TONICITY S 



moisture, gravitation, temperature, electricity, electro-magnetism, 

 light, X-rays, and other manifestations of radiant energy. The 

 irritable influence of any of these forces depends upon variations 

 in their intensity rather than upon the actual intensity which' they 

 exert upon their organism. 



5. Reaction of Organisms to Internal Forces. The activity 

 of any group of substances, or of any organ in the protoplast 

 may set free forces which act as stimuli in setting up irritable 

 reactions in other parts of the protoplast, or organism. The 

 molecular motion set up by such stimuli may traverse long dis- 

 tances and incite reactions in portions of the body distant from 

 the place of origin. It is this mechanism which correlates the 

 activities of the entire body of the plant and gives it an automatic 

 control over the functions of all of its organs (See correla- 

 tions). 



The character of the internal stimuli, and the method of trans- 

 mission of their molecular effects is most imperfectly understood. 

 Manifestations of such automatism are most plainly apparent in the 

 behavior of growing points, diverse carpotropic phenomena, and the 

 general axial arrangement and development of the members of the 

 body. Thus the transferrence of food-material from one part of 

 the body to another, the deposition of reserve matter, the activity 

 of buds, the formation of enzymes, the division and behavior of 

 embryonic tissues are not explainable by reference to the simple 

 chemical and physical activities of living matter, but are controlled 

 by its self-regulatory mechanisms. 



6. Tonicity. The principal forces necessary for the continued 

 activity of living matter, which may be designated as trophic fac- 

 tors, are moisture, food, and various forms of radiant energy. 

 Protoplasm is in a condition for the normal performance of its 

 functions only when these forces act upqn it with a degree of 

 intensity to which its accumulated experiences have accustomed 

 it. Thus a cell carries on its entire group of vegetative functions 

 only at certain temperatures, in which it is said to be in a state of 

 thermotonus. 



