6 NATURE AND RELATIONS OF AN ORGANISM 



7. Critical Points in the Action of External Forces. Within 

 the tonic range of any force there is a degree of intensity at which 

 the organism carries on the functions, most directly affected by 

 this force, the most rapidly and to the greatest amount. This 

 point is the optimum of the agent in question. If the intensity of 

 the agent is increased a point is reached, the maximum, where the 

 functions concerned are inhibited. If the intensity is decreased 

 from the optimum, a point is reached where the functions cease, 

 and the minimum is determined. These critical points are by no 

 means identical in regard to different organisms, or the different 

 stages of the same individual, and vary with the complex of all of 

 the incident forces. 



8. Rigor. A decrease of the intensity below the minimum or 

 an increase above the maximum, exercises diverse effects upon 

 living matter. In some instances mechanical injury is produced, 

 in other instances disintegrating chemical action ensues, or in 

 response to some forces the protoplasm becomes rigid and unre- 

 sponsive at the unfavorable conditions. Death follows the undue 

 and rapid increase or decrease of most of the trophic factors. The at- 

 tainment of less favorable conditions of intensity by sudden changes 

 within the tonic range, also brings about a rigor in which the organ- 

 ism is unresponsive to stimuli. The rapid repetition of changes in 

 an incident force backward and forward over a given range of 

 tonic intensity may induce a state of tetanus, or rigid inactivity. 



9. Irritability. ' Sudden variations of the intensity of an incident 

 force may induce changes in the activity of the organism greatly 

 disproportionate to the amount of change in the incident force 

 (3). Generally such responses ensue only when the organism 

 is in a state of tonicity to the force in question if it is a trophic one. 

 The amount of variation of any given force acting upon a plant 

 necessary to produce a response or constitute a stimulus varies 

 geometrically with the amount of the force acting upon the organ- 

 ism at the time the change is made. Furthermore the amount 

 or amplitude of the response varies with the total amount of the 

 stimulating force (Weber's law). 



