202 



GROWTH HORMONES IN PLANTS 



method of controlling the stimulus is by the use of centrifuging, 

 which can be accurately graded by changing the rate of revolution 

 or the length of the radius; Pekelharing (1909) investigated the 

 threshold reaction of Lepidium roots (see Table 16) by this 



Table 16. — The Threshold Value of Centrifugal Force for 

 Curvature of Lepidium Roots, Illustrating the Quantity-of- 



sTiMULUS Principle 



method. It appears from her data that the product of the pre- 

 sentation time by the force is a rather constant value, within 

 certain limits. 



Growth in Geotropic Curvature. — For the production of a mini- 

 mum curvature, a definite amount of stimulus is necessary. If 

 the amount of stimulus is increased, the size of the reaction is also 

 increased. Lundegardh (1918) made a quantitative study of 

 curvature in the root tip after different amounts of geotropic 

 stimulation (Table 17). The relationship between the increase 



Table 17. — Increase in Amount op Curvature of Avena Coleoptiles 

 WITH AN Increase in the Amount of Centrifugal Force 

 Amount of Stimulus, g Minutes Curvature, Degrees 



5 12 



10 22 



20 47.8 



of curvature with increasing quantity of stimulus is approxi- 

 mately linear. With great amounts of stimulus, a negative 

 instead of a positive curvature takes place in the root (Jost and 

 Stoppel, 1912; Jost and Wissmann, 1924). 



It is important to know whether or not the average rate of root 

 growth during geotropic curvature remains constant. Sachs 

 (1887) concluded that the geotropic reactions in seedhng roots 



