CHAP, xi Sensitivity and Stimulation 473 



by Sachs, who objected to both his experiments and con- 

 clusions, insisting that the removal of the root apex is too 

 drastic treatment and causes various irregular curvatures. 

 He insisted, therefore, that conclusions drawn from such 

 experiments must be valueless. Detlefsen, in 1882, sup- 

 ported Sachs in his objections and joined him in denying 

 that roots with amputated tips have lost their power of 

 curvature when placed horizontally. Wiesner, in 1884, 

 supported Sachs up to a certain point, but admitted that 

 amputation diminishes subsequent geotropic curvature, as 

 did Molisch in 1883. 



The controversy on this point became very acute, but 

 events have proved that Darwin was right in his con- 

 clusions. Experimental evidence of some weight was 

 furnished in 1894 by Czapek, by his classical experiment 

 of causing young roots to grow into small glass slipper- 

 shaped tubes, bent at a right angle at about 2 mm. from 

 the pointed apex. He brought about in this way a natural 

 bend in the root, the tip only being diverted through an 

 angle of 90 from the rest of the axis. In Czapek's experi- 

 ments the root so treated underwent no geotropic curvature 

 so long as the tip was directed downwards, though, of 

 course, the rest of the axis was horizontal. 



F. Darwin's demonstration by a similar method that the 

 cotyledon of Setaria and Sorghum is the region sensitive 

 to gravitation was made in 1899. 



The method of response was shown to be different for 

 growing and for adult organs. In the case of the former 

 Hofmeister showed, in 1863, that no curvature can be 

 observed except in the actively growing zones, and that the 

 curve is due to difference of growth upon the opposite 

 sides. Sachs claimed, in 1865, that increased growth of 

 the convex side brings about the curvature in all cases. 

 Darwin gave a similar localization. A more exact de- 

 limitation of the area of most active curvature was given 



