INFLUENCE OF EXTERNAL CONDITIONS ON THE PLANT 57 



Darkness on Growth and Development." Memoirs 

 of the New York Botanical Garden, No. 3.) 

 C. The Influence of Light on the Direction of Growth of Roots 



and Stems: 



Experiment 41. To ascertain how one-sided illumina- 

 tion affects the direction of growth of roots and 

 stems. 



1. Fix a vigorous young seedling of white mustard 

 with the root extending through the mesh of a piece 

 of cheese-cloth stretched over the mouth of a large 

 salt-mouthed bottle nearly filled with tap-water. 

 The seedling should be as straight as possible, and 

 stand vertically at the beginning of the experiment, 

 with root extending well into the water. 



2. Place the plant thus prepared into a box with a 

 tightly fitting cover and a narrow, vertical slit at 

 one side to admit the light. (A pasteboard shoe- 

 box with the cover on, and the slit cut vertically 

 in the cover will answer.) 



3. Set the box and plant in a well-lighted window, 

 with the slit toward the light. 



4. Make a diagram of the entire apparatus and plant, 

 in longitudinal section. 



5. At the next laboratory period carefully remove the 

 cover from the box and observe the position of the 

 root and stem. 



6. Draw another diagram similar to, and by the side 

 of the first one, showing what you observe. 



7. Compare the manner of response of the root and 

 stem to one-sided illumination. 



