58 INFLUENCE OF CHEMICALS 



substances also exercise an effect. Moreover many trophic sub- 

 stances such as sugars, asparagin, etc., which do not undergo 

 electrolytic dissociation produce chemotactic effects. Every 

 organism has acquired the power of reacting to certain substances 

 which are of importance in its existence. The mechanisms of re- 

 sponse may be set in action by other substances of related chemical 

 structure, or other forces, of which the plant had had no previous 

 experience. 1 



93. Relation of the Organism to Trophic and Other Compounds. 

 The three critical points may be noted in the relation of a plant 

 to trophic substances. It is to be said however that the minimum 

 of intensity is generally very low. A correlation is to be found 

 between the optimum and maximum and the irritable influence 

 of a substance. When a free-moving organism finds itself in a 

 medium at a point where any trophic substance is below the 

 optimum in concentration, or below a certain standard of experi- 

 ence, it begins to move toward the point where the concentration 

 is greater. This positive action is sustained until the point is 

 reached where the tonic optimum is reached. If on the other 

 hand, the organism should be under the influence of a concentra- 

 tion above its optimum, or standard of experience, it will move 

 away from this concentration toward a point where this optimum 

 may be attained. It appears probable that the positive chemo- 

 tactic movement is due to the attractive power of the ions of the 

 radicle, and that tfie negative action is due to the hydrogen ions 

 in dissociated substances. The positive response is continued 

 until the repelling power of the hydrogen overbalances the con- 

 trary influence of the radicle, and then the negative reaction is 

 shown. It has been held by some investigators that the repel- 

 lant power of concentrated solutions was due to osmotic action. 

 The influence of the non-trophic substances is most varied. Thus 



1 In this connection see, Jennings, H. S. On the movements and motor reflexes 

 of the flagellata and ciliata. Amer. Jour. Physiol. 3 : 229. 1900. And, Carrey. The 

 effects of ions upon the aggregation of flagellated infusoria. Amer. Jour. Physiol. 

 3:291. 1900. 



