53] CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 225 



taneous or induced, is the motility of the protoplasm. With re- 

 gard 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 movement 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. 208) ; 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. It is clear that if the turgidity, that is the hydrostatic 

 pressure of the cell-contents, increases, the cell will expand pro- 

 vided 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 cell-walls possess 

 these physical properties: hence, the pulvinus of mature motile 

 leaves consists mainly of parenchymatous cells with unlignified 

 walls, the lignified tissue being reduced as much as possible : 

 similarly, in the elongating region of growing-members the cell- 

 walls are thin and unlignified. But whilst the movements of 

 variation (p. 207) are the result of a sudden loss of turgidity, 

 which is either spontaneous or the effect of stimulation, the move- 

 ment of growth depends upon the maintenance of turgidity, and 

 the variations in the rate of growth (see p. 208) are the expression 

 of variations in the degree of turgidity. 



The following instances will serve to illustrate the foregoing 

 considerations. 



A simple case is offered by the induced movement of the stamens 

 of the Cynarese (p. 176). When at rest, the cells of the filaments 

 are expanded in the direction of their length, and are turgid ; on 

 stimulation, the cells suddenly shorten and become flaccid, having 



M.B. Q 



