MECHANISM OF IRRITO-MOTILITY 7 



10. Reactions may be Morphologic or Physiologic. The reac- 

 tions which follow the reception of any stimulus may occur im- 

 mediately, within a few seconds, or may be delayed for hours or 

 even days. These reactions may consist in the alteration of the 

 intensity, rapidity and direction of growth ; of the intensity and 

 character of the metabolic processes ; of the rate, rapidity and 

 method of reproduction, alteration of the position of the body 

 by flexions or locomotive action ; of the character and extent 

 of the nuclear and cell cleavages, and of the formation of new 

 tissues, entailing changes in form and mechanical relations to 

 environment. 



11. Motile Reactions. Motile reactions are most easily ap- 

 prehended and estimated, and the organization of irritability 

 reaches its highest visible development in the tissues devoted or 

 concerned in the movements and orientations of the body; it 

 will be most profitable therefore, to begin the study of the rela- 

 tion of the plant to incident forces by a consideration of this form 

 of activity. Such procedure is of still further advantage, be- 

 cause suitable material for the experimental demonstration of 

 these phenomena is most easily procurable in the autumn or the 

 opening of the collegiate year. 



12. Mechanism of Irrito-motility. Two general forms of 

 mechanisms for the performance of movement and other manifes- 

 tations may be mentioned, which differ chiefly in complexity. 

 One has been developed to the greatest extent in the animal 

 kingdom, while the other alone is exhibited by plants. The first 

 receives the stimulus in sensory organs, communicates some kind 

 of molecular motion to a central organ of the nervous system 

 where it may come to consciousness, and is complicated with 

 psychical processes before it traverses a second series of conduc- 

 tors to the organs exhibiting the phenomena of reaction. The 

 other, exhibited by plants, receives the stimulus in sensory re- 

 gions which may or may not be differentiated morphologically, 

 and in which the stimulus gives rise to a second kind of molecu- 

 lar motion which is transmitted more or less directly to the organ 



