768 General and Applied Biology 



growing regions of the tip of roots and stems from which they are trans- 

 ferred to the growing areas to promote cell elongation and possibly 

 mitosis. When plants are stimulated by light coming from one direction, 

 the o-rowth hormones flow down the shaded side and decrease on the 

 lighted side. Thus the growth is hastened on the shaded side and 

 retarded on the lighted side. The light displaces the specific hormone 

 toward the shaded side but does not enter into its formation. It is sug- 

 gested that light changes the electrical potential, the shaded side being 

 the positive side electrically. Since these growth hormones are acid, they 

 would be displaced toward the positive (shaded) side. The respective 

 growth of stems toward light and of roots away from light may be ex- 

 plained at least partially on this basis. 



Certain hormones in plants conduct stimuli from one part of a plant 

 to another. This is known as the hormone theory of conduction. These 

 hormones are of great importance in the correlation phenomena in plants. 

 Plant hormones might be defined as chemical substances naturally pro- 

 duced in minute quantities in certain regions of the plant and either 

 stored or transported to other regions to regulate the growth, develop- 

 ment, or reactions of that organism. The tropic responses of plants to 

 light and gravity are definitely associated with specific hormones. Sev- 

 eral diflferent plant hormones have been isolated, such as auxin A, auxin 

 B, and heteroauxin. These are considered in detail in another chapter. 

 Another plant hormone, traumatin (Gr. trauma, wound), seems to initi- 

 ate and influence healing of plant wounds. Plant hormones are trans- 

 ported (1) by diflfusion, (2) by protoplasmic streaming, (3) by plant 

 circulatory systems if they are present, and (4) by electrical phenomena 

 by which they are moved toward a positively charged area because of 

 changes in electrical potential within the plant. A plant hormone 

 (indole-3-acetic acid) produces a tumorlike growth in certain plant tis- 

 sues. In spite of the fact that plant hormones can be isolated from 

 plants, we have no chemical test which provides a simple and efficient 

 means of qualitative and quantitative detection of minute amounts of 

 them in living plants. However, certain physiologic methods are now 

 used to determine their concentration. 



Many data have been collected in connection with hormones in higher 

 animals. Hormonelike substances are present at the nerve endings of 

 nerve fibers in vertebrates, where they may transfer impulses from nerve 

 endings across synapses to responding tissues. Recent data associate hor- 

 mones with the conduction of impulses in nerves. Recent experiments 



