764 PART IV. THE PHYSIOLOGY OF PLANTS. 



motile members and that of growing members is essentially the 

 same, there is this secondary difference between the two cases. 

 The change of position which is the result of the movement of 

 mature members, is reversible ; they can return to their former 

 position : the change of position, curvature for instance, of growing 

 members is reversible only so long as it has not been rendered 

 permanent by actual deposition of substance. Thus the changes of 

 .position due to the nutation (p. 741) of growing members are only 

 temporary, for they are of brief duration ; but changes of position 

 due to some directive influence acting for a considerable time 

 become permanent, for instance, the light-positions (p. 748) assumed 

 by growing members. 



The general conclusion to be drawn from the consideration of 

 the phenomena of movement is that motile protoplasm may be in 

 one of two states of aggregation ; either in the state of relaxation, 

 or in the state of contraction ; and that it is capable of passing from 

 the one state to the other: in fact the term " motility " simply 

 means the property in virtue of which the protoplasm can thus 

 change its state of aggregation. In the case of cellular members, 

 the state of contraction is that in which the protoplasm is imper- 

 meable to the cell-cap, and the cell is turgid ; whereas the state 

 of relaxation is that in which the protoplasm is permeable to the 

 cell-cap, and the cell is consequently flaccid. The state of con- 

 traction may be regarded as the normal condition : the state of 

 relaxation is a more or less sudden deviation from this condition, 

 the result of the action of internal or external causes ; of internal 

 causes, as in the case of the spontaneous movements ; of external 

 causes, as in the case of the induced movements. 



This account of the mechanism of the movement of cellular members applies 

 equally to the production of the root-pressure (see p. 701) and to the opening 

 and closiug of the stomata (see p. 698). 



The ultimate point to be considered is the conception to be 

 formed as to the nature of the change of state of the protoplasm. 

 It has been ascertained, in the more striking cases of induced 

 movements, such as those of the leaves of the Sensitive Plant, that 

 the change of state of the protoplasm of the stimulated cells of the 

 pulvinus is accompanied by a rise of temperature, and by certain 

 electrical phenomena : and it is probable that all movements as- 

 sociated with sudden change in the state of aggregation of proto- 

 plasm are attended by a dissipation of energy in the form of heat 

 and electricity : this indicates that it is intimately connected with 



