_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 " | +-------------------+-----------------------------+---------------+ The effect of mechanical stimulus on growth is thus similar to that induced by electrical stimulus. Moderate stimulus of rough contact induces an incipient contraction, seen in retardation of growth, the recovery being complete in the course of an hour; but intense stimulation, induced by wound, gives rise to greater and more, persistent retardation of growth. SUMMARY. Mechanical stimulus induces incipient contraction or retardation of rate of growth, the effect being similar to that induced by electric stimulus. Stimulus by contact or friction induces a retardation which is, relatively speaking, moderate. On the cessation of stimulus the normal rate of growth is restored within an hour.