CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 765 



some catabolic process. It appears probable that the sudden 

 change of state of the protoplasm is determined by the explosive 

 decomposition of some complex substance. In the spontaneous 

 or automatic movements, the decomposition takes place spon- 

 taneously ; in the induced movements, the decomposition is de- 

 termined by the action of the stimulus. During the gradual 

 resumption of the normal condition, a process of anabolism pro- 

 bably goes on which leads to the formation of a fresh quantity 

 of the decomposable substance, in consequence of which the proto- 

 plasm again becomes irritable and capable of undergoing another 

 sudden change of state. 



The effect of various external conditions, such as exposure to 

 light, long-continued darkness, deprivation of oxygen, etc., in 

 retarding or inhibiting movement, may be ascribed generally to 

 an interference with the motilitv of the protoplasm, that is, to an 

 interference with some condition upon which change in its state of 

 aggregation depends. For instance, the retardation of the rate of 

 growth by light, is the result of a diminished degree of turgidity 

 as is proved by the diminution of the tissue-tensions ; arid this 

 lower level of turgidity in growing cells when exposed to light is 

 probably to be attributed to a modification of the molecular 

 constitution of their protoplasm. The arrest of movements of 

 variation by darkness is probably to be ascribed to the sup- 

 pression of the chemical processes active in the formation of sub- 

 stances upon the presence of which the sudden passage of the 

 protoplasm from the state of contraction to that of relaxation 

 depends : and it may be that the phototonic effect of light (see 

 p. 760) is due to its influence in promoting these chemical pro- 

 cesses. 



In conclusion, the mechanical work done in connexion with 

 movement remains to be considered, especially in relation to 

 cellular members. In the movement of such a member, the work 

 is done by those cells whose protoplasm is in a state of contrac- 

 tion, not by those whose protoplasm is in a state of relaxation. 

 This is a point of special physiological interest ; and, on analogy 

 with animals, this is what might be assumed to be the case. For 

 instance, the great force which can be exerted by a root (see p. 

 763) is the outcome of the turgidity of its cells, that is, of the 

 contracted condition of their protoplasm. Similarly, in the induced 

 movement of the leaf of a Sensitive Plant, the work is done, not 

 by the flaccid, but by the still turgid cells of the pulvinus j and 



