24 RESEARCHES ON IRRITABILITY OF PLANTS 



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 con- 

 tention is not justified will be fully demonstrated in a sub- 

 sequent chapter. We have, then, in correspondence to the 

 nerve and muscle preparations of the animal, plant-speci- 

 mens, 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 inten- 

 sity of which might be maintained uniform in successive 



