166 GROWTH HORMONES IN PLANTS 



displacement of growth substance with illumination on one side, 

 for it was found that the substance could be drawn toward the 

 shaded side of a decapitated hypocotyl cylinder. 



This investigator came to the conclusion that the phototropic 

 curvature of the Raphanus hypocotyl is brought about by the 

 combined action of two different effects: (1) by the differential 

 retardation of growth, which is inhibited more upon the lighted 

 side, and (2) by the displacement of growth substance to the back, 

 which must produce an increase in growth upon the dark side and 

 a retardation of growth upon the lighted side. He concluded, 

 further, that the Blaauw theory and the growth-substance theory 

 are not to be considered as antitheses but that the fundamental 

 ideas of both are complementary. 



The question remains how these two effects share quantita- 

 tively in producing the curvature. The miequal distribution of 

 growth substance and of light must be considered together with 

 the fact that the retarding effect of hght is influenced by the 

 growth-substance concentration. Although the computations 

 become somewhat involved, it may be concluded that the curva- 

 ture comes about mainly through the fact that growth on the 

 lighted side is checked markedly, while growth of the 

 darkened side is slightly increased in comparison with the normal. 



THE PHYCOMYCES SPORANGIOPHORE 



The growth-substance explanation has not yet been applied to 

 positive phototropism in the sporangiophores of Phycomyces. 

 Another explanation, however, has been attempted on the basis of 

 the unequal distribution of light wdthin the cylindrical stalk 

 bearing the sporangium. Blaauw (1914, 1919) studied the light- 

 growth reaction of Phycomyces and found that illumination 

 increased the rate of growth. Wiechulla (1932) found that 

 colored lights of equal phototropic effectiveness produce about 

 the same size and type of accelerative growth response. Blaauw 

 explained the positive (not negative) response which follow^s 

 unilateral illumination, by pointing out that the parallel rays of 

 light are concentrated on the back side owing to the lenticular 

 effect of the sporangiophore. In agreement with this theory is 

 the well-known experiment of Buder (1918), who immersed the 

 fungus in paraffin oil to overcome the lens effect and cause the 

 front side to be more strongly illuminated than the back side. A 



