Page:Life Movements in Plants.djvu/253

Rh stimulus is positive, i.e., by expansion, enhancement of turgor,. and acceleration of the rate of growth. (‘ontinuous stimulation also raises the sub-tonic growing tissue. to a condition at par, converting the response from abnormal positive to normal negative.

It was also explained that every stimulus gave rise to dual reactions, positive and negative, and that. in a highly excitable tissue the positive is masked by the predominant negative. The positive. or A-etl‘ect, is generally described as a “building up” process. By choosing a sub—tonic specimen, I have. been able to unmask the positive, A. In the case of sub-tonic growing organs the positive, A, is literally a building up process, giving lies to an accelera- tion of growth.

From these facts and others given previously it will be seen that the abnormal response of acceleration of growth under stimulus is by no means accidental or fortuitous but is a deﬁnite expression of an universal reaction, characteristically exhibited by all tissues in a condition of sub-tonicit)’.

A given plant-tissue may exist in widely diti'erent conditions of tonicity. Let us take two extreme. condi- tions, the optimum and the minimum. The tonic level will be at its lowest at the minimum, where growth will be at a standstill. The range between the optimum and minimum will be very extended; hence strong and long continued stimulation will be necessary to raise the tissue from the tonic minimum to the optimum level. There are innumerable grades of tonicity between the optimum and minimum. Within this wide range the characteristic response will be the abnormal positive. As we approach the Optimum, the range for positive response. will