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. 

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

 ,;mature members, is reversible ; they can return to their foi'mer 

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

 members is reversible only so long as it has not b een rendered 

 perm anent 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 expansion, 

 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 change 

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

 state of expansion is that in which the protoplasm is impermeable 

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

 traction is that in which the protoplasm is permeable to the cell- 

 sap, and the cell is consequently flaccid. The state of expansion 

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

 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 closing 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 that, in the more striking cases of induced 

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

 passage from the state of expansion to that of contraction is accom- 

 panied by a rise of temperature, and by certain electrical phe- 

 nomena : this indicates that it is intimately connected with some 

 catabolic process. It appears probable that the rapid passage 

 from the expanded to the contracted state is determined by the 



