78 RESEARCHES ON IRRITABILITY OF PLANTS 



condition reaches an optimum, with the attainment of 

 highest degree of excitability. Here impinging stimulus 

 has evoked the maximum response. 



We see in a general way that in these responses the 

 accession of stimulus has given rise to two kinds of effects, 

 external and internal, whose relative values have been 

 progressively changing. At the beginning a portion of the 

 stimulus was utilised to improve the tonic condition, the 

 complementary portion inducing external response. Hence 

 at the beginning the response was small. At the end of 

 the series, however, where the maximum tonicity has been 

 attained, the whole blow of the stimulus is utilised in 

 giving external response, which now therefore is maximum. 

 After this attainment of maximum excitability the usual 

 fatigue-decline is seen to have taken place. 



We must nevertheless be on our guard against drawing 

 too hasty a conclusion, as regards the tonic condition, from 

 the relaxation or contraction seen in the record ; we should 

 remember that a relaxed condition is not only indicative of 

 a-tonicity, but may also be brought about by fatigue due 

 to over-stimulation. The changing position of the leaf, 

 owing to daily periodicity, should also be taken into 

 account. Bearing in mind, however, the immediately pre- 

 ceding history of the given plant, the experimenter will not 

 find it difficult to guard himself against wrong inferences. 



Being desirous of ascertaining how far the theoretical 

 considerations here advanced would be borne out in extreme 

 cases, I tested a specimen which from appearances was not 

 at all vigorous and likely to be a-tonic. The record it gave 

 at the beginning, of increasing relaxation, probably indicated 

 its growing a-tonicity (fig. 39). That it was lacking in tone 

 at once became evident from the fact that the first stimulus 

 — applied at the point shown by the thick dot — did not 

 evoke any response. But that this nevertheless did cause 

 improved tonicity, is seen from the fact that the former 

 rate of relaxation underwent a diminution, the record 

 tending to become more horizontal. The second stimulus 



