INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 297 



Thus Biophytuni, which normally speaking gives ordinary 

 single or multiple response, may under favourable circum- 

 stances exhibit automatic response. On the other hand, 

 Dcsiitodiuiii, which normally speaking exhibits automatic 

 response, will under unfavourable circumstances become 

 converted into an ordinarily responding plant. 



As already stated, in the case of Biophytitm, the point of 

 application of stimulus becomes also the source of subse- 

 quent multiple excitations. An observer, unacquainted with 

 the position of this point, might succeed in determining it, by 

 watching the order of pulsation of the leaflets. For if we 

 suppose the stimulus to have been applied at some point in 

 the middle of the leaf, the observer will in that case notice 

 periodic waves of excitation proceeding in opposite directions, 

 and giving rise to the closure of successive leaflets from a 

 common point outwards, thus clearly indicating the position 

 of the point from which successive excitations are initiated. 

 Had the stem, on the other hand, been stimulated, and thus 

 become the source of excitation, these successive impulses 

 would have begun by arriving at the first pair of leaflets, 

 and thence would have passed through all the leaflets to 

 the tip of the leaf, in a centrifugal order. Again, had the 

 stimulus remained latent at the tip of a particular leaf, the 

 successive excitatory waves would then have proceeded 

 through the leaf in a centripetal order, and on reaching the 

 stem would have radiated outwards, or in a centrifugal 

 sequence once more, throughout the other leaves. There 

 are other ways also, presently to be described, by whose 

 means we can obtain an idea of the direction in which the 

 excitatory wave is travelling. 



Localisation of seat of origin of autonomous excitation 

 in Desmodium. — Our next inquiry is into the very obscure 

 question of the point of origin of the so-called autonomous 

 excitation of Dcsmodiiiiii. I have already shown that, if the 

 plant absorbs a certain amount of energy in excess of that 

 required for immediate response, the surplus is stored up, to 

 be given out subsequently in the form of pulsating waves of 



