164 PHYTOHORMONES 



E. The Mechanism of Geotropic Perception 



It is interesting to recall that Czapek's theory of geotropism (1902) 

 comes remarkably close to the auxin explanation in one particular 

 respect. According to this theory, on geotropic (or phototropic) stim- 

 ulation of plants, the reducible substances inside the cells increase, 

 this being due mainly to the formation of homogentisic acid from 

 tyrosine. The remarkable fact about this is that homogentisic acid, 



HOn 



-CHo COOH is closely related to phenyl-acetic acid, 



H 

 — CH2 COOH which has definite growth-promoting activity 



("primary activity") in the pea stem test. 



A number of attempts have been made to explain the 

 way in which gravity may cause the unequal distribution 

 of auxin. Two pieces of evidence indicate that the same 

 basic principle underlies all geotropic reactions. In the 

 first place, Navez (1929) has compared the temperature 

 coefficient for the respiration of Vicia Faba seedlings with 

 the temperature coefficients for the geotropic presentation 

 and reaction times of the roots. Between about 5° and 21° 

 all have the same value {fx = 16,200), from which he 

 concludes that the geotropic response is governed by an 

 oxidative or respiratory reaction. The role of a respiratory 

 reaction in the response to auxin has been discussed in 

 Chapter VIII. 



In the second place, the percentage of the total auxin 

 which diffuses out of the lower half of horizontally placed 

 shoots or roots of different plants varies within rather narrow 

 limits, as Table XIII (ampHfied from that of Boysen Jensen, 

 1936a) shows. It may be concluded from the uniformity 

 of the results with different plants that the basic factor 

 inducing lateral auxin transport is the same throughout. 



A number of theories as to the mechanism of auxin dis- 



