Rustic Sounds, and Other Studies in Literature and Natural History

by Sir Francis Darwin

All these cases of plants executing certain useful curvatures, which occur when the plant is displaced as regards the vertical, and cease when the habitual, relation is reached, all these, I say, seem to me only explicable on the theory that gravitation does not act as a mechanical influence, but as a signal which the plant may neglect entirely, or, if it notices, may interpret in any way; that is, it may grow along the indicated line in either direction or across it at any angle. It may be said that this is no explanation at all, that it only amounts to saying that the plant can do as it chooses. I have no objection to this, if the meaning of the word ‘choice’ be defined.

I am now going to deal with the subject of movement from a somewhat different point of view, namely, with the object of showing that it is possible to discover the part of the plant which reads the signal: and this is not necessarily the part that executes the correlated movement. In the reflex movement of an animal (for instance, a cough produced by a crumb going the wrong way), we distinguish the irritation of the throat and the violent action of the muscles of the chest and abdomen; and further, the nervous machinery by which the stimulus is reflected or switched on, by way of the central nervous system, from the throat to the muscles concerned in coughing. In the plant, too, if we are to compare its movements to the reflexes of animals (as has been done by Czapek), we must distinguish a region of percipience, another of motility, and the

transmission of an influence from the percipient to the motor region.

Transmission of a stimulus has long been known in Mimosa, but, in the far more important curvatures which we are now considering, it was not known to

My father and I made an attempt to prove the same thing for the gravitation-sense of roots, that is, to prove that the tip of the root is the region in which the force of gravity is perceived by the plant. Our method of proof does not hold good, but our conclusions are true after all. When gravitation is the stimulus, the experiment is much more difficult than when light is in question, because now that fairy godmothers are extinct we must not hope for a substance opaque to gravitation, a substance with which we might shelter the root-tips from the force of gravity as the tips of the Setaria seedlings were sheltered from light.

The plan adopted by us was simply to cut off the extreme tip of the roots, and fortunately (or unfortunately) the result was just what was expected—the tipless roots had lost the sense of gravitation, and were unable to curve downwards towards the centre of the earth. It was natural to believe that the tipless roots failed to bend because their sense-organs—their percipient parts—had been removed. As a matter of fact they had been removed, but it was fairly objected that the operation of removing the delicate tissues at the tip of the root is a severe one, and that the roots which refused to grow downwards were suffering from shock, and not from the absence of their sense-organs.

The subsequent history of the inquiry is an instance of the unwisdom of prophesying unless you know. In 1894 an able summary of the question was published in a German journal, in which the impossibility of solving the problem of the gravitational sensitiveness of the root-tip was dwelt on, and immediately afterwards Section K of the British Association had the satisfaction of hearing Pfeffer read a brilliant paper giving the long-hoped-for proof that the tip of the root is a sense-organ for gravitation. [47]

Like many other experiments, it depends on a deception or trick played on the plant. The root is forced to grow into a minute glass tube closed at one end and sharply bent in the middle, resembling a little glass boot; the extreme tip being thus

kept at right angles to the main body of the root. If the theory we are testing is the right one, a root with its motor region horizontal and its tip vertical ought to continue to grow horizontally, because the tip being vertical is not stimulated by gravity; it is in a quiescent, or, as it were, a satisfied condition, and no bending influence is being sent to the motor region. And this is what Pfeffer and Czapek found. On the other hand, if the main body of the root points vertically down while the sensitive tip is horizontal, a curvature results, because as long as the tip is horizontal it is stimulated, and the stimulus is transmitted to the motor region.