598 LECTURE XXXIV. 



with the enormous velocities with which cause and effect may follow one another in 

 the domain of pure physics and chemistry, even the perceptions of our senses are but 

 very slow processes. 



Respecting the propagation in space of stimuli in plants, then, we may here 

 refer to quite similar relation's among purely physical processes. A physical effect is 

 scarcely ever confined exactly to the spot where the effective cause acts, and this is to 

 be explained simply from the fact that the material parts of the body in which the 

 action takes place are mutually held together and arranged by means of forces. If a 

 stretched string is jerked at one point, the whole string vibrates; if one end of a long 

 train of gun-powder is set on fire, the whole train explodes progressively, and so on. 

 And we have to conceive of the propagation of a stimulus in an organ in a similar 

 manner. If, for example, a single place of an irritable tendril is stimulated by contact 

 with a solid body, this means that at this spot the unstable equilibrium in the molecular 

 structure of the organ is disturbed ; but the neighbouring parts are also connected with 

 those immediately disturbed, and stand in a certain equilibrium with these, so that if this 

 equilibrium is disturbed at one point, the disturbance is propagated to the neighbouring 

 points also, and the propagated disturbance acts again and again on neighbouring 

 points, so that at last parts of the organ far distant from the points originally 

 stimulated are set in motion; and it is just in these things that the previously 

 mentioned disproportionality between stimulus and stimulation comes out particularly 

 clearly. But in this respect also the effects of stimulation in animals usually surpass 

 those in plants. In tendrils, irritable Mimosse, and in many other cases, the propagation 

 of the stimulus proceeds only exceedingly slowly : several seconds, minutes, or even 

 hours pass before the local stimulation has traversed a path of 10 to 20 or 30 cm. 

 This takes place much more rapidly in the case of animals, where, moreover, special 

 organs exist — viz. the nerves — ^for the rapid propagation of the stimulations : these 

 are wanting in plants. But even in the nerves of man himself the stimulus advances 

 only about 30 meters per second, and this must be termed an exceedingly slow 

 movement, compared with the inconceivable velocity of an electric current or of a ray 

 of light, or even compared only with the velocity of sound in air. 



We may, as already mentioned, regard the numerous periodic movements in the 

 vegetable kingdom as a special category of phenomena of irritability. In cases 

 where the external conditions are quite constant, where the temperature, light or dark- 

 ness, or the degree of moisture is perfectly constant, where no vibration occurs, and 

 so forth, periodically motile organs, as found particularly often among leaves, perform 

 movements of such a kind as to bend sometimes to one, sometimes to the other side, 

 either with considerable velocity or very slowly. Thus we find continual vibrations 

 to and fro in the case of the small lateral leaflets of the compound leaf of Hedy- 

 sarum gyrans (an Indian papilionaceous plant allied to our Sainfoin) when the 

 temperature and illumination are high and constant; and in the same way the 

 leaves oi Mimosa, Oxalis, Trifolium, Acacia, and other plants go on moving in constant 

 darkness — i. e. their motile organs slowly bend upwards and downwards by turns. 



The most astonishing point in such cases, to those unacquainted with science, 

 lies in the fact that effects are here taking place for which no apparent causes are 

 present. When a movement is induced by a mechanical shock, however slight, or by 

 a mere alteration in the intensity of the light or pf the temperature,^&c., this obviously 



