IRRITABILITY; POLARITY AND CORRELATION 343 



their environment by responding to changes in moisture, heat, 

 light, etc., is called irritability. A living plant or animal is conse- 

 quently irritable. A tree growing in a wood is subject to a flood 

 of stimuli : the light which falls upon it is varying in direction and 

 intensity through the day and from season to season ; the moisture 

 content of the soil and air varies from a comparatively small 

 amount on a dry day during a long drought to saturation during 

 a rainy spell; and in the summer the temperature is high, while 

 during the winter the water between the cells may be freezing. 

 If all these many factors are combined in all the possible ways, 

 one may get some conception of the varying conditions under 

 which a plant carries on its work. To adjust itself to all these 

 changes, is the chief problem of living protoplasm. Under the 

 normal conditions, when these factors are well combined, the 

 organism is thriving and in good health. If life means the adjust- 

 ment of the organism to the environment, then sickness and poor 

 health are accompanied by a partial loss of irritability as the 

 organism loses the power to make the proper adjustments under 

 unfavorable conditions, and death is the permanent loss of ir- 

 ritability. Anaesthesia, in which certain phases of irritability are 

 temporarily but not permanently lost, should not be confused 

 with the state of " suspended animation' 5 as seen in seeds and 

 trees in winter. In these latter cases, the plant is inactive, but 

 this inactivity is in itself a form of response; as soon as water, 

 heat, etc., are present, the plant becomes as active as ever. In 

 anaesthesia and death, the power to respond is gone; in suspended 

 animation the power to respond is as great as ever. 



Fatigue, Tetanus, and Rigor. — If an organ is stimulated and 

 the stimulus is withdrawn, the organ will respond and then re- 

 turn shortly to the normal condition, when it is ready to respond 

 again. If several stimuli are given in succession before time for 

 complete recovery has elapsed, the response diminishes. The 

 organ is becoming fatigued and lacks the power to respond as at the 

 beginning. This condition of fatigue may continue, if the stimuli 

 are repeated, until the organ lacks the power to respond at all. 

 Unlike the original unstimulated position, it assumes a rigid fixed 

 one from which no stimulus can arouse it. This condition is 

 that of tetanus, and when this has set in a long rest is required 

 for complete recovery before the plant or organ becomes again 

 irritable or sensitive. 



