INTRODUCTION AND HISTORICAL SKETCH 



Went, 1928 

 Growth hormone is given off from plant 

 tissue (coleoptile tips) into agar. When a small 

 block of this agar is placed unilaterally on a • 

 decapitated Avena coleoptile, the resulting 

 curvature, a, is proportional, within limits, to 

 the concentration of growth hormone pres- 

 ent, b. 



Went, 1928 

 When unilateral light falls 

 upon an excised Avena coleoptile 

 tip, a, placed in contact with two 

 agar blocks, 6 and c, separated by 

 a razor blade, d, growth hormone 

 is displaced toward the shaded 

 side; block 6 receives 65 per cent 

 and block c 35 per cent of all the 

 recoverable growth hormone 

 given off from the tip. 



Went, 1928, ANb 

 X.KN Der Weij, 1932 

 Transport of growth sub- 

 stance from an agar block, 

 f, through a segment from 

 an Avena coleoptile into 

 another agar block,/, takes 

 place only toward the mor- 

 phological base. Qualitative 

 proof of the same phenome- 

 non was given bv Beyer 

 (1928). 



A 



V 



Van Overbeek, 1933 

 An agar block, a, containing 

 ■ growth hormone is placed upon the 

 . upper cut surface of a Raphanus 



hypocotyl segment standing upon 

 " two plain agar blocks, 6 and c. 

 . Exposure to unilateral light causes 



displacement of growth hormone 

 'toward the shaded side; the re- 

 - coverable portion is present in the 



two blocks as indicated. 



Hormone Explanation of 



Phototropism 

 The growth hormone is displaced 

 by unilateral light into the shaded 

 portion of a hypocotyl, petiole, or 

 similar organ. Its presence in 

 greater concentration promotes 

 growth more rapidly there, and the 

 organ bends toward the light. 



IIIU 



Gravitv 



DoLK, 1929 



When an excised coleoptile tip, a, is placed in a horizontal position in 

 contact with two agar blocks, b and c, growth hormone is displaced 

 toward the lower side. It accumulates in greater concentration in block c. 



When a cylindrical segment of an Avena coleoptile is placed hori- 

 zontally and supplied with growth hormone at the morphological apex, 

 d, transport takes place toward the lower side of the morphological 

 base, the hormone accumulating in block e. 



U 1 1 i 1 



BoYSEN Jensen, 1933 

 When an excised root tip of Viaa Faba. a, is placed horizontally in contact with agar blocks, 

 b and f, containing 10 per cent glucose, growth hormone is displaced to the lower side and accumu- 

 lates in the lower block, c. 



illU 



IIUU 



DlJKMAN, 1934 



Growth hormone supplied 

 in agar to the cut apex of a 

 [-— J segment of Lupinus by po- 

 t's colyl, placed in a horizontal 

 "tjy position, is transported to- 

 ward the lower side of the 

 morphological base. 



Hormone Explanation 

 OF Geotropism 

 Tropic bending results from dis- 

 placement of hormone to the lower 

 side of the plant axis. The shoot 

 curves upward because its growth is 

 promoted, and the root 

 turns downward be- 

 cause its growth is in- 

 hibited by the hor- 

 mone (Cholodny). 



1 Hi 1 11 



Fig. 2.— Outline of recent contributions to our knowledge of plant growth 



hormones. 



