ADJUSTMENT TO LIGHT 89 



Experiment 29. Phototropic movements. Select four sunflower 

 seedlings in which the plumules have not j^et unfolded. Remove the 

 cotyledons from one. WrajJ the stem of another with black paper or 

 tinfoil from the base to the cotyledons, and wrap the tip of the stem 

 of the third in similar fashion. Use a thread moistened with ink to 

 mark the upper part of the stem of the fourth at 2 mm. intervals. Place 

 the plants in a light-tight box at equal distances from a small hole in 

 one end. Remove them after a few days, and explain the results noted 

 in each. 



1 10. Nyctotropism. Many plants change the position of their 

 leaves at night. This is particularly true of plants with compound 

 leaves, such as the bean, clover, oxalis, sensitive plant, and others. 

 The leaflets of these plants close at night, and the entire leaf often 

 changes its position, usually by drooping. This phenomenon is 

 called nyctotropism. It is thought to be due to the influence of 

 light, but it seems that temperature plays at least as active a part 

 in bringing it about. The movement occurs at night and morning, 

 when the changes in light intensity are the most rapid, but at this 

 time the changes in temperature are marked also. The opening 

 and closing of many flowers takes place at these times also. These 

 have likewise been supposed to be influenced by light, but it is 

 now known that they are due to heat. 



Experiment 30. Nyctotropic movements. Grow sensitive plants, 

 beans, clover, and oxalis. Compare the position of the leaves at night 

 and in the daytime. Change the plants from sunlight to darkness, and 

 the reverse, and note the results. 



