CHAPTER VI 

 IRRITABILITY 



The preceding chapters have taught us that living or- 

 ganisms are composed of chemical compounds and that they 

 work by physical force. The body of a living plant consists 

 of living protoplasm and of lifeless substances The func- 

 tions of a living plant consist in chemical changes some of 

 which liberate energy, or store it, while others result in the 

 accumulation of matter, lifeless and living. These functions 

 are carried on by the living organism, they do not simply 

 take place ; but the organism lives and carries on these func- 

 tions only by using chemical compounds and physical forces. 

 Just as chemical and physical processes are affected by pre- 

 vailing conditions, so the living organism is affected by each 

 factor of its environment. When the factors change, the 

 organism is differently affected, just as, with changing^ 

 conditions, ordinary chemical and physical processes also 

 change correspondingly. As there is an optimum condition, 

 which consists hi temperature, illumination, supply of water 

 and of other substances, etc., for each chemical reaction 

 taking place in the laboratory and in nature, so there is an 

 optimum condition for that complex of chemical reactions 

 constantly taking place hi the actively living organism. - 

 Any departure from the optimum modifies some or all of the 

 chemical changes in the organism so that there is a different 

 and less favorable balance in the complex. Conversely, any 

 approach to the optimum condition so modifies some or all 

 of the chemical changes that their balance is more favor- 

 able. When we conceive a living organism, even the simplest 

 and smallest, as being a definite structure (protoplasm) 

 consisting of simple water molecules and of other molecules 

 highly complex and therefore comparatively destructible, 



