384 NATURAL SCIENCE [June 



municatecl, both in Germany and England, by Professor Pfeffer 

 (10 & 11), in whose laboratory and under whose guidance the 

 researches had been carried on. All who were present at the 

 Oxford meeting of the British Association in 1894 will remember with 

 what eagerness Professor Pfeffer's spirited address was listened to. 



The plan which was adopted in this research was as ingenious 

 as it was simple. The end of the root to be experimented on was 

 directed into a small glass tube of suitable calibre, which had been 

 bent on itself at right angles. The apex of the root was thus 

 obtained at right angles to the part which carries out the curvature. 

 Accordingly, if the apex is placed vertically the growing (and 

 curving) part is horizontal and vice versa ; we have, thus, a means 

 to hand by which we can determine whether it is the 'tip' of the 

 root or the growing region lying behind the apex which is perceptive 

 of the stimulation due to gravity. Two batches of roots, which had 

 been treated in this way but which were otherwise normal, were 

 taken. The members of the one batch were placed so that their 

 apices were vertical, whilst those of the other group were arranged 

 so that the growing region was vertical. The result was that the 

 first set with vertical root-tips grew downwards without exhibiting 

 any curvature, whilst those plants which had their root-apices 

 horizontal, curved in the growing region, so as to bring the apices 

 into the vertical position which is normal to them, notwithstanding 

 the fact that the growing region was itself displaced from the 

 vertical by this movement. The long disputed question was at 

 length set at rest by these experiments and it can no longer be 

 doubted that Darwin's views were correct. The apex of the root 

 receives the stimulation of gravity and transmits it to the growing 

 region behind, which reacts to the stimulus by a curvature. The 

 enthusiasm with which Professor Pfeffer's communication was 

 received at Oxford was perhaps not altogether uninfluenced by a 

 feeling of pleasure that our great naturalist had been in the right 

 through these long years of controversy. 



In the following year Czapek (1) published his " Untersuchungen 

 iiber Geotropism " in which the above-mentioned matters were set 

 down at length, together with many other results of importance. 

 The demonstration that the perception by the root-apex of the 

 stimulus due to gravity was influenced by the various conditions of 

 environment otherwise than was the reaction itself, is of great 

 interest. Under a temperature too low for the geotropic curvature 

 (reaction) to take place, the perception of the stimulus by the 

 root-tip was nevertheless manifest. The curvature of the root, 

 which is the end result of the stimulation of gravity, is directly 

 dependent upon the growth of the organ. Stop the growth of the 

 root and you at the same time stop the reaction. 



