GROWTH SUBSTANCES FOR GEOTROPISM 



175 



Table 11. — The Quantity-of-stimulus Principle in the Curvature of 

 the avena coleoptile in response to centrifugal force 



The Course of Geotropic Curvature. — Before discussing the 

 course of geotropic curvature in detail, it will be well to mention 

 the question of the total growth taking place during the curvature. 



With the help of the micropotometric method, Cholodny (1930) 

 compared the rate of growth during the period of geotropic 

 curvature with that of a normal upright-growing Avena coleop- 

 tile. He concluded that "all these experiments clearly point to 

 the fact that geoinduction has absolutely no influence upon the 

 growth rate of a coleoptile." It is true that Cholodny 's figures 

 for half-hour growth increments fluctuate considerably, and it is 

 probable that small growth changes cannot be demonstrated by 

 this method. 



By means of cinematographic photographs, Weber (1931) 

 investigated the course of growth in barley seedlings which lay 

 continually in a horizontal position. Immediately after having 

 been placed in the horizontal position, a difference between the 

 rate of growth of the two sides of the coleoptile became apparent ; 

 an increase on the lower side and a decrease on the upper side 

 occurred. The total growth was not demonstrably changed 

 (Fig. 52), When barley seedlings were stimulated by being 

 placed in a horizontal position for 30 minutes, a negative curva- 

 ture of the coleoptile appeared, followed by several back-and- 

 forth curvatures in the upper zones, where growth promotion 

 on one side alternated with growth retardation on the other. 



