124 PLANT PHYSIOLOGY 



Boysen-Jensen (1910) observed a fact of great importance, 

 that growth of a decapitated coleoptile may be appreciably 

 hastened if the cut tip is stuck back on the cut end by means of 

 a drop of water or gelatin. 



However, if a thin piece of tin foil or mica is placed between 

 the cut end and the tip, acceleration will not be observed. 

 Hence Boysen-Jensen drew the conclusion that the tip of the 

 coleoptile excretes a special substance that accelerates growth 

 and is capable of diffusing through a layer of water 

 or gelatine. 



This stimulating effect may be easily demonstrated 



if the cut tip is placed on one side of the cut end. 



This side will obtain more of the growth-promoting 



substance than the opposite one and will grow more 



rapidly. In consequence, the seedling will curve 



Fig. 29.— toward the opposite side (Fig. 29) . Such a curvature 



fhe^'oit'cole- i^ certainly much more easy to perceive than the 



optile under general steady increase in length. 



the influence rpj^ development of the ideas of internal secretion 



of auxin, 1, ^ 1 ' £ ' £ 



contained in in animal physiology and the isolation of a series oi 

 com'''al''fd animal hormones contributed greatly to further 

 ^vith agar advancement in the concept of growth-promoting 

 ''^^'''^^ ^' substances. At present, the term ''hormone," intro- 



duced to science in 1905 by the animal-physiologist Starling, 

 designates a fairly large group of substances elaborated by the 

 organism and possessing the capacity of directing and regulating 

 the course of different vital processes. In the animal organism, 

 the hormones are produced by special glands of internal secretion, 

 endocrine glands, and are introduced into the lymphatic and 

 blood system, which carries them throughout the body. The 

 most widely known of the hormones are adrenalin, excreted by 

 the suprarenal gland; insulin, secreted by the pancreatic gland; 

 folliculin, elaborated by the ovaries; and many others. In their 

 chemical structure, the hormones do not show much chemical 

 relationship, except that a number are related in structure to 

 phenanthrene. In common with enzymes, they possess a very 

 high activity at exceedingly low concentrations, but they are 

 distinguished by a greater stability in respect to heat and other 

 destructive agents. 



