328 TEXTBOOK OF PLANT PHYSIOLOGY 



plants have been subjected recently to reconsideration. Experi- 

 ments by Cholodny show that decapitated roots regain their lost 

 geotropic sensitivity if the tips of roots or coleoptiles of corn are 

 placed on the surfaces of the cut. Similarly, a section of the hypo- 

 cotyl of lupine, which had lost its geotropic sensitivity (Art. 86), 

 regains it if supplied with the growth-promoting substance from 

 the coleoptile. It is evident, therefore, that the main factor influ- 

 encing curvature and bending is the presence and distribution of 

 this hormone. In a horizontally placed organ, or in case of one- 

 sided illumination, the hormone may accumulate on one side, in 

 this case the lower or ventral side, which, according to Browner 

 (1927), becomes negatively charged. The cause of this deviation 

 in flow of the substance, therefore, is probably the electrical 

 polarization of the cells. The falling of the starch grains that carry 

 a negative electrical charge may be considered as one of the factors 

 of increasing polarization and not merely as the inevitable result 

 of the force of gravity. Geotropical sensitivity of decapitated 

 organs can be restored by fastening upon the surfaces of the cuts 

 not only growing tips that manufacture the growth hormone but 

 also pieces of paper or gelatin saturated with saliva, which may act 

 as a substitute for the hormone (Art. 86). 



The very important and interesting question as to why roots 

 react differently to the force of gravity than do stems is explained 

 by Cholodny on the basis of the different action of the hormone on 

 roots as compared to stems. Under the action of the growth hor- 

 mone, the rate of growth of stems is increased while that of 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 roots. Moreover, experiments have 

 shown that the growth hormone which is secreted by the tip of 

 the coleoptile of corn slows down the growth of positively geotropic 

 organs and increases that of negatively geotropic organs. Although 

 the growth hormone in horizontally placed stems and roots flows 

 to the lower side, the result of its action is different. The accumu- 

 lation 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 down- 

 ward. Thus the difference in geotropic reaction can be explained 

 by a dissimilar reaction of the cells of roots and stems to the hor- 

 mone that diffuses into them. Not all authors agree with the 



