Page:Life Movements in Plants.djvu/178

148 The modifying influence of tonic condition on response I find to be of universal occurrence. In vigorous speci- mens the electric response to stimulation is tiegatiue ; but tissues in sub-tonic condition give positive response and after long-continued stimulation the abnormal positive is convert- ed into the normal negative. It is very interesting that under condition of sub-tonicity diverse expressions of phy- siological 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, exces- sive stimulus has the effect of diminishing the conduct- ing 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 stimu- lation 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 blood- less muscle, shows a preliminary staircase response. Stimu- lus induces simultaneously both " A " and " D " effect.^-, with their attendant positive and negative reactions.

A tissue in optimum condition exhibits only the result- ant negative response, the comparatively feeble positive be- ing masked by the predominant negative. With decline of tone, the "D" eft'ect diminishes and we get "A" effect unmasked.

In extreme sub-tonic specimen, we get first only the "A" effect, with its positive response. Successive stimu- lation converts the pure positive into diphasic and ulti- mately into normal n-^gative response.