Power to Adjust Parts to Surroundings 237 



pearance presented by those plants at night. The same plants, 

 moreover, can be put to sleep very easily, even at midday, by 

 simply covering them up from the light. Now the exact meaning 

 of the sleep movement is somewhat in doubt, as our chapter on 

 Protection will show; but there is no question at all that light is 

 the stimulus concerned. This response has, however, an interest 

 in another direction, for the motor-mechanism is not growth, but 

 a simple hydraulic contrivance contained in the clear little 

 swellings at the bases of the sleeping leaflets. In the daytime, 



FIG. 81. The cliff-dwelling plant, Linaria Cymbalaria, showing the positive phototropism 

 of its flowers and the negative phototropism of its seed capsules, which thus are 

 brought Into advantageous positions for the deposition of the seeds. (Copied, sim- 

 plified, from Kerner's Pflanzenleben.) 



under stimulus of light, their cells become strongly turgescent 

 and hold the leaves stiffly expanded ; but at night the turgescence 

 is lessened, and the spring of the tissues, aided more or less by 

 gravitation, causes them to droop. It is perhaps simply a high 

 degree of development of sleep movement which gives us the 

 remarkably-balanced leaf mechanism of the Sensitive Plant, 

 later to be considered. 



In viewing these sensitive responses, and others of similar 

 sort, one soon comes to wonder what the limits may be to the 

 changes they can cause in the construction of the plant. This, 

 like most of our problems, is amenable to experiment. If the 

 most favorable possible conditions for one-sided stimulation are 



