GROWTH HORMONES — THDVIANN 



395 



be used as a test for the hormone (fig. 2). Agar pieces of controlled 

 size were used, and the curvature of the plants measured after a 

 definite time. The curvature was then proportional, within certain 

 limits, to the amount of hormone which must have entered the agar. 



Instead of placing the agar on one side of the coleoptile stump 

 it can be placed symmetrically on it, thus taking the place of the 

 tip, with the result that the coleoptile grows faster on all sides. 

 With a traveling microscope the straight growth can also be used 

 for the assay of the growth hormone. This is important in prin- 

 ciple, but the curvature method has certain technical advantages 

 for use as a routine test. 



Now there are a good many natural conditions under which plants 

 curve. Plants are not free to move about as the higher animals 

 are, since their base is usually fixed. When one is confined to bed 

 by doctor's orders, one's base is similarly fixed, and about all that 

 one can do, when receiving visitors, is to curve in various ways. 



m\im: 



FiGUKE 2. — Oat seedlings with tips removed and blocks of agar containing growth 

 hormone applied. Photographed 100 minutes later. 



Plants curve in particular in response to light and gravity. In these 

 curvatures there is a characteristic difference between the response of 

 the shoot and that of the root. Shoots curve toward a weak light, 

 while the roots are either indifferent or (in some plants) curve away 

 from the light. Shoots curve upward away from the earth, roots 

 typically downward. As mentioned above, it was from studies of 

 the curvature toward light that the role of the growth hormone was 

 discovered. Naturally, therefore, it occurred to these workers that 

 the curvatures caused by asymmetric application of the growth hor- 

 mone are probably related to those due to light and gravity. Cho- 

 lodny, in Russia (1927), suggested that all such curvatures were due 

 to a displacement of the hormone within the plant, more going to 

 the lower side when the plant is placed horizontal, or to the shaded 

 side when exposed to a one-sided source of light. That this is the 

 correct explanation was proved in the following way: tips were cut 

 off and placed on two small pieces of agar so that the hormone 

 diffusing from the two sides would be collected in separate pieces. 

 On now exposing to light from one side, the agar on which the dark 

 side rested was found to contain more growth hormone than the 



