GROWTH SUBSTANCES FOR PHOTOTROPISM 



153 



back than to the front side during the phototropic curvature, the 

 growth-substance concentration upon the back side of tlie tip will 

 be greater than that upon the front side. There are several other 

 possible ways of explaining the origin of this difference in con- 

 centration. Some of these are shown in Fig. 47. The line D 

 in the figure shows the growth-substance concentration in the tip 

 of plants grown in the dark, while the different possibilities of its 

 distribution which can lead to positive curvatures are shown by 

 the lines I to VI. 



Co7itrasting Theories of Boysen Jensen 

 and Padl. — As a result of studies on the _ 



transmission of the stimulus in the Avena • 



I 



I 



coleoptile, Boysen Jensen concluded that 

 phototropic curvature is brought about 

 by an increase in the rate of growth upon 

 the back side of the coleoptile, caused by a 

 downward migrating substance (line VI in 

 Fig. 47). In opposition to this view, Paal 

 (1918) presented the hypothesis that the 

 phototropic curvature may be caused by a 

 retardation of growth upon the front side. 

 He held that the growth-promoting sub- 

 stances, which show an unlocalized migra- 

 tion from the tip of plants in the dark, 

 are either partly destroyed by the light on 

 the front side of the coleoptile tip or are 

 impeded in their movement in such a man- 

 ner as to produce a growth retardation upon the front side of the 

 coleoptile. Paal's suggestion can be represented by the line I or 

 II in Fig. 47. 



Paal's theory is not consistent with the investigations of 

 Boysen Jensen, and Purdy (1921), who demonstrated that the 

 transmission of the stimulus can be almost completely inhibited 

 by a transverse incision upon the dark side. Furthermore, the 

 theory is refuted by certain experiments of Boysen Jensen and 

 Nielsen (1925). In these experiments with coleoptiles 2 to 

 3 cm. long, a 4 mm. tip and the vipper portion of the primary leaf 

 were removed. The empty part of each coleoptile was then split, 

 and a thin, rectangular piece of platinum was inserted in the 

 incision. Two coleoptile tips were then placed symmetrically 



I 

 I 



Fig. 48. — Diagram of 

 a decapitated Avena cole- 

 optUe with two coleoptile 

 tips applied symmetri- 

 cally. A platinum plate 

 is inserted vertically in 

 the stump between the 

 two tips. This permits 

 illumination of a single 

 tip from one side. 

 {From Boysen Jensen.) 



