CHAP. IX. SUMMARY OF CHAPTER. 487 



is therefore no improbability in this power having been 

 specially acquired. In several respects light seems to 

 act on plants in nearly the same manner as it does 

 on animals by means of the nervous system.* With 

 seedlings the effect, as we have just seen, is trans- 

 mitted from one part to another. An animal may be 

 excited to move by a very small amount of light ; and 

 it has been shown that a difference in the illumination 

 of the two sides of the cotyledons of Phalaris, which 

 could not be distinguished by the human eye, sufficed 

 to cause them to bend. It has also been shown that 

 there is no close parallelism between the amount of 

 light which acts on a plant and its degree of curva- 

 ture; it was indeed hardly possible to perceive any 

 difference in the curvature of some seedlings of Phalaris 

 exposed to a light, which, though dim, was very much 

 brighter than that to which others had been exposed. 

 The retina, after being stimulated by a bright light, 

 feels the effect for some time ; and Phalaris continued 

 to bend for nearly half an hour towards the side which 

 had been illuminated. The retina cannot perceive 

 a dim light after it has been exposed to a bright one ; 

 and plants which had been kept in the daylight 

 during the previous day and morning, did not move 

 so soon towards an obscure lateral light as did others 

 which had been kept in complete darkness. 



Even if light does act in such a manner on the 

 growing parts of plants as always to excite in them 

 a tendency to bend towards the more illuminated 

 side a supposition contradicted by the foregoing 

 experiments on seedlings and by all apheliotropic 



* Sachs has made some striking See his paper * V?ber orthotrope 



remarks to the same effect with, und plagiotrope Pflanzentheile,' 



respect to the various stimuli * Arb. des. Bot. Inst. in Wiirzburg,' 



which excite movement in plants. ] 879 B. ii. p. 282. 



