CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 761 



instance, when a Sensitive Plant is kept in continuous darkness, 

 the leaves first lose their power of responding to stimuli, and 

 later their spontaneous movements cease. 



Irritability may also be abolished by special means. For in- 

 stance, exposure to the vapour of chloroform or ether destroys the 

 irritability of the leaves of the Sensitive Plant. Again, it may be 

 abolished by repeated stimulation, the interval between the stimu- 

 lations being very short. This has been observed in the case of 

 the leaves of the Sensitive Plant and of Dionsea. 



17. Mechanism of the Movements. The ultimate factor 

 in the mechanism of the vital movements of plants, whether spon- 

 taneous or induced, is the mntility of the protoplasm (see p. 670). 



Though the intimate mechanism of the motility of protoplasm 

 is not fully understood, yet a consideration of the mechanism of 

 the movements described above will be instructive. With regard 

 to the streaming movement of the protoplasm, it is probably due 

 to wave-like contractions and expansions of the protoplasm. The 

 mechanism of the movements of the contractile vacuoles appears 

 to be this : the systole of the vacuole is due to the sudden active 

 contraction of the protoplasm, the contained liquid being expelled; 

 the diastole, to the active but gradual expansion of the protoplasm, 

 the cavity of the vacuole, as it enlarges, being filled with liquid. 

 The protrusion and retraction of pseudopodia in amoeboid move- 

 ment may be accounted for in the same way ; the protrusion is 

 probably analogous to the diastole of the contractile vacuole, the 

 retraction to the systole. A similar explanation may be offered 

 of ciliary movement. 



The movements of cellular members take place in a definite 

 region, which may be distinguished as the motile region ; this is, 

 in growing members, the region of elongation (see p. 738) ; and, in 

 mature members, is a more or less well-marked region of motile 

 tissue which may constitute a distinct motile organ (e.g. pulvinus 

 of a motile leaf). The movements depend essentially upon varia- 

 tions in bulk of the cells, and these, in turn, upon variations in 

 turgidity (see p. 668). It is clear that if the turgidity, that is 

 the hydrostatic pressure of the cell-contents, increases, the cell 

 will expand provided that the wall be extensible ; and conversely, 

 that if the turgidity diminishes, the cell will shrink, provided the 

 wall be elastic. Movement can only take place when the eel - 

 walls possess these physical properties : hence, the pulvinus of 

 mature motile leaves consists mainly of parenchymatous cells with 



