70 RESEARCHES ON IRRITABILITY OF PLANTS 



be induced in the moist thread by drying. The horizontal 

 rod holding the cork of the contact-maker is soldered to a 

 tube which can move up and down a vertical rod. These 

 adjustments enable the point of electrolytic contact to be 

 brought to a level with the point of connection on the 

 specimen. The rod is insulated on ebonite. 



All the practical difficulties having thus been eliminated, 

 I shall now proceed to show the various records obtained 

 under this mode of periodic stimulation of uniform intensity. 

 In fig. 32 is given a photograph of the apparatus with its 

 accessories. The recorder is of duplex type, for taking two 

 sets of records at the same time. 



Before entering upon the detailed consideration of the 

 results of these experiments, I may say that at the beginning 

 of this investigation my attention was roused by the 

 apparently capricious variations in the responses obtained 

 under conditions which were rigidly uniform. 



A long-continued investigation through the different 

 seasons of the year has given me the clue to what at first 

 appeared to be so anomalous. The outward response, it is 

 obvious, is dependent on two factors — the intensity of the 

 impinging stimulus and the capacity for reply possessed by 

 the plant itself. It is easy to see that the second of these 

 factors must be dependent on the vigour of the plant, or 

 in other words, on its tonic condition, which in its turn is 

 modified by the environmental condition. Thus in unfavour- 

 able circumstances the plant may fall into an atonic or 

 sluggish condition. The absorption of energy from with- 

 out, by whatever form of stimulation, will improve the tonic 

 condition of the plant, with consequent enhancement of 

 excitability. 



Taking a plant in a subtonic condition, then, we may 

 expect that any application of stimulus will increase its 

 excitability, a fact which will find expression in a growing 

 amplitude of response. This enhancement of excitability 

 will reach a limit at which the plant will be in an optimum 

 condition. After reaching this climax there may be a 



