694 PLANT RESPONSE 



teristics, such as the relative quickness of the up or down 

 movement, it may be exhibited in opposite ways, according 

 to the intensity of stimulus. 



The swimming movements of ciliated organisms. — It 

 is the automatic character of ciliary movements which has 

 hitherto rendered them a subject of such great perplexity, 

 for these movements are independent of any nervous system 

 for their initiation or control. The impulses that lead to 

 ciliary motion arise in the cilia themselves. There is, again, 

 some difficulty in understanding the mechanics of these 

 movements. 



Their automatism may, however, be explained by those 

 considerations which are now familiar to us in similar in- 

 stances — that is to say, initiation of movement by external 

 stimulus, and its maintenance by excess of latent energy. 

 There is, again, the closest resemblance, from a mechanical 

 point of view, between the movements of the cilia and the 

 movements of Dcsmodium leaflets. In both cases alike, one 

 of the component strokes is quicker than the other. Again, 

 though in both cases we meet with instances in which the up 

 and down movements take place in the same plane, yet there 

 are also others in which more complicated paths, whether 

 circular or elliptical, are described. The fact that one move- 

 ment is quicker than the other, and that the strokes are 

 lateral, shows that we have in the cilium an anisotropic 

 organ, essentially similar to the pulsating pulvini of the 

 lateral leaflets of Desmodium. The only difference between 

 the two cases lies in the fact that we have in the pulvinus a 

 multicellular, and in the cilium a unicellular, organ. 



Let us suppose a detached petiole of Dcsmodium, bearing 

 the lateral leaflets, to be thrown into water contained in 

 a glass trough ; let us further suppose these leaflets to be in 

 a sub-tonic condition, that is to say in a state of standstill. 

 If this vegetable organism be now stimulated by sunlight 

 of moderate intensity, striking it horizontally from one side 

 of the vessel, it will be found that rhythmic excitation is 

 initiated, and that the stroke backwards is much quicker and 



