464 SENSITIVENESS TO LIGHT. CHAP. XX. 



light, continue to bend in the same direction for 

 between a quarter and half an. hour. 



In the two experiments just given, the cotyledons 

 moved backwards or from the window shortly after 

 being subjected to darkness; and whilst tracing the 

 circumnutation of various kinds of seedlings exposed 

 to a lateral light, we repeatedly observed that late in 

 the evening, as the light waned, they moved from it. 

 This fact is shown in some of the diagrams given in 

 the last chapter. We wished therefore to learn whether 

 this was wholly due to apogeotropism, or whether an 

 organ after bending towards the light tended from 

 any other cause to bend from it, as soon as the light 

 failed. Accordingly, two pots of seedling Phalaris 

 and one pot of seedling Brassica were exposed for 8 h. 

 before a paraffin lamp, by which time the cotyledons 

 of the former and the hypocotyls of the latter were bent 

 rectangularly towards the light. The pots were now 

 quickly laid horizontally, so that the upper parts of 

 the cotyledons and of the hypocotyls of 9 seedlings 

 projected vertically upwards, as proved by a plumb-line. 

 In this position they could not be acted on by apo- 

 geotropism, and if they possessed any tendency to 

 straighten themselves or to bend in opposition to their 

 former heliotropic curvature, this would be exhibited, 

 for it would be opposed at first very slightly by apogeo- 

 tropism. They were kept in the dark for 4 h., during 

 which time they were twice looked at ; but no uniform 

 bending in opposition to their former heliotropic 

 curvature could be detected. We have said uniform 

 bending, because they circumnutated in their new 

 position, and after 2 h. were inclined in different 

 directions (between 4 and 11) from the perpendicular. 

 Their directions were also changed after two additional 

 hours, and again on the following morning. We may 



