GROWTH SUBSTANCES FOR GEOTROPISM 



193 



greater part of the growth substance disappeared, as did geo- 

 tropic sensitivity, and the shoots grew horizontally. The small 

 amount that was left w^as unequally distributed, more being 

 found in the upper half. Since the quantity remaining after 

 treatment with ethylene was very slight, the measurements may 

 not be significant. 



14 



V2 



Time in hours 



PiQ. 59.— Negatively geotropic curvature and differential distribution of 

 growth hormone in horizontally placed seedling stems of Vicia Faba. A, the 

 course of curvature. B, the growth hormone is equally distributed in the upper 

 and lower halves during the first half hour in the horizontal position; in the 

 ensuing 15 minutes there is a shift in the concentration, so that the lower side 

 contains 65 per cent, the upper side, 35 per cent. The higher concentration 

 on the lower side is correlated with the negatively geotropic curvature. (.AJter 

 Van der Loan, 1934.) 



Boysen Jensen (1936), using the chloroform extraction method, 

 investigated the distribution of growth substance in seedling 

 axes of Phaseolus and Vicia during geotropic curvature. In 

 accord with previous work, he found a greater concentration 

 in the under side than in the upper, although the difference in 

 concentrations was not so great as might have been expected 

 from the observed rates of growth on the two sides. 



According to Beyer (1932), geotropic curvatures observed in 

 stalks of Taraxacum, hypocotyls of Impatiens and Cucurbita, and 



