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290 antecedent cause. In some of these cases the origin of the impulse can be traced, but there are others in which no external source of stimulus can be assigned. And here we find ourselves passing imperceptibly into the obscure region of automatism.

There is much resemblance between such semi-automatic phenomena in Biophytum and the well-known instance of spontaneous movements exhibited by Desmodium gyrans, the successive mechanical responses of which exhibit all the peculiarities of multiple response as seen in Biophytum. In Desmodium as in Biophytum we have similar cyclic changes of period and of amplitude. Just as in the case of Biophytum, when we cannot determine the source of stimulus, we are tempted to regard the phenomenon as automatic, so in the case of Desmodium gyrans it is our own inability to trace out the origin of stimulus that leads us to regard the periodic movements of the plant as automatic.

Biophytum, then, under ordinary circumstances, exhibits a single response to a single stimulus; but when the stimulus is strong, a single stimulation will produce multiple responses, and these will persist long after the cessation of the primary stimulus. Under exceptionally favourable circumstances periodic movements occur, apparently without any exciting cause. I have again found, as will be described more fully in Chapter XXIV., that Biophytum itself under favourable circumstances of light and warmth exhibits persistent and long-continued autonomous pulsations, which are in no way distinguishable from those of Desmodium. Even the periods of vibration are, generally speaking, similar, inasmuch as in both these plants, under different circumstances, I have recorded pulsations, the periodicities of which vary from one minute or less to four or five minutes. Thus Biophytum forms a connecting link between those plants which exhibit only ordinary response (single stimulus, single response) and those in which mechanical movements appear to be automatic. Biophytum is also particularly interesting, because in its case we find the same plant, under different