64 THE METHOD OF DARWIN. 



Movement in Plants " was written, it has been 

 shown that the conclusion of the authors that 

 "an object which yields with the greatest ease 

 will deflect a radicle " is wrong. The whole 

 superstructure of reasoning which resulted in 

 the notion that the tips of radicles are sensitive 

 to contact was therefore built on sand. The 

 experiments and reasoning will be discussed 

 from the point of view of the authors at the 

 time they were made, and afterwards attention 

 will be called to the corrections that have since 

 been made by others. In their work on the 

 movements of radicles, Charles and Francis 

 Darwin found that "an object which yields 

 with the greatest ease will deflect a radicle." 

 Extremely thin tin-foil on soft sand was not at 

 all impressed, and deflected the root at right 

 angles. Hence, they reasoned, the cause "of 

 the deflection could not be mechanical contact. 

 A conceivable hypothesis was that " the gentlest 

 pressure might arrest growth and the apex grow 

 only on one side; but this view leaves unex- 

 plained the curvature of the upper part, extend- 

 ing for a length of 8-10 mm. . . . We were 

 therefore led to suspect that the apex was sen- 

 sitive to contact, and that an effect was trans- 

 mitted from it to the upper part." 1 By the 



1 Power of Movement in Plants, pp. 131-140. 



