Page:Researches on Irritability of Plants.djvu/70

Rh other. Hence lateral movement, dependent on differential contraction, cannot take place. But if we take a hollow tubular organ of some ordinary plant, say the peduncle of daffodil, it is clear that the protected inner side of the tube must be the more excitable. When this is cut in the form of a spiral strip and excited by means of an electric shock, We observe a responsive movement to take place by curling, due to the greater contraction of the inside of the strip. This mechanical response is at its maximum at that season which is optimum for the plant. When the plant is killed its response disappears.

It will be seen that the division of plants into sensitive and insensitive is without any justification. Moreover, by adopting the electric mode of investigation, I have shown that every plant and every organ of the plant is sensitive and responds to stimulus by a definite electric variation.

We have hitherto referred but vaguely to the question of the intensity of the induction-shock employed as stimulus to induce response. We have observed that on making and breaking a current in the primary coil, instantaneous currents are induced in the secondary. The intensity of the induction-current employed for giving a shock depends in the first place on the intensity of the primary current; secondly, on the suddenness with which the primary current is made or broken; and lastly, on the relative distance separating the secondary from the primary coil. The intensity of the current can be maintained uniform if we always employ the same battery, say a 4-volt accumulator or storage-cell. As the break of a current is accomplished more quickly than make, the break-shock, as we have seen, is more intense than the make-shock. The plant may, therefore, be excited by a single make-shock, or by a single break-shock or a double make-and-break shock, or by a sequence of make-and-break shocks, of definite duration, according to the particular requirements of the experiment.

The intensity of the shock moreover may, as already