CHAP. fX. SENSITIVENESS TO LIGHT. 463 



almost stationary for 10 m. ; then crossed 5 divisions 

 of the micrometer in 6m.; and then 8 divisions in 

 11 m. This unequal rate of movement, interrupted 

 by pauses, and at first with occasional retrogressions, 

 accords well with our conclusion that heliotropism 

 consists of modified circumnutation. 



In order to observe how long the after-effects of 

 light lasted, a pot with seedlings of Phalaris, which 

 had germinated in darkness, was placed at 10 . 40 A.M. 

 before a north-east window, being protected on all 

 other sides from the light; and the movement of a 

 cotyledon was traced on a horizontal glass. It cir- 

 cumnutated about the same space for the first 24 m., 

 and during the next 1 h. 33 m. moved rapidly towards 

 the light. The light was now (i.e. after 1 h. 57 m.) 

 completely excluded, but the cotyledon continued 

 bending in the same direction as before, certainly for 

 more than 15 m., probably for about 27 m. The doubt 

 arose from the necessity of not looking at the seed- 

 lings often, and thus exposing them, though momen- 

 tarily, to the light. This same seedling was now kept 

 in the dark, until 2.18 P.M., by which time it had 

 reacquired through apogeotropism its original upright 

 position, when it was again exposed to the light from 

 a clouded sky. By 3 P.M. it had moved a very short 

 distance towards the light, but during the next 45 m. 

 travelled quickly towards it. After this exposure of 

 1 h 27 m. to a rather dull' sky, the light was again 

 completely excluded, but the cotyledon continued to 

 bend in the same direction as before for 14 m. within 

 a very small limit of error. It was then placed in 

 the dark, and it now moved backwards, so that after 

 1 h. 7 m. it stood close to where it had started from at 

 2 . 18 P.M. These observations show that the coty- 

 ledons of Phalaris, after being exposed to a lateral 



