I. On the mechanical effects and on the electrical disturbance consequent on excitation of the leaf of Dionæa muscipula

Abstract

The mechanism by which the leaf of Dionæa closes after mechanical excitation has been already studied by Mr. Darwin and many other naturalists. It was, however, necessary, in order to connect the electrical phenomena which form the principal subject of this paper with this mechanism, to study the successive changes of form which the leaf undergoes in the act of closing. The investigations we have made relating to this subject have brought to our knowledge facts which have an important bearing on the general question of the nature of the excitocontractile process in plants and animals. The smooth green outer surface of a leaf of Dionæa in full vigour is concave, and the marginal hairs are thrown back so that they are nearly in the same plane with the lobe from the edge of which they spring. If one of the sensitive hairs of a leaf in this condition is carelessly touched the leaf usually closes. If, however, a hair is touched very cautiously, with the aid of a camel-hair pencil, it can be predicted with certainty that no visible effect will be produced; and a similar gentle contact may be repeated several times before the leaf begins to answer to the irritation by any movement. Sooner or later, however, the marginal hairs bend inwards, and if the leaf is carefully watched it may be observed that each touch is followed by a slight approach of the lobes to each other. If the observation is continued, it is seen that each approach exceeds its predecessor in extent, until at last the lobes suddenly come together in the manner which is, by this time, familiar to every one.