Page:Researches on Irritability of Plants.djvu/101

78 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 preceding 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