Page:Researches on Irritability of Plants.djvu/122

Rh indicate the beginning of death-change, I first took a specimen of Mimosa and subjected it to a gradual rise of temperature in a water-bath. The leaf was attached to the recording-lever in the usual manner. The recording apparatus employed was of the oscillating type, where the plate oscillates to and fro by an electro-magnetic contrivance, thus producing a series of dots in the response-curve. In the present investigation the electro-magnetic circuit is completed for a brief period, at every degree rise of temperature in the bath. Successive dots thus represent intervals of temperature of 1° C. The ordinate of the curve indicates expansive or contractile movement of the leaf: down-curve representing the expansion, and up-curve the contraction.

The temperature of the bath is continuously raised by the application of gas or spirit flame. For certain reasons, to be presently explained, it is necessary to raise the temperature gradually and continuously, without any sudden variation. There should also be no mechanical disturbance of water in the bath during heating, as that would disturb the leaf and vitiate the record. These difficulties are overcome by constructing the heating-bath of two vessels, one placed within the other. Heating the water of the outer vessel raises the temperature of the water in the inner in a very even manner, and without any mechanical disturbance.

It is necessary to subject the plant to gradual rise of temperature in order to protect it from excitation. Any sudden variation, due either to lowering or raising of temperature, causes excitatory movement of the leaf. This is seen in the following records (fig. 57), obtained with Mimosa. The first response is of excitation due to application of a drop of ice-cold water on the pulvinus; the second response is due to the very opposite treatment of application of a drop of hot water. In both cases we obtain the excitatory fall of the leaf.

The effect of temperature as such is, however, very definite: gradual rise of temperature inducing progressive