Page:Life Movements in Plants.djvu/175

Rh ''Staircase response in Mimosa : Experiment 48. ''— The theoretical considerations will be found experimentally verified in the record obtained with a specimen of Mimosa in a sub-tonic condition (Fig. 53). Owing to the lack of favourable ' tone ' the leaf was relax- ing as seen in the first part of the curve. The stimulus of electric shock, applied at the thick dot in the curve slanting downwards, gave no response but raised the tone of the tissue by arresting the growing relaxation. Subsequent stimuli gave rise to staircase responses. Stimulus

Fig. 53.-Record showing j^^s, through the A-eflFect, raised the the effect of stimulus modify- functional activity of the tissue to ing tonicity and producing "^ staircase effect. {Aiimofa.) a maximum.

It has been shown that while favourable tonic condi- tion has the efi'ect of raising the excitability and enhancing the negative response with the associated D-change, a con- dition of sub-tonicity, on the other hand, induces depression of excitability, a diminution of negative response and of the attendant D-change. In this condition the positive element in the response with the A-change will come into greater prominence. These considerations led me to experiment with specimens exhibiting increasing sub-tonicity, with a view of ummasking the positive element in the response, i.e., the A-change. In the last experiment a specimen was found which happened to be in a sub-tonic condition on account of the unfavourable condition of its surroundings. I was next desirous of securing specimens in which I could induce increasing sub-tonicity at will.