CHAPTER XXXII 

 GROWTH HORMONES 



Among the substances which markedh' influence the reactions and metab- 

 olism of the animal body are those internally produced compounds called 

 hormones. Many of the hormones of the higher animals are secreted by the 

 ductless glands; examples are adrenalin, thyroxin, and insulin. Within the 

 last decade or two convincing evidence has accumulated that hormone-like 

 substances also occur in plants. These compounds are called growth sub- 

 stances, hormones, groivth hormo?ies, or phytohormones. Like animal hor- 

 mones plant hormones quite commonly affect parts of the organism other 

 than those in which they are produced. It is a characteristic of both plant 

 and animal hormones that they usually exert their physiological effects while 

 present in minute concentrations; it is principally on this basis that they differ 

 from compounds ordinarily classified as foods. 



Relation of Auxins to Growth of the Oat Coleoptile. — The auxins are 

 the best known and most comprehensively studied group of plant hormones. 

 Their action has been most clearly demonstrated in the leaf sheath or coleoptile 

 of the oat plant (Jvena sativa) . This is a tubular, leaf-like structure, closed at 

 the top, which is the first organ of the plant to emerge from the soil. Similar 

 coleoptiles are produced during the germination of seeds of other members 

 of the grass family. The coleoptile encloses the first leaf, and is eventually 

 pierced at the tip as a result of the growth of this leaf, soon after which all 

 growth in length of the coleoptile ceases. Oat coleoptiles are approximately 

 1.5 mm. in diameter, and when illuminated seldom attain a length of more 

 than 2 cm. In the dark they may attain heights ranging up to 6 cm. Cell 

 divisions cease relatively early in the life history of an oat coleoptile, and 

 during approximately the last three-fourths of its growth period all increase 

 in its length is due to elongation of its constituent cells (Avery and Burk- 

 holder, 1936). 



If the tip of a coleoptile is removed by a clean cut made several millimeters 

 below the apex, the rate of growth of the stump is immediately retarded 

 (Soding, 1925). If, however, the cut-off tip of the coleoptile or a similar 

 tip from another coleoptile if afllixed on the stump, its growth will be resumed, 



572 



