Page:Researches on Irritability of Plants.djvu/65

42 not commence immediately; there is a latent period of ·1 second. The responsive movement then begins and proceeds for a time with increasing speed, the maximum contraction being attained about 3 seconds after the shock. The pulvinus remains in the contracted position for a short period. After this the recovery is initiated. The rate of recovery at the beginning is relatively rapid, and very slow towards the end. The maximum rate of recovery is ·09 mm. per second, in contrast with the maximum rate of contraction, which is 24 mm. per second. The movement of recovery is thus about three hundred times slower than the movement of contraction. The recovery is completed in about 16 minutes.

A stronger stimulus, generally speaking, requires a longer period for recovery. The influence of season is also a factor to be taken into consideration. Under the physiological depression induced by winter, the responsive process is appropriately modified. The excitability of the tissue becomes depressed. An intensity of stimulus which in summer was effective, becomes in winter ineffective. To evoke response much stronger stimulus has to be employed. The latent period is prolonged and the amplitude of response reduced. And lastly, in winter there is, generally speaking, a great prolongation of the period of recovery. In summer, with vigorous specimens, recovery may be practically complete in as short a time as 8 minutes. But owing to sluggishness induced in winter, on the other hand, the recovery may be prolonged to 25 minutes or more. In a severe winter response may even be abolished altogether.

I have hitherto dealt in some detail with the responsive movement of Mimosa. In contrast with this may be cited other examples in which the excitatory reaction may be either more rapid or extremely sluggish.

Response of Biophytum.—As an instance of relatively quick reaction I give (fig. 15) a record of response of leaflet of Biophytum. The maximum fall was here attained in the