Page:Researches on Irritability of Plants.djvu/26

Rh as that in the animal? Is there to be found in plants any tissue that might twitch persistently, like the cardiac tissue of the animal? If so, are these rhythmic pulsations characteristically similar? Is there, again, any general resemblance between responsive actions in plant and animal? Going deeper, since the same protoplasmic basis underlies them both, are these reactions to be regarded as essentially the same, though different in degree? If this last were true, then since the simpler explains the more complex, might not the physiological reactions of the plant be expected to elucidate many of the obscurities in the similar reactions of animal tissues?

We return, then, to the question, Is the plant capable of furnishing any such responsive indications as we have supposed? I have shown elsewhere that all plants give response to impinging stimulus by a definite electrical change, which can be recorded by means of suitable apparatus. For the purpose of the present work, however, it will be convenient to employ the more conspicuous motile indications afforded by certain plants, pre-eminent amongst which is Mimosa pudica.

The most prominent motile organ in Mimosa consists of a mass of tissue known as the pulvinus, at the joint or articulation of the primary leaf-stalk. The swollen mass on the lower side of this organ is very conspicuous. Under excitation the parenchyma, in this more effective lower half, undergoes 'contraction,' in consequence of which there is a fall of the leaf. This sudden movement constitutes the mechanical response of the leaf to the impinging stimulus, just as the contractile movement of a muscle in similar circumstances forms its characteristic mechanical response.

Digressing for a moment to consider the phenomenon of excitatory contractions in general, it may be said that our present knowledge is not complete as to the minutiæ