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 minutes. ^ a P h . P lant U'?"*; 
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 variations 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. 
FIG. 684. Leaf of tele- 
graph plant (Desmodium 
