58 GROWTH HORMONES IN PLANTS 



in a "bound form" and could not diffuse out. Later work by 

 Soding (1935c, 1936), using the Cephalaria and the Avena tests, 

 has shown that if several pieces of the basal portion of Avena cole- 

 optiles (6 mm. in length) are placed in succession on an agar block, 

 an amount of growth hormone great enough to give test curva- 

 tures diffuses out. He concluded that the growth substance is 

 present in the coleoptile base in the same free form as it is in the 

 tip. 



Cholodny (19356), Laibach and Meyer (1935), and Pohl (1935) 

 have shown that growth hormone is present in the endosperm. 

 Pohl's experiments show a decrease in growth in length of the 

 coleoptile by as much as 25 per cent, if the seed coat and aleurone 

 layer are removed or punctured so as to allow the outward 

 diffusion of growth hormone from the endosperm. Appropriate 

 tests showed that wounding was not responsible for the decreased 

 growth. Addition of growth-hormone paste to the wounded 

 seeds increased the elongation of the coleoptile to almost the 

 normal length. When wounded seeds were placed for 12 hours 

 in an electric field with the injured region toward the anode, 

 growth hormone was demonstrable in the fluid surrounding the 

 anode, and the coleoptiles averaged only 30 mm. in length while 

 the controls were 40 mm. Growth hormone added to the seeds 

 that had their supply removed caused normal growth in length 

 of the coleoptiles. It was concluded that growth hormone is not 

 produced in the coleoptile tip but is given off by the endosperm 

 (presumably moved upward in the vascular bundles) and acti- 

 vated by the tip. 



When an Avena coleoptile is decapitated, the center from which 

 growth substance is distributed is cut off; hence the growth rate 

 of the stump is reduced for some time. After a few hours, growth 

 is renewed due to a "physiological regeneration" at the upper end 

 of the coleoptile stump. This phenomenon has been investigated 

 by Soding (1925), Dolk (1926), Gorter (1927), Tendeloo (1927), 

 Beyer (1928a), Soding (1929), Tsi-Tung Li (1930), etc. Soding 

 (1929) showed that the uppermost millimeter of the "physio- 

 logically regenerated" tip is the new center from which growth 

 substance is dispersed. "Regeneration" takes place to the same 

 extent whether 1 to 1.5 mm. or 5 to 6 mm. is removed (see p. 90). 



Heyn (1935) collected the growth substance from "physio- 

 logically regenerated" Avena coleoptiles into agar blocks. 



