Page:Life Movements in Plants.djvu/168

138 been able to devise a direct experimental test to settle the question.

In the last experiment the stimulus was applied at the moderate distance of 50 mm. Let us now consider the respective effects, first, of an increase, and second, of a decrease of the intervening distance. In a tissue whose conducting power is not great, the excitatory impulse is weakened, even to extinction in transmission through a long distance. Thus the negative impulse may fail to reach the responding organ, when the stimulus is feeble or the intervening distance long or semi-conducing. Hence, under the above conditions, stimulus applied at a distance will give rise only to a positive response.

A reduction of the intervening distance will give rise to a different result. As the negative response is the more intense of the two, the feeble positive will be masked by the superposed negative. The separate exhibition of the two responses is only possible by a sufficient lag of the negative impulse behind the positive. This lag increases with increase of length of transmission and decreases with the diminution of the length. Hence the application of stimulus near the responding organ will give rise only to a negative response, in spite of the presence of the positive, which becomes masked by the predominant negative.

These inferences have been fully borne out by results of experiments carried out with various specimens of plants under the action of diverse forms of stimuli. In all cases, application of stimulus at a distance causes a pure positive response; moderate reduction of the distance induces a diphasic response—a positive followed by a negative; further