GROWTH HORMONES IN PLANTS 



A 



A 



Darwin, ISSO 

 Light falling from one side 

 upon the tip of a grass coleoptilc 

 (Phalaris) causes some influence 

 to be transmitted downward; 

 the coleoptile curves toward the 

 light. 



When the coleoptile tip is re- 

 moved, phototropic response 

 does not occur. Localized sensi- 

 tivity to light and conduction of 

 a stimulus was observed early in 

 many kinds of plants. 



Fitting, 1907 

 In a room .saturated with 

 water vapor, lateral incisions 

 either on one or on both sides 

 of the Avena coleoptile do not 

 prevent its bending toward 

 light from one side. 



A 



BoYSEN Jensen, 1910-1911 

 When an excised coleop- 

 tile tip was replaced with 

 gelatin inserted between it 

 and the stump, phototropic 

 curvature resulted as in nor- 

 mal coleoptilcs; the tropic 

 stimulus passed over the 

 incision. 



£\ 



n 



a 



BoYSEN .Jensen, 1910-1(111 

 Insertion of mica plates on the shaded side pre- 

 vented curvature following unilateral illumination of 

 the tip. When the mica insert was made on the illumi- 

 nated side, curvature resulted in the usual way. It was 

 concluded that a substance migrates down the back side 

 promoting growth curvature toward light. 



A 



Paal, 1918 

 When an excised tip is re- 

 placed on one side of the Avena 

 coleoptile stump, accelerated 

 g:o" th beneath the tip results in 

 curvature. 



SUDING, 1925 



Decapitation results in 

 diminished growth of the 

 Avena coleoptile, but when 

 the excised tip is replaced, 

 growth in length is renewed. 



a 



Negative 

 curvature 



Stahk, 1917-1921 Seubekt, 1925 

 Expressed sap from Avena coleoptiles was put into agar blocks which were 



■■ipplied unilaterally to coleoptile stumps; curvatures resulted. 

 .Seubert (1925) found by using this method that some substances promote 



while others inhibit growth, as indicated by negative and positive curvatures. 



Positive 

 curvature 



LOEB, 1910 

 The presence of vig- 

 orous leaves on a hori- 

 zontally placed Bryo- 

 pliylluni stem increases 

 geotropie bending (also 

 the production of roots 

 is stimulated). The 

 action of hormones 

 was suggested as the 

 explanation. 



Went, 1928 

 When an Avena coleoptile is 

 decapitated its growth in length 

 cea.ses, a. The addition of a plain 

 agar block, b, has no effect, but 

 growth is renewed by the addi- 

 tion of a block, c, containing 

 juice extracted from the excised 

 tip. 



Fig. 1. — Historical outline of the early discoveries concerning plant growth 



hormones. 



