LECTURE XVIII. 



c. 10 radicles had their tips greased for a length of only i m.m. 



of these, four curved to the sieve within 24 hours, and four more 

 curved in the succeeding 24 hours. 



d. 5 radicles cauterised to a length of about i m.m. with nitrate of silver : 



in 24 hours one had curved into contact with the sieve, another 

 was coming towards it, and the remaining three were vertical. 



He suggests that in those cases in which the radicles became curved, 

 it is possible that the layer of grease was not sufficiently thick wholly to 

 exclude moisture, or that a sufficient length was not thus protected, or, 

 in the case of caustic, was not destroyed ; and he accounts for the fact 

 that in some cases curvature took place after an interval of one or two 

 days by pointing out that when radicles with greased tips are left to 

 grow for several days in damp air, the grease becomes drawn out into the 

 finest reticulated threads with narrow portions of the surface left clean 

 which would probably be able to absorb moisture. 



The validity of Darwin's conclusion as to the localisation 

 of the hydrotropic sensibility in the root-tips has been ques- 

 tioned by Wiesner and Detlefsen. Wiesner urges, as he did 

 also with reference to geotropism, that the greasing or 

 cauterising of the root-tips induces altogether abnormal 

 modes of growth. He points out that though Darwin indeed 

 mentions that the roots with greased tips continued to grow 

 satisfactorily, yet they do not grow nearly so actively as 

 uninjured roots, but he admits that there is no ground for 

 asserting a proportionality between hydrotropic curvature 

 and growth in length. Both he and Detlefsen observed, in 

 experiments with decapitated roots, that some of them curved 

 hydrotropically, but the great difficulty of obtaining accurate 

 results in experiments of this kind, as mentioned above in 

 speaking of geotropism (p. 468), must be borne in mind. 



On the other hand, Darwin's observations are confirmed by 

 those of Molisch. The method which Molisch employed is 

 not open to the objections which may fairly be made against 

 the methods of greasing and decapitation, for it does not 

 inflict any injury upon the root. It consists in enveloping 

 the whole root, except the tip which is left uncovered, in wet 

 paper, and exposing it to the influence of a moist surface. If, 

 now, a curvature of the root takes place, it can only be due to 

 the action of the moist surface upon the apex, for the rest of 



