210 GROWTH HORMONES IN PLANTS 



curvature. Amlong tried to explain this discrepancy on the 

 assumption that the opposing action of gravity is not excluded 

 in electrotropic curvatures. Future experimental investigations 

 must determine whether this explanation is valid. 



Koch (1934) has given some valuable evidence in support of the 

 electrical potential theory of growth-substance displacement in 

 roots. The roots of Pisum were placed horizontally in conduc- 

 tivity water, and an electric current from a small battery was 

 applied to the system. It was found that the roots conducted the 

 current; they curved toward the positive pole (Fig. 60). The 

 same electrical potential applied to wet agar containing growth 

 substance caused movement of the latter to the positive pole. 

 In other experiments where growth substance was applied uni- 

 laterally to the tip of a seedling root, Koch found that a bending 

 toward the side of growth-substance application resulted. When 

 he extracted growth substance from Avena coleoptiles and 

 applied it in different concentrations to the opposite sides of the 

 tips of vertical Pisum roots, curvatures resulted which were about 

 equal to the difference between the angles of curvature obtained 

 when these same concentrations were separately applied to just 

 one side of the root. 



In seeking an explanation for the positively geotropic curvature 

 of the root, it has been shown that growth substance has a 

 retarding effect upon the growth of the root, although up to the 

 present time this has been proved only for definite growth-sub- 

 stance concentrations. Since the rate of growth in the root is 

 increased by decapitation, it may be concluded that in normal 

 roots, growth is retarded by the growth substance which is 

 present in the root tip. It follows that the rate of growth of the 

 upper and lower sides of the root must be different because of the 

 unilateral accumulation of growth substance. The lower side 

 grows more slowly, and a downward curvature is produced. It is 

 difficult to say whether or not the difference which is found in the 

 amount of growth substance in the upper and lower sides is 

 sufficiently great to produce a normal geotropic curvature. The 

 quantitative relationship between fluctuating growth-substance 

 concentration and root-growth rates and the precise determina- 

 tion of grow^th-substance distribution in the root have not been 

 investigated with methods entirely free from objection. It is 

 highly probable, however, that the positively geotropic curvature 



