CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 765 



sudden explosive decomposition of some complex substance. In 

 the spontaneous or automatic movements, the decomposition takes 

 place spontaneously ; in the induced movements, the decom- 

 position is determined bj the action of the stimulus. Though the 

 phenomena in question have only been observed in cases in which 

 the contraction is sudden and violent, yet the explanation is 

 applicable to all cases of contraction. It appears, therefore, that 

 all movements associated with contraction are attended by a 

 dissipation of energy in the form of heat and electricity. During 

 ithe gradual resumption of the expanded condition, a process of 

 'anabolism probably goes on which leads to the formation of a fresh 

 quantity of the decomposable substance, in consequence of which 

 the protoplasm again becomes irritable and capable of performing 

 another movement. 



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 motility of the protoplasm, and more 

 especially to an interference with the change in its state of aggre- 

 gation upon which the variation in turgidity, leading to movement, 

 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. The arrest of movements 

 of variation by darkness is probably to be ascribed to the suppres- 

 sion of the chemical processes active in the formation of substances 

 upon the presence of which the passage of the protoplasm from 

 the state of expansion to that of contraction depends : and it may 

 be that the effect of light in inducing the condition of phototonus 

 (see p. 760) is due to its influence in promoting these chemical 

 processes. 



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 expansion, 

 not by those wliose protoplasm is in a state of "contraction. This 

 lis a point of spe(;ial physiological interest ; for, on analogy with 

 lanimals, the contrary might be assumed to be the case. For 

 Jinstance, 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 

 expanded condition of their protoplasm. Similarly, in the induced 

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



