40 INFLUENCE OF CHEMICALS 



functions, or may bring about a rearrangement of the molecules 

 or units of organization of living matter in such manner as to cause 

 morphological changes, exaggerating or retarding development. . 



Then again the non-trophic compound may exert a purely 

 physical effect upon the protoplasm, such as increasing or de- 

 creasing the amount of water content by changing osmotic pres- 

 sure, or it may interfere with the exchange between the organism 

 and the trophic factors of its environment. 1 The intensity of the 

 influence of all chemical substances upon the organism is condi- 

 tioned to a great extent upon the temperature, concentration, 

 structure, and pressure of the incident compounds. 



57. Oxygen. Oxygen is a constituent of the protoplasm, 

 and as it is combined with other elements to form compounds 

 which are more or less constantly excreted from the body, a con- 

 stant supply of the free element is necessary for the existence of 

 every organism. This supply may be taken from the air as it is 

 needed, from compounds in the medium or substratum, or it may 

 be obtained from compounds absorbed and stored in the body of 

 the protoplast. The most important form of energy release in 

 living matter is that which is brought about by the oxidation of 

 certain compounds in the protoplast ; it is not definitely deter- 

 mined whether this oxidation concerns the material of which pro- 

 toplasm is actually constructed, or merely the saturating and in- 

 terlocking compounds. The preponderance of evidence seems to 

 favor, the former view however (See respiration). 



If the supply of oxygen is reduced, a correspondent diminu- 

 tion of the amount of energy released and convertible to various 

 uses will ensue, and the performance of the organism lessened, ex- 

 cept in anaerobic organisms (see above). The continuation of only 

 a partial supply of this element during a period of active growth 

 and development will generally result in death. During the rest- 

 ing period, however, such as that shown by seeds, long periods 

 of total vacuity may be endured without deterioration. On the 



1 Livingstone, B. E. On the nature of the stimulus which causes the change of 

 form in polymorphic green algae. Bot. Gazette, 30 : 289. 1901. 



