Page:Researches on Irritability of Plants.djvu/85

62 with gradually increasing stimuli. We have already seen, in fig. 12, the marked heightening of the response under an increased intensity of stimulus. In muscle, in the narrow range between minimal and maximal stimulation, there is increasing amplitude of response with increasing stimuli. But this soon attains a limit beyond which there is no further increase of responsive contraction, whatever be the stimulus-intensity employed.

In order to demonstrate a similar progressive increase in the response of Mimosa, I first determined the minimal

stimulus that was barely effective in inducing a feeble response. Starting from the particular position of the secondary which gave this minimal intensity of stimulus, I very gradually increased the intensity, by moving the secondary only 5 mm. at a time nearer to the primary. At each step I took a corresponding record.

In fig. 29 a series of seven such records is shown, the successive responses being taken, as previously mentioned, under slightly increasing stimuli and at intervals of 15 minutes. It will be seen how the height of response is progressively increased. This increase is at first marked, but towards the end we note that a limit is being approached, the difference between numbers 6 and 7 of the series