182 GROWTH HORMONES IN PLANTS 



a geotropic reaction which results in bringing the organ back into 

 a position of equiUbrium. 



Nemec (1901) showed that movable starch grains are present 

 in the parenchymatous tissue of the Avena-coleoptile tip. 

 Whether or not they are significant for the perception of the 

 geotropic stimulus was investigated by von Guttenberg (1912), 

 who found that their presence is associated with geotropic 

 sensitivity, although the relationship does not constitute definite 

 proof for the statolith theory. 



The literature on this theory has been brought together in an 

 extensive monograph on geotropism by Rawitscher (1932), who 

 came to the following conclusion: 



If we look back over the results of the main experiments, we must 

 admit that a very close relationship seems to exist between the presence 

 of starch and georeception. We must not overlook the role that carbo- 

 hydrate metabolism plays in the reception of the stimulus of gra\aty. 

 Whether the starch grains actually function in the sense of the statolith 

 theory as conveyors of pressure has become extremely doubtful in view 

 of more recent observations. 



If starch grains actually function as statoliths, it still remains to 

 be explained how their pressure can lead to the unequal distribu- 

 tion of growth substance. 



Electrical Theories and Experiments. — Whatever the explana- 

 tion of the response to gravity may be, the primary reaction is 

 initiated by a traction or pressure effect. According to the 

 electrical theories of geotropic stimulation, differences in potential 

 are produced by the action of gravity upon electrically charged 

 particles which presumably initiate the chain of reactions leading 

 to geotropic response. Small's explanation (1920a, h) was based 

 upon the colloidal nature of protoplasm in relation to isoelectric 

 points. The assumption was made that the protoplasmic 

 particles in the root are electropositive while those of the stem are 

 electronegative. When the plant organs are placed in a hori- 

 zontal position, these particles rise to the upper side and in this 

 way bring about radial potential differences. Cholodny (1918, 

 1923a, b, c, d, 19316) assumed that negatively charged microsomes 

 move downward under the influence of gravity. The difference 

 in potential which arises therefrom was supposed to displace the 

 metal ions and lead to a change in the relationship between 



