Chap. XII. CONCLUDING EEMAEKS. 573 



in the case of radicles this was ascertained in only a 

 single instance) the adjoining part bends from the 

 light ; hut when excited by gravitation the same part 

 bends towards the centre of gravity. In almost every 

 case we can clearly perceive the final purpose or advan- 

 tage of the several movements. Two, or perhaps more, 

 of the exciting causes often act simultaneously on the 

 tip, and one conquers the other, no doubt in accord- 

 ance with its importance for the life of the plant. 

 The course pursued by the radicle in penetrating the 

 ground must be determined by the tip; hence it 

 has acquired such diverse kinds of sensitiveness. It 

 is hardly an exaggeration to say that the tip of the 

 radicle thus endowed, and having the power of 

 directing the movements of the adjoining parts, acts 

 like the brain of one of the lower animals ; the brain 

 being seated within the anterior end of the body, 

 receiving impressions from the sense-organs, and 

 directing the several movementa. 



