7 14 PLANT RESPONSE 



hand, practically the whole of the incident energy is expressed 

 in direct response, there being little or no absorbed element 

 (p. 460). A similar series of considerations may be applied 

 to the response of mature pulvinated organs. In this case, 

 however, the indirect effect of stimulus will find expression 

 in an enhancement of the rate of recovery of the organ. It 

 is, however, difficult always to discriminate with certainty 

 between the natural and an enhanced rate of recovery ; 

 but on turning to growth-response we find that, by using 

 the balanced method of record, it is easy to distinguish 

 between the direct and indirect effects of stimulus, since 

 these are here shown by curves in opposite directions. 

 This method moreover affords us some means of measuring 

 the relative magnitudes of the two factors in the response. 



Excitability. — It is interesting to find that an agency 

 which induces a variation of excitability produces a similar 

 modification of all the different forms of response. In this 

 way the long-continued application of cold has the effect of 

 lessening excitability, and the response of a motile organ is 

 thus found to be temporarily diminished or abolished. At 

 the moment of application, however, owing to the fact that 

 any sudden variation of environment acts as a stimulus, its 

 effect is the induction of an excitatory movement. These 

 phenomena are repeated with curious exactness in the case 

 of suctional response. On the application of ice-cold water 

 to the root, the immediate effect is a transient exaltation of 

 suction, followed later by depression and arrest (p. 375). In 

 growth-response, also, growth is diminished or arrested by 

 this agency. 



Another effect of the moderate application of cold is to 

 induce a molecular sluggishness by which the latent period is 

 increased. A moderate rise of temperature, on the other 

 hand, increasing the molecular mobility, has the contrary 

 effect, of reducing the latent period. 



Anaesthetics, again, induce a diminution or abolition of 

 excitability, as is seen by their effect on the various forms of 

 response. 



