154 



GROWTH HORMONES IN PLANTS 



upon the coleoptile stump (Fig. 48), and one of these was unilat- 

 erally illuminated in the usual fashion. Curvature toward the 

 light was produced as shown in Fig. 49. This experiment 

 indicates that the flow of growth substance upon the darkened 

 side is increased above the normal, and from this it follows that 

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

 is increased by unilateral illumination. 



Purdy's Theory. — What is known regarding the growth- 

 substance concentration upon the front side of a unilaterally 

 lighted tip? According to one hypothesis (indicated by line VI 



Fig. 49. — Avena coleoptiles as diagrammed in Fig. 48. Light came from the 

 right-hand side, so only the tips on the right were illuminated; positive curva- 

 tures resulted. {From Boysen Jensen and Nielsen, 1925.) 



in Fig. 47), the phototropic curvature may arise solely through 

 the increase in the growth-substance concentration upon the back 

 side, while it remains unchanged upon the front side. The 

 experiments of Purdy, however, indicate that even when the 

 transmission of the stimulus upon the back side is blocked, a slight 

 phototropic curvature occurs. To explain this, one must assume 

 that the rate of growth upon the lighted side is slightly retarded. 

 According to this hypothesis (represented graphically by line V in 

 Fig. 47), the growth-substance concentration upon the front side 

 is slightly decreased. 



The Went Theory. — Went (1928a) determined the amount of 

 growth substance given off on the illuminated and shaded sides of 

 unilaterally illuminated coleoptile tips by placing the tip upon 

 two agar blocks, so that the lighted side stood upon one block and 

 the darkened side upon the other. It was found that less growth 

 substance was given off by the lighted side (about 46 per cent as 

 much as from the darkened side). According to Went, the 



