Chapter Twenty 

 PLANT HORMONES 



The nature of growth has been as eagerly sought as has 

 been the explanation of life itself. They are certainly closely 

 related. But how are roots, shoots, and flowers formed.? 

 Why do most shoots grow upward and most roots down- 

 ward.? During the past fifty years the discoveries of hor- 

 mones, and their effect on growth, have explained some 

 things about growth but have stimulated even greater inter- 

 est in the problem. 



Hormones are substances which are produced in one part 

 of an organism, and are transported to another part where 

 they influence a specific physiological process. Many hor- 

 mones are known in animals, such as those secreted by the 

 thyroid gland, sex organs, etc. 



Plant hormones have been called growth regulators, 

 growth substances, auxins, growth hormones, and phytohor- 

 mones. It should be noted that they are used to condition 

 the growth of the organism rather than to build its structure, 

 as mineral salts, sugars, and proteins are used. Hormones 

 are used in extremely small quantities; for example, if a tril- 

 lion oat plants were treated with a dilute solution made from 

 an ounce of auxin, a pronounced growth curve would occur 

 on each of them. Auxin-a has been shown to be active in 

 water solution in one part to 110,000,000 parts of water. 



Besides hormones, other substances which influence 

 growth are common in plants in small quantities, as enzymes, 

 vitamins, and bios. These differ from hormones in that they 

 usually remain where they are formed to influence a special 

 process. Enzymes increase the rate of an action already 

 taking place. They are widely distributed through the 



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