280 PLANT BIOCHEMISTRY 



PLANT HORMONES AND REGULATORS 



All multicellular organisms are integrated so that the various parts 

 function together as a closely coordinated unit. By careful regulation 

 of growth, development, and function, an organism is produced which 

 is always characteristic of the species. Moreover this organism is 

 capable of responding to external stimuli and therefore is sensitive 

 to its environment. In plants this response is accomplished without 

 benefit of a nervous system and specific sense organs, such as occur in 

 most animals. 



Plants, like animals (page 312), have hormones that are produced 

 in one part of the organism and that influence a specific physiological 

 process when transported to another part of the organism. Contrary 

 to animals, plants do not produce hormones in any specific glands, 

 but can manufacture them in a variety of plant tissues. Plant hor- 

 mones also differ greatly in chemical structure from those of animal 

 origin. 



Substances which act like hormones but are not known to be pro- 

 duced by the plant are called plant regulators. The study of plant 

 hormones and regulators is one of the most interesting fields of plant 

 physiological research, and their use in agriculture has become very 

 important. 



Growth Hormones 



The first postulation of the existence of a plant-growth hormone was 

 advanced by Charles Darwin in 1881. As a result of some simple 

 experiments using canary-grass seedlings, a light source, and a razor 

 blade, Darwin concluded that the tip of the grass shoot governs the 

 phototropic response. Excision of the tip of the sheath-like structure 

 which encloses young leaves of grasses (coleoptile) resulted in the loss 

 of sensitivity of the remaining portion to unilateral light. When the 

 tips of intact coleoptiles were covered with little tin-foil caps, a similar 

 loss of sensitivity to light occurred. When the lower part of the 

 coleoptile was wrapped with tin foil and only the tip was exposed to 

 unilateral light, the tip bent towards the light. From these findings 

 Darwin concluded that "when seedlings are freely exposed to a lateral 

 light, some influence is transmitted from the upper to lower part, 

 causing the latter to bend." 



In 1919 Padl discovered that, if he cut off the tip of an oat coleoptile 

 and applied it against the side of the cut stump, the coleoptile was 



