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The question whether the transmitted effect in Mimosa is due to a hydro mechanical or excitatory impulse is held to be of much interest in Plant Physiology. I shall therefore deal with the subject in some detail and adduce results of additional investigations that I have carried out recently. The general belief that the transmitted impulse in the plant is hydro-mechanical has been largely based on two well-known experiments of Pfeffer and Haberlandt. In the former of these the effect of strong stimulus was found to travel over chloroformed parts of the stem. Pfeffer assumed that the conductivity of this portion must have been abolished, since chloroform is known to abolish motile excitability. In the experiment of Haberlandt, an intervening tissue was killed by scalding; in spite of this, stimulus was found to be transmitted across the scalded area. From these two experiments it was inferred that the impulse which was transmitted could not have been of a true excitatory nature. It was held, on the contrary, that the strong stimulus had given rise to a variation, whether of increase or diminution, of hydrostatic pressure. This variation of pressure, it was assumed, had been hydromechanically transmitted, and, on reaching the distant pulvinus, had inflicted on it a blow which had proved as effective as if a mechanical stimulus had been applied locally. It is thus held that in Mimosa there is a mere transmission of stimulus but no transmission of excitation.

III. An automatic method for the investigation of velocity of transmission of excitation in mimosa