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- FIG. 684. Leaf of tele- 

 and-down movement requires 2-4 minutes, graph plant (Desmodium 



7 gyrans), natural size: /, /, 



The fall is more rapid than the rise (for ex- lateral leaflets which show 

 ample, 45 sec. as against 70); and as the tur- autonomous movements; the 



terminal leaflet in the 



gor vanations tend to fluctuate regularly to depressed position. After 

 right and left of the vertical plane, the tip of PFEFFER. 

 each leaflet describes a narrow ellipse. The reason for these move- 

 ments is unknown, nor are they known to be of any value to the plant. 



