Stimulation and Correlation. 



117 



in which curvature has ensued. This demonstration may 

 also be repeated with seedlings of bean or stems of grass. 



127. Influence of light on form. Grow two lots of 

 seedlings of pea or bean in pots. Place one lot near a win- 

 dow where it will receive the greatest amount of sunlight 

 possible, and set the other in a dark corner of the room, 

 where it will be exposed to only a weak diffused light. Take 

 precautions that the two preparations are kept at about the 

 same temperature. Two weeks later take up the plants from 

 the two lots, and 



note the differences 

 between specimens 

 grown in intense and 

 in faint illumination. 

 Measure the length 

 and thickness of the 

 stems, the area of 

 the leaves, and note 

 difference in texture 

 of leaves. 



128. Move- 

 ments due to in- 

 tensity of light. 

 Observe the posi- 

 tions of the leaflets of seedlings, of bean, or of Oxalis in 

 the morning, mid-afternoon, and late in the evening, in a 

 room kept at ordinary temperatures. On a second day cover 

 the plant with a box which shall totally exclude light, and 

 note positions of leaves as before. Repeat on a second, 

 third, and fourth day. 



The leaflets will be found to exhibit periodic movements, 

 assuming certain positions at regular portions of the day. 

 These movements are protective in their purpose and are 

 designed to shield the leaves from the effects of intense 



Fig. 87. Oxalis in darkness, showing adaptive 

 positions of leaflets. 



