GROWTH OF PLANTS 149 



zone of geotropical sensitivity of different organs with the 

 zone that produces growth hormones served as "a point of attack 

 for the further solution of the question. 



Experiments by Cholodny (1926) have shown that decapitated 

 roots regain their lost geotropic sensitivity if tips of roots or 

 even tops of the coleoptile of corn are placed on the surfaces of 

 the cut. Similarly, a section of the hypocotyl of lupine that 

 owing to the removal of its conductive tissue has lost its geotropic 

 sensitivity (see Art. 29) regains it if supplied with the growth- 

 promoting hormone from the coleoptile. Hence it is evident 

 that the main factor influencing curvature is the presence and 

 distribution of the growth hormone. In the case of a hori- 

 zontally placed organ, a unilateral distribution of the hormone 

 flow may be observed, viz., in the lower side, which according 

 to Browner (1927) is negatively charged. Thus the cause of the 

 deviation in the flow of the hormone is probably the electrical 

 polarization of the cells. 



The very important and interesting question as to why roots 

 react differently from stems to the force of gravity, in spite of the 

 fact that both produce the same hormone, is explained by 

 Cholodny on the basis of the different action of hormones on the 

 roots and stems. Under the action of the growth hormone, the 

 rate of growth of stems is increased, while that of the roots is 

 decreased. This is shown also by the fact that decapitated 

 stems grow more slowly than uninjured ones, while decapitated 

 roots grow faster than normal ones. 'Moreover, experiments 

 have shown that one and the same hormone, e.g., the hormone 

 secreted by the tips of the coleoptile of corn, retards the growth 

 of positively geotropic organs and increases that of negatively 

 geotropic organs. Although the growth hormone in horizontally 

 placed stems and roots flows mainly to the lower side, the result 

 of its action is different; the accunaulation of the hormone in 

 stems increases the growth of the lower side and causes the 

 organ to bend upward, while in the case of roots, the growth 

 of the lower side is decreased and the bending is downward. 

 Consequently, the difference in the geotropic reaction can be 

 explained by a dissimilar reaction of the cells of roots and stems 

 to the hormone that diffuses into them (see Art. 30). 



Geotropic sensitivity is not quite constant and may be influ- 

 enced by external as well as by internal conditions. Owing to 



