+--------------------------------------+ | Successive points | Susceptibility | | in the curve. | for excitation. | +-------------------+------------------+ | 1 ... ... | 0 | | 2 ... ... | 0·187 | | 3 ... ... | 0·44 | | 4 ... ... | 0·625 | | 5 ... ... | 0·875 | | 6 ... ... | 1·25 | | 7 ... ... | 1·87 | | 8 ... ... | 3·12 | | 9 ... ... | 5·0 | | 10 ... ... | 6·25 | | 11 ... ... | 8·75 | | 12 ... ... | 8·87 | | 13 ... ... | 8·12 | | 14 ... ... | 6·6 | | 15 ... ... | 4·4 | | 16 ... ... | 2·5 | | 17 ... ... | 1·87 | | 18 ... ... | 1·5 | | 19 ... ... | 1·12 | | 20 ... ... | 0·937 | | 21 ... ... | 0·75 | | 22 ... ... | 0·562 | | 23 ... ... | 0·375 | | 24 ... ... | 0·25 | | 25 ... ... | 0·187 | | 26 ... ... | 0·062 | +--------------------------------------+ The induced excitation is seen to be increased very gradually from the zero point of susceptibility, known as the latent period at which no excitation takes place. In the second part of the excitation curve, the rate of increase is vary rapid; the maximum rate is nearly reached at point 11 of the curve and remains fairly constant for a time. This is the median range where equal increment of stimulus induces equal increment of excitation. The susceptibility for excitation then falls rapidly, and increase of stimulus induces no further increase of tropic curvature. The maximum tropic curvature was, in the present case, reached in the course of nine minutes. The attainment of this maximum depends on the excitability of the tissue, and the intensity of incident stimulus. The characteristics that have been described are not confined to the phototropic curve but exhibited by tropic curves in general. Similar characteristics have been found in the curve for electric stimulus (Fig. 130a), and will also be met with in the curve for geotropic stimulus (Fig. 161). I may here refer incidentally to the three types of responses exhibited by an organ to successive stimuli of uniform intensity; these appear to correspond to the three different regions of tropic curve; in the first stage, the plant exhibits a tendency to exhibit a 'staircase' increase of response; in the intermediate stage, the response is uniform; and in the last stage, the responses show a 'fatigue' decline.