the root-tip is sensitive to mere contact, since a certain amount of injury to the tissues was inflicted by the method employed; and this objection has not so far been fully met. Whatever may be the true explanation, it is a fact that roots find their way into worm-burrows, and otherwise follow in the earth lines of least resistance, in a way that is strongly suggestive of a power to discriminate between harder and softer regions of the soil.
Fig. 7.—A Seedling of Pea, with radicle extended horizontally in damp air, with a little square of card affixed to the lower side of its tip, causing it to bend upward in opposition to gravity. The deflection of the radicle after twenty-one hours is shown at A, and of the same radicle after forty-five hours at B. (From Darwin's Power of Movement in Plants.
An electric current passed through the tip induces curvature, and in some cases roots have been found to bend away from the light. Although it can hardly be supposed that sensitiveness to these stimuli is of any special use to the plants, such behavior, taken in connection with the highly useful modes of sensitiveness above described, surely indicates an almost animal-like irritability of the organ in question.
From what has been said of the curvature of young roots, it is obvious that, whenever the tip proper is stimulated, the effort must be transmitted to the part above, since it is only this upper portion which curves. A similar transmission of stimulus takes place in the leaf of the sensitive-plant, and both suggest an analogy with the propagation of an impulse along the nerves in animals. Nevertheless, in the absence of all proof that anything resembling nerves entered into the structure of plants, the analogy referred to was deemed rather fanciful, and certain mechanical explanations of the phenomena were offered as more in keeping with what was known. A few years ago, however, Gardiner's demonstration of the continuity of protoplasm in plants rendered the mechanical theories superfluous, by showing that the living matter of adjacent cells was connected by delicate protoplasmic threads which might fairly be considered the analogues of nerves. The essential similarity of many plant movements with those of animals is thus seen to be even closer than was at first supposed,
- Philosophical Transactions, 1883, p. 817.