In the matter of mechanical stimulation, some specimens are more irritable than others. The persistence of after-effect of irritation in retardation of growth will be demonstrated in the following experiments, where the stimulus employed was more intense.
A prick causes an intense excitation in _Mimosa_. I tried the effect of this form of stimulation on responsive variation in growth.
_Experiment 77._--The specimen was the same as had been employed in the last experiment. After moderate stimulation due to friction it had, in the course of an hour, completely recovered its normal rate of growth of 0.18 µ per second. I now applied the stimulus of pin prick; the actual injury to the tissue due to this was relatively slight; but the retardation of growth induced by this more intense mode of stimulation was very great. With moderate mechanical friction the rate had fallen from 0.18 µ to 0.11 µ per second, _i.e._, to three-fifths the normal rate: in consequence of prick the depression was from 0.18 µ to 0.05 µ per second, _i.e._, to less than a third of the normal rate. After 15 minutes the rate recovered from 0.05 µ to 0.07 µ per second. After moderate friction the recovery was complete after an hour; but in this case the recovery after an equal interval was only three-fourths of the original, the rate being now 0.12 µ per second (Fig. 74b). I next applied the more intense stimulus caused by a longitudinal cut This caused a depression of growth rate to 0.04 µ per second. A transverse cut, I find, gives rise to a more intense stimulation, than a longitudinal slit.
TABLE XVII.--EFFECT OF MECHANICAL IRRITATION AND OF WOUND ON GROWTH.
(_Zephyranthes._) +-------------------+-----------------------------+---------------+ |Nature of stimulus.| Condition. |Rate of growth.| +-------------------+---------------------------------------------+ |Mechanical friction|Normal rate |0.18 µ per sec.| | |Immediately after stimulation|0.11 µ " | | |15 minutes after stimulation |0.14 µ " | | |60 minutes after stimulation |0.18 µ " | +-------------------+---------------------------------------------+ |Prick with needle |Normal rate |0.18 µ per sec.| | |Immediately after stimulation|0.05 µ " | | |15 minutes after stimulation |0.07 µ " | | |60 minutes after stimulation |0.12 µ " | +-------------------+-----------------------------+---------------+