EXERCISE 64 

 HOW DO STEMS AND LEAVES REACT TO LIGHT? 



Materials. Seedlings of pea, bean, peanut, or any other seedlings 

 available. Older plants may also be used. 



Directions for work. Place the seedlings in a small box which 

 is tight enough to exclude light except for a small opening on 

 one side at about the level of the leaves of the seedlings 'to 

 be used. Note the position of the stem and principal leaves of 

 the plants and make a memorandum of the facts. Place the 

 inclosing box as near to the window as possible, with the open- 

 ing toward the window. 



Make daily observations on the plant for several days, noting 

 any change of position of the leaves and stem with relation to 

 the light. What position does the stem assume with relation to 

 the direction of incoming light? Is this equally true of the 

 older parts of the stem as well as of the newer parts? What 

 position do the leaves assume with reference to the light which 

 reaches them ? Is there any difference in the action of old and 

 young leaves ? 



Reverse the position of the plant; that is, rotate it through 

 180 degrees, so that the side which was formerly toward the 

 light is turned away from it but the plant remains standing at 

 the same place. How do leaf and stem react at this time ? 



Is the position assumed by the leaves such as secures the 

 best exposure to the light entering the opening? 



References 



BERGEN and CALDWELL. Practical Botany, pp. 56, 57, 64, 65. 

 BERGEN and CALDWELL. Introduction to Botany, pp. 72-76. 



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