600 LECTURE XXXIV. 



in the present case on the contrary the first problem is to seek for the impulse, 

 because we premise on the basis of the principle of causality that such an impulse 

 must actually exist. The preceding considerations will at least have made it clear 

 that the constant course of molecular and atomistic processes which constitute 

 life in general may give rise to periodic alterations in the interior of the cells, 

 and we may regard these latter again simply as stimuli, from which the visible 

 periodic movements arise as effects. 



I have repeatedly had cause to refer to certain resemblances between the 

 phenomena of irritability in the vegetable kingdom and those of the animal body, 

 thus touching a province of investigation which has hitherto been far too little 

 cultivated. In the last instance, indeed, I might say animal and vegetable life 

 must of necessity agree in all essential points, including the phenomena of irri- 

 tability also, since it is established that the animal organism is constructed entirely 

 and simply from the organic substances produced by plants, and ultimately it is 

 simply from the properties of these substances that all vital movements both of 

 plants and animals are to be explained. Since it is possible for not only other 

 animals but even man to be nourished by seeds and tubers of plants, and the 

 substance of the human body produced by this nutriment is able to carry out all 

 sensitive perceptions, all the periodic motions of the heart, and finally also the 

 functions of the entire nervous system, including those of the brain, we must refer 

 it to the properties of those substances which have been produced by plants from 

 mineral matters, water, and carbon dioxide under the influence of sunlight. 



Returning from these general considerations to definite comparisons between 

 the animal and the plant, I would make special mention of that exceedingly 

 remarkable phenomenon in animal hfe, termed by its great discoverer, Johannes 

 Müller, the specific energies of the sensory nerves. As is well known, we under- 

 stand by this the fact that for instance the optic nerve responds to any given 

 excitation whatever with the sensation of light : true, this sensation is as a rule called 

 forth by the vibrations of the luminiferous ether, but even electric currents or mere 

 concussion or diseased conditions impel the optic nerve to the sensation of light. 

 In the same way the auditory nerve is impelled to the perception of sound not 

 merely by waves of sound, but by every change which aff"ects it, and similarly 

 with the remaining organs of sense. 



Now 1 pointed out several years ago that even the organs of plants are 

 provided with similar specific energies. Irritable organs in plants are indeed, like 

 the sense-organs of animals, sensitive to a definite category of stimuli, but they 

 can very often be affected by other stimuli also, and in this case the stimula- 

 tion is always the' same. This appears most distinctly, for example, in the case 

 of growing internodes and leaves. If they are illuminated from one side they 

 become curved, and if brought out of their normal position they are caused to 

 make exactly similar curvatures : the one mode possible for responding to any 

 stimulus whatever is simply this curving. The matter only obtains its full signi- 

 ficance by means of the fact, which I rendered clear, that every individual plant- 

 organ responds to the influence of light as well as to that of gravitation in a manner 

 specifically peculiar to it, and it is upon this that the previously mentioned ani?otropy 

 of the parts of plants depends. No less clear is the specific energy of tendrils : as 



