372 LECTURE XVI. 



evolution of energy is accompanied by a change in form of 

 the protoplasm, and that the performance of a movement by 

 an organ is the external expression of a change in form of 

 the protoplasm of some or all of its cells. We can, in fact, 

 only obtain satisfactory evidence of the existence of irritability 

 in organs which are so constructed that they can respond by 

 movements to changes in the form of the protoplasm of their 

 cells. Such are growing organs, and organs, the motile organs 

 par excellence, which retain, after they have ceased to grow, 

 such a structure that movement is possible to them. But we 

 must be careful not to assume that irritability is restricted to 

 growing and to motile organs. For all we know to the con- 

 trary, it is possessed by the protoplasm of all plant-organs, 

 and, if in any case the action of a stimulus is not followed 

 by a responsive movement, we must, before we assume the 

 absence of irritability, assure ourselves that the structure of 

 the organ is such that a movement is a mechanical possibility 



(P- 340. 



Movement, then, depends essentially upon the irritability 

 and motility of the protoplasm, and it is upon these properties 

 that external conditions exert their tonic influence and thus 

 affect movement. Under the most favourable external con- 

 ditions the evolution of energy and the concomitant change 

 in form of the protoplasm take place most actively, and move- 

 ment follows ; but any variation in these conditions will induce 

 retardation or arrest of movement, the retarding effect being 

 attributable either to a diminished evolution of energy, or to 

 a diminished motility of the protoplasm. To take a single 

 illustration. We have seen (p. 293) that growth takes place 

 with greatest rapidity at a certain optimum temperature, and 

 that at temperatures either above or below this temperature 

 the rapidity of growth is perceptibly less, and that at extreme 

 temperatures growth is altogether arrested. 



We pass now to consider the stimulating action of external 

 agents. Movements, we have seen, are either spontaneous or 

 induced. With regard to the former, we have ascribed them 

 (p. 301) to the action of internal stimuli, but we may perhaps 

 account for them more simply and generally by referring 



