CHAPTER IV 



THE OCCURRENCE AND FORMATION OF GROWTH 



SUBSTANCES 



The presence and activity of growth substance were demon- 

 strated first in the phototropic and geotropic curvatures of the 

 Avena coleoptile. Later it became clear that growth-regulating 

 substances, that is, hormones, are widely distributed in the plant 

 kingdom. Their presence in any part of an organ or in a culture 

 substratum may be shown in any one of several ways, e.g., by 

 their growth-promoting effect on decapitated Avena coleoptiles. 

 A survey of the distribution of these growth hormones in different 

 organs of higher plants and in different groups of higher plants 

 will be given briefly. 



OCCURRENCE 



Higher Plants. Growth Substance in Coleoptiles. — Padl (1918) 

 showed that a growth substance, influencing phototropic curva- 

 ture, is present in the tip of the Avena coleoptile. In a typical 

 experiment, a coleoptile was decapitated, and one-half of the cut 

 surface was covered with a foil plate. When the tip was replaced 

 in its original position, contact was made only between half of the 

 tip and basal region of the organ. In a short time, curvatures 

 resulted toward the inserted tin-foil plate. From this experi- 

 ment it was concluded that growth substance present in the non- 

 illuminated tip moves downward unilaterally and produces a 

 curvature. A similar bend is obtained when the excised tip is 

 replaced unilaterally upon the coleoptile stump. Paal carried out 

 these latter experiments with Coix, and later Neilsen (1924) 

 performed similar tests with Avena. 



Soding (1924, 1925, 1929) investigated the occurrence of growth 

 substance in the tip of nonilluminated coleoptiles. It may be 

 mentioned here that the growth rate of a decapitated Avena 

 coleoptile was found to be increased by replacement of the tip. 



Using coleoptiles of different species of grasses. Stark and 

 Drechsel (1922) and Stark (1924) showed that phototropic 



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