Tomato plants laid on their sides, with the light coming from the right. 



turn their tips upward, and also toward the light 



If now the lighted side in the erect plant and the upper sides in the horizontal plants are treated 

 with indole-acetic acid. 



the plants bend away from the light and also turn downward 



p. W. Zimmerman and A. E. Hitchcock, contributions from Boyce Thompson Institute 



RESPONSE OF PLANTS TO LIGHT, GRAVITY AND GROWTH SUBSTANCES 



Is the tomato plant's turning toward the light and away from the earth deter- 

 mined by auxins, or growth substances? Is it possible to remove auxins — ^ if any 

 — from the plant? Both extracted auxins and synthetic compounds reverse the 

 plant's responses to light and gravity. Can we change the behavior of animals by 

 chemical means? 



