of the root-tip in relation to geotropism. 135 



fixed into a tube or other support, and since the cotyledons are of 

 relatively great weight the difficulty appears insuperable without 

 some special device. This has been supplied by Mr H. Darwin 

 who devised a " Root-Lever " which has been made for me by the 

 Scientific Instrument Company. It will be described in a fuller 

 paper ; it will be sufficient for the reader to know that while 

 the weight of the cotyledons is supported, they are free to move 

 in any direction in obedience to the curvature of the root. The 

 cotyledons being supported by the root-lever, the tip of the root is 

 inserted into a fixed horizontal tube and the root is kept from 

 withering by water slowly and gently dripping on it. The great 

 difficulty is to keep the tip of the root from slipping out of the 

 fixed tube. A great majority of the experiments failed from this 

 cause, but this does not invalidate the theory, although it points 

 to the need of improving the method. Omitting from consider- 

 ation these cases and a few in which the roots did not curve 

 geotropically, my results point clearly to a strong tendency in the 

 root to continue curving. In several cases the cotyledons travel 

 through 180° and in one case through about 360°, so that the root 

 looked as though a knot had been tied in it. 



In the case of peas the difficulty of keeping the root in place is 

 less, the failures are fewer, and the tendency to continued curva- 

 ture in the root is strongly marked. These experiments are still 

 in progress and will be described in detail in a fuller publication. 

 There too will be fully discussed the interpretation of the results 

 which presents some difficulties depending on the distribution of 

 growth in roots (as compared with stems) and on the existence of 

 other forms of irritability in the root-tip. One consideration must 

 not however be omitted. The tip of the root is sensitive to 

 unequal pressure or unilateral injury, and it may be supposed that 

 the continued curvature of the root is due to pressure exercised on 

 the tip of the root by the fixed tube. But the results of a number 

 of experiments in which a root was placed tip downwards in a 

 vertical tube do not support this view. There seems no reason 

 why a vertical tube should not be equally effective with a hori- 

 zontal one in producing contact stimulation. But there was no 

 tendency to continuous curvature when the tube was vertical. 



