GROWTH SUBSTANCES FOR NORMAL GROWTH 



87 



rapidly throughout most of the coleoptile while it is young, the 

 region of greatest elongation is basal (Fig. 26). As the coleoptile 

 nears maturity, growth slows down throughout its length, ceases 

 at its base, and becomes relatively greater near its apex. At the 

 time the foliage leaf bursts through, all basal growth has ceased, 

 but a localized region of slow elongation at the tip below the 

 pore may persist for as much as two or three days after the leaf 

 bursts through the coleoptile. These same facts apply, in gen- 

 eral, to the coleoptile of Triticum, on which similar observations 

 were made. 



Light-groivfh Reaction. — The inhibiting effect of light upon the 

 rate of growth in plants was recognized in the older plant phys- 



0» 234 5 6 7 89 



Fig. 27. — The course of growth in the Avena coleoptile in different intensities 

 of light. Ordinate: growth; abscissa: 12-hour periods, a, growth of coleoptiles 

 in weak light for 4^-^ days; b-e, growth of other coleoptiles kept for 1 day in 

 the same weak light as in a, and then transferred to different intensities of light, 

 increasing successively from b to e. Greater light intensity shortens the period 

 of growth and causes the size of coleoptiles to be smaller at maturity. (After 

 Sierp, 1918.) 



iology literature (deCandolle, 1832; Sachs, 1874). Since the 

 time when Blaauw (1914) attempted to use Ught-growth reactions 

 as the foundation for a theory of phototropic curvatures, the role 

 of light in growth has been the subject of many investigations. 



Although Blaauw did not perform any experiments on the 

 light-growth reactions in the Avena coleoptile, such investigations 

 were carried out by Vogt (1915), Sierp (1921), Lundegardh 

 (1921, 1922), Koningsberger (1922), Renner (1922), Brauner 

 (1922), Erman (1923, 1930), Went (1926, 1928a), Dillewijn (1925, 

 1927a, 6), Pisek (1926), Beyer (1926, 1927a, c, 19286), Priestley 

 (1926c), Gradmann (1930), Bergann (1930), Nuernbergk and 

 duBuy (1930), Cholodny, (1931rf, 19326, 1933a), and duBuy 

 (1933). Sierp (1918) studied the development of the Avena 



