Page:Plant Response.djvu/335

Rh In connection with this, I have observed, among the types of cyclic variation in multiple response in plants, an instance in which every third response was missing, the period of each second beat being thus approximately twice as long as that of the first. It was easy to see that this was an instance of alternating fatigue, causing the particular recording

leaflet to just miss the response every third time (p. 122). That the excitatory wave arrived in regular sequence, was seen by the fact that the neighbouring leaflets pulsated at regular intervals.

Semi-automatism.—The plant Biophytum growing in the open, under favourable conditions of heat. and light, sometimes becomes so excessively sensitive that motile impulses are generated, the stimulus causing which it is often difficult to localise. A particular leaflet may have been moved by a puff of wind; or the alighting of a small insect, or the accidental grazing of an adjacent blade of grass, may have been the original source of the impulse. But this is enough to set all the leaflets of the plant quivering in an extraordinarily lively manner. For from the excited leaflet, the impulse travels inwards, the leaflets falling in centripetal succession. The excitatory wave then reaches the stem and overflows to the other leaves. But this time the progressive closure of the leaflets proceeds in a centrifugal or outward direction. Before the first discharge, however, going through the numerous avenues, can exhaust itself, the second impulse of the multiple response may begin; and in this way the leaflets exhibit most lively movements without any immediate