more peripheral; while the peripheral position of the thin-walled cells of the cortex is such that any variation in their turgor will produce a cur- vature, the side with less turgor becoming concave, since its cells no longer oppose fully the turgid cells of the opposite side. Correspond- ingly, the parts beyond the curving motor organ will be displaced by it. These turgor variations, due to modified permeability, being usually restricted to the upper and lower sides of the motor organ, the distal parts are moved up and down. Since the relaxed cells may recover tur- gidity and the turgid cells become flaccid, the notable feature of all such movements is that the changes in the cells are reversible; whereas the cell changes involved in growth are irreversible (or soon become so). The motor organs of stigmas and stamens are essentially similar to those of foliage leaves, but simpler, since vascular tissues are slightly or not at all developed, and almost the whole tissue is parenchymatous. Autonomic movements. — The variations in turgor are sometimes autonomic, that is, determined by causes unknown and apparently in- ternal to the plant, but more commonly they are controlled by external stimuli. Autonomic movements are not at all uncommon, but they are mostly too slow to be observed easily without apparatus, and, when sought, are often masked by more obvious movements (see p. 457). The classical and almost the only striking example of easily seen move- ments is offered by Desmodium gyrans, whose lateral leaflets (fig. 684) are constantly rising and falling under favorable conditions. These movements, sometimes uniform, but usually jerky, are not very rapid, for a complete up- and-down movement requires 2-4 Fig. 684. — Leaf of te
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