i62 SENSITIVENESS TO LIGHT. Chap. IX. 



placed under a microscope provided with an eye-piece 

 micrometer, of which each diyision equalled rls'^^ oi an 

 inch (O'OSl mm.) ; and they were at first illuminated 

 by light from a paraffin lamp passing through a solu- 

 tion of bichromate of potassium, which does not induce 

 heliotropism. Thus the direction in which the coty- 

 ledons were circumnutating could be observed inde- 

 pendently of any action from the light ; and they could 

 be made, by turning round the pots, to circumnutate 

 transversely to the line in which the light would strike 

 them, as soon as the solution was removed. The fact 

 that the direction of the circumnutating movement 

 might change at any moment, and thus the plant 

 might bend either towards or from the lamp indepen- 

 dently of the action of the light, gave an element of 

 uncertainty to the results. After the solution had 

 been removed, five seedlings which were circumnutat- 

 ing transversely to the line of light, began to move 

 towards it, in 6, 4, 7^, 6, and 9 minutes. In one of 

 these cases, the apex of the cotyledon crossed five 

 of the divisions of the micrometer (i.e. j^th. of an 

 inch, or 0-254 mm.) towards the light in 3 m. Of two 

 seedlings which were moving directly from the light at 

 the time when the solution was removed, one began to 

 move towards it in 13 m., and the other in 15 m. 

 This latter seedling was observed for more than an 

 hour and continued to move towards the light; it 

 crossed at one time 5 divisions of the micrometer 

 (0-254 mm.) in 2 m. 30 s. In all these cases, the 

 movement towards the light was extremely unequal in 

 rate, and the cotyledons often remained almost sta- 

 tionary for some minutes, and two of them retrograded 

 a little. Another seedling which was circumnutating 

 transversely to the line of light, moved towards it in 

 4 m. after the solution was removed ; it then remained 



