GROWTH REGULATION 281 



induced to grow on that side and to bend away from the tip. From 

 this he conchided that the curvature was due to an unequal distribu- 

 tion of a growth-regulating substance, and that the tip of the coleoptile 

 is the source of this substance. 



In 1928 F. W. Went at Utrecht came to the conclusion that a 

 growth-hormone system is essential to all plant growth. He demon- 

 strated that the growth hormone can be isolated by diffusion of the 

 substance into an agar block. If the agar block containing the hor- 

 mone was pressed against the side of a coleoptile stump, the coleoptile 

 bent away from the block. When the hormone from more than one 

 tip was transferred to an agar block, Went found that the degree of 

 bending was proportional to the number of tips and hence to the 

 amount of hormone in the block. Thus, Went provided a means of 

 measuring the concentration of the hormone and a method for the 

 isolation of the active substance. 



When the amount of a biologically active substance is measured 

 by application to or injection into a living organism, the technique 

 is called a bioassay. This method is used frequently to evaluate quan- 

 titatively very small amounts of substances, although refined chemical 

 techniques rapidly are replacing the bioassay as increased information 

 allows their development. 



The principal growth substance isolated from higher plants is 

 3-indoleacetic acid. It has been isolated from a large number of mono- 



.^^^N 



— CH2COOH 



H 



3-indoleacetic acid 



cot and dicot species and has been identified in certain fungi. Indole- 

 acetic acid is produced in greatest concentration in the apex of the 

 shoot and in the tip of the root, although it is also found in other 

 actively dividing tissues, such as expanding leaves, flowers, and fruits. 

 As the leaf, flower, or fruit matures, the amount of growth hormone 

 produced decreases materially, although it never completely disappears. 

 In fact, the formation of growth hormones by mature leaves seems to 

 indicate that growth is not the sole prerequisite of hormone produc- 

 tion, since a sizable quantity of the growth substance is formed even 

 when there is little change in size. 



The movement of the growth hormone in a plant is from the tip 

 of the shoot to the older portions of the plant. So powerful is the 

 movement of the substance towards older tissue that if a section of the 



