Page:Researches on Irritability of Plants.djvu/67

44 was the duration of this particular record. The recovery was completed after a further period of 20 minutes, that is to say, the total period of recovery was an hour. The apex time or period of contraction is shortened by the application of a stronger stimulus, but the period of recovery then becomes very much prolonged. The following tabular statement will display the range of variation in the speed of reaction of these sensitive plants:—

The arbitrary distinction that is generally drawn between the so-called sensitive and ordinary plants may briefly be referred to here. In the case of Mimosa, it is generally supposed that the lower half of the pulvinus is alone sensitive; this however is not an accurate statement. By local application of stimulus it can be shown that the upper half also undergoes a feeble contraction, causing an 'up' movement of the leaf.

The localised stimulus may be applied by means of the electro-thermic stimulator. Application of stimulus of moderate intensity on the upper half of the pulvinus will be found to give rise to an erectile response. Another practical method of local application of stimulus is by means of sunlight. A narrow beam may thus be thrown on the upper half of the pulvinus; this will be found to give rise to erectile or 'up' response. Two such responses are