8 MOVEMENT 



beyond which the effect of the stimulus gradually diminishes and dis- 

 appears. 



The duration of the latent period lies between a few seconds and a few 

 hours in the case of the slow movements usual in plants. A stimulus 

 impressed upon a plant continues to act for a time after it has been 

 removed, and the greater the difficulty of producing an irritable response 

 the longer will be the persistent after-effect. Hence a stimulus may 

 produce a response some time after it has ceased to act. Similar relation- 

 ships hold good for the rapid movements of the leaves of Mimosa pndica 

 and of the staminal filaments of Cynareae or of Berberis, although they 

 become more immediately perceptible when the duration of the reaction 

 is lengthened by low temperatures. The irritability of the plant, as well 

 as its power of response, are dependent upon the external conditions, 

 although naturally the most favourable external conditions cannot increase 

 the response beyond a certain limit. 



When the reaction is rapid, all the phases of stimulation must be 

 passed through in a very short time, but when it is slow the delay may 

 occur either in the perception of the stimulus or in the responding 

 mechanism, or in both. It is presumably owing to the lessened power 

 of movement that the nutation of the older parts of stems and roots is 

 a little later in time, and also less pronounced than in the younger parts. 

 In many cases a stimulus is only gradually perceived, and frequently 

 a long time elapses before the motor-mechanism begins to be called into 

 action. This is especially well shown when a conducting zone intervenes 

 between the percipient and responding organs. A prolongation of the 

 latent period in an organ capable of rapid response is probably in most 

 cases the result of slow perception. 



Except in the case of motile organisms, the movements of plants have 

 almost always the purpose of gradually bringing the organs into a definite 

 functional position, and it is only rarely that for special purposes a power of 

 rapid movement is developed. In such cases we are usually dealing with 

 transitory reactions produced by sudden changes, as, for example, when 

 a blow or a sudden change of transpiration causes the leaves of Mimosa 

 to close. Reactions of this kind may be termed temporary, transitory, 

 or shock effects, whereas the slower movements involve a condition of 

 permanent or stationary stimulation. Here a condition of equilibrium 

 is maintained so long as the external conditions and the properties of the 

 organism remain unaltered, and a response of this kind is possible not only 

 to orienting stimuli such as gravity or light, but also to diffuse ones such as 

 temperature. No sharp distinction can however be drawn, for often both 

 forms of stimulation act together, and it is in fact in this way that the 

 peculiar sequence observed in thermonastic and photonastic movements 

 is produced. The leaflets of Mimosa pndica return to their original 



