26o FOOT-NOTES TO EVOLUTION. 



duties the same structure had to perform in the life of 



the creature's ancestors. 



A plant may be defined as a sessile animal. It is an 



organic colony of cells, with the power of motion in 



parts but not that of locomotion. The 



/" ° plant draws its nourishment from inor- 



the plant. . . 



game nature — from air and water. Its 



life is not conditioned on a search for food, nor on the 



movement of the body as a whole. 



The plant searches for food by a movement of the 

 feeding parts alone. In the process of growth, as Dar- 

 win has shown, the tips of the branches and roots are in 

 constant motion. This movement is in a spiral squirm. 

 The movement of the tendrils of the growing vine is 

 only an exaggeration of the same action. The course 

 of the squirming rootlet may be deflected from a regu- 

 lar spiral by the presence of water. The moving branch- 

 lets will turn toward the sun. The region of sensation 

 in the plant and the point of growth are identical be- 

 cause this is the only part that needs to move. The 

 tender tip is the plant's brain. If locomotion were in 

 question the plant would need to be differently con- 

 structed. It would demand the mechanism of the ani- 

 mal. The nerve, brain, and muscle of the plant are all 

 represented by the tender growing cells of the moving 

 tips. The plant is touched by moisture or sunlight. It 

 "thinks" of them, and in so doing the cells that are 

 touched and "think" are turned toward the source of 

 the stimulus. The function of the brain, therefore, in 

 some sense exists in the tree, but there is no need in 

 the tree for a specialized sensorium. 



The many-celled animals from the lowest to the 

 highest bear in their organization some relation to loco- 

 motion. The animal feeds on living creatures, and these 

 it must pursue if it is to thrive. It is not the sensitive 



