Page:Researches on Irritability of Plants.djvu/47

24 course of the present chapter that plant tissues also may be excited by similar methods.

In all these cases excitation may be either direct or indirect. In the case of muscle, with its attached nerve, we may cause excitation directly by applying the various forms of stimulus on the muscle itself, or indirectly by applying them on the nerve. In the latter case excitation is transmitted by the conducting-tract—the nerve—and reaching the muscle after a brief and definite interval, induces there the usual contraction.

Taking the case of Mimosa, we may similarly have either direct or indirect excitation. Excitation is direct when it is applied, say, on the contractile pulvinus itself. It is indirect when it is applied on the petiole, at a distance from the pulvinus. Certain tissues in the petiole conduct this excitation, which, reaching the pulvinus after a definite interval, induces a responsive contraction.

It is usually maintained that in the case of Mimosa there is no true conduction of excitation, but that this contention is not justified will be fully demonstrated in a subsequent chapter. We have, then, in correspondence to the nerve and muscle preparations of the animal, plant-specimens, consisting of petiole and pulvinus. Indirect excitation for specific experiments is effected in the animal through the nerve, and in the plant through conducting-strands embedded in a tissue, as in the petiole.

Although we thus have various forms of stimulus at our disposal for inducing individual and isolated responsive contractions in Mimosa, yet we are confronted with very great difficulties when we wish to obtain a series of uniform excitations for quantitative investigation. It is obvious that chemical forms of stimulus would be impossible for successive excitations. The objection to a mechanical blow, as the stimulus to be employed, lies in its liability to cause a mechanical jar and thus to disturb the record.

The ideal form of stimulation would be one the intensity of which might be maintained uniform in successive