Page:Life Movements in Plants.djvu/178

The modifying influence of tonic condition on response I find to be of universal occurrence. In vigorous specimens the electric response to stimulation is negative; but tissues in sub-tonic condition give positive response and after long-continued stimulation the abnormal positive is converted into the normal negative. It is very interesting that under condition of sub-tonicity diverse expressions of physiological reaction exhibit similar change of sign of normal response. Thus in my measurement of the velocity of transmission of excitation in the conducting tissue of Mimosa, I find that, when the tissue is in an optimum condition, exhibiting high velocity of transmission, excessive stimulus has the effect of diminishing the conducting power. But in a depressed condition of the tissue the effect is precisely the opposite. Thus in a given case the velocity of transmission was low; strong electric stimulation enhanced the rate by 33 per cent. In extreme cases of sub-tonicity, where the conducting power was in abeyance, the excessive stimulus caused by wound not only restored the power of conduction but raised the velocity of transmission to 25 mm. per second (Expt. 37).

The excitability of a plant is found to be modified by its tonic condition.

A sub-tonic specimen of Mimosa, like an excised bloodless muscle, shows a preliminary staircase response. Stimulus induces simultaneously both "A" and "D" effects, with their attendant positive and negative reactions.

A tissue in optimum condition exhibits only the resultant negative response, the comparatively feeble positive being masked by the predominant negative. With decline of tone, the "D" effect diminishes and we get "A" effect unmasked.

In extreme sub-tonic specimen, we get first only the "A" effect, with its positive response. Successive stimulation converts the pure positive into diphasic and ultimately into normal negative response.