has been observed in Spirogyra by Nägeli and in Nitella by Hofmeister.
Whatever theory may be held, the undoubted fact in these two cases, of plant and animal alike, is the occurrence of a fundamental excitatory protoplasmic change which finds external expression in alteration of form. If we now record the responsive movement, we shall be recording what is an effect of excitatory change, either in plant or animal. Whether or not this fundamental change is similar in the two cases can only be decided by comparing the records due to excitation, in plant and animal tissues, under all possible variations of external conditions.
The fall of the leaf of Mimosa is brought about in consequence of the contraction of cells in the lower half of the pulvinus. I shall for convenience describe the fall as the contractile movement, in contradistinction to the erectile movement brought about by the recovery of cells into normal turgid and expanded condition.
In studying the excitatory reactions of the plant, under external stimulus, we have to determine, first, what time elapses between the incidence of the shock and the initiation of a perceptive responsive movement. This constitutes the determination of the Latent Period. We have next to find out at what rate this responsive movement of the leaf takes place, and after what time the contractile phase of the movement is exhausted. After a short pause the plant gradually recovers from the effect of the shock, and the leaf is re-erected to its former position. We therefore want to know the various rates at which recovery gradually takes place. In order to secure these data, it will be necessary to make a graphic record of the entire responsive movement of the plant organ. This record, further, must furnish us not only with the amount, but also with the time-relations, of this movement. This would involve the construction of a writing-lever which, deflected by the pull of the falling leaf, would be capable of tracing on a writing-surface, moving at a known uniform rate, the
