ACTION OF THE NERVOUS SYSTEM. 69 



the fundamental property of irritability. As related 

 in character to this irritability, may be classed the 

 phenomena of some of the experiments of the pre- 

 vious chapter, in which the forces, — for example, the 

 sunlight, — may be supposed to act directly upon the 

 parts of the plant which are affected and changed. 

 But there is another class of phenomena, in which 

 the connection of the change of forces and the 

 change in manner of growth, while none the less 

 certain, is much more obscure and complicated, and 

 indicates a co-ordination of the internal forces, which 

 must partake of the nature of nervous co-ordination. 

 An illustration of this class is found in an experi- 

 ment referred to by Professor Detmer, in which the 

 tip end of a young pine tree is broken off. The re- 

 sult of the removal of the tip is that one of the side- 

 branches turns upward, and, losing its own function 

 and flat-spreading shape, it assumes the function and 

 symmetrical shape of the lost tip of the vertical pine- 

 stem. The side branch thus undergoes a complete 

 change in the manner of its growth. Another strik- 

 ing experiment of this class relates to a habit of 

 growing plant-stalks, according to which the stalk 

 bends sideways or downwards, while the tip end 

 swings slowly in a circle around the main stem, — 

 a process called nutation. The following description 



