96 



GROWTH HORMONES IN PLANTS 



The observations of Thimann and Skoog (1934), working with 

 epicotyls of Vicia Faba, show that a growth-promoting substance 

 is formed by the action of hght upon the green portions of the 



>> 



a 

 •a 



u 

 a 

 O 



o 



Xw 



Zones 123456789 

 Fig. 32. — Distribution of growth in the seedling stem of Phaseolus multiflorus 

 (hypogeal cotyledons) at 24°C. The stem was marked into nine 5 mm. zones, 

 beginning at the level of the first foliage leaves. Ordinate: growth in millimeters 

 in 24 hours; abscissa: 5 mm. zones. {After Boysen Jensen.) 



I 2 3 4 5, 



TIME IN DAYS 

 Fig. 33. — Growth of defoliated and decapitated plants of Vicia Faba in the 

 Hght and in the dark, with and without growth hormone. A, growth in the 

 dark, following the addition of 1600 units of growth hormone per cubic centi- 

 meter. B, growth in the light with the same amount of growth hormone as in 

 A. C, growth in the light following the application of plain agar. D, growth 

 in the dark with plain agar. {Adapted after Thimann and Skoog, 1934.) 



plant. The rate of growth, however, is more rapid in darkness 

 than in Hght in experiments where comparable amounts of 

 growth substance are added (Fig. 33). 



