14 PLANT PHYSIOLOGY 



whole, only until the death of the cells composing the body. 

 The cells need free oxygen during life; they remove it by 

 combining it with complex combustible compounds. So 

 long as the removal of free oxygen by combining it with 

 other substances takes place, so long will oxygen continue, 

 in obedience to the laws governing the diffusion of gases, to 

 enter the cells. The combination of oxygen with combusti- 

 ble substances at temperatures below those at which these 

 substances would spontaneously oxidize, takes place in liv- 

 ing cells and is accomplished only by them. 



Though the living cell is supplied with oxj-gen by purely 

 physical means, by diffusion, the continued supply of new 

 molecules of oxygen is contingent upon its continued con- 

 sumption by the cell. The supply must make good the lack 

 produced by oxidation, the rate of oxidation must equal the 

 demand for energy, and the amount of work done in the 

 cell will always be directly proportioned to the amount of 

 energy it can liberate. The initiative, however, wiU always 

 come from the living cell, because oxidation by respiration 

 is not dependent solely upon the mutual affinities of oxygen 

 and the substances to be oxidized. The activity of the cell 

 controls the activity of respiration (not rice versa), and the 

 supply of oxygen, normally exactly equal to the consump- 

 tion, is also controlled by the cell. An excessive supply of 

 oxygen, which for experimental purposes may be artificially 

 furnished, will affect the respiratory only as it, at the same 

 time affects the other activities of the cell. Each kind of 

 cell and each kind of organism will have its own optimum, 

 maximum, and minimum, any departures from which will 

 characteristically affect the cell and the organism. The 

 percentage of oxygen in atmospheric air (about 20^?) is 

 approximately the optimum for the majority of land organ- 

 isms, though for a small number this amount, and even 

 much less than this, is fatal. The amount of available free 

 oxygen necessarily varies with the habitat, plants living on 

 mountain-tops, and in water, having less than those living on 

 the land at ordinary elevations. But the successful existence 

 of plants at different elevations and depths shows that they 

 are capable of supplying themselves with what they need. 



