100 Prof. W. Pfeffer. The Nature and 



directly follows that the consumption of the material to be elaborated, 

 as well as the absorption of free oxygen by the organism, is regu- 

 lated in accordance with its requirements. Consequently, when 

 these requirements are fully satisfied, an increased supply of food 

 material or of oxygen results in no essential acceleration of the func- 

 tional metabolism. For this reason plants do not breathe any more 

 vigorously in pure oxygen than in ordinary air, for even in the latter 

 much more oxygen penetrates into the cell than is consumed in 

 normal respiration. 



If, however, the supply is not sufficient to fully satisfy the demand, 

 then the functional metabolism, and with it the whole activity of the 

 organism, is unavoidably reduced, just as a fire can no longer burn 

 properly when insufficiently supplied with fuel or with oxygen. 

 Most plants, however, can completely meet their demand for oxygen 

 in an atmosphere in which the proportion of oxygen (at ordinary 

 atmospheric pressure) is reduced to 5 8 per cent., so that on the 

 highest mountains vegetable organisms find a more than sufficient 

 density of oxygen. If its density be still further diminished, then, 

 after a transient disturbance, the respiration and the total activity of 

 the plant are depressed, so that in an atmosphere containing only 2 4 

 per cent, of oxygen the plant, though it survives, breathes and 

 works in a diminished degree. 



Since functional metabolism depends on the vital activities, a satis- 

 factory causal explanation of the former will only be possible after 

 we have gained a sufficient insight into the latter. In general, how- 

 ever, we may say that the same processes which effect intramolecular 

 respiration also develop those affinities, by means of which free 

 oxygen, when supplied, is drawn into metabolism. For intra- 

 molecular respiration is at once stopped on access of oxygen, and 

 after the withdrawal of oxygen is immediately resumed. In much 

 the same way as the development of the spontaneously inflammable 

 phosphuretted hydrogen brings about the fixation of a certain 

 amount of oxygen, may physiological combustion be determined 

 and regulated through the continual formation of a single 

 autoxydable body. In respiration, however, we have evidently to do 

 with complicated reactions and reciprocal changes, which come into 

 play between the constituent parts of the protoplasmic body. And 

 further, the respiratory process must take a somewhat different form 

 when, instead of a carbon compound, ammonia or ammonium nitrite, 

 sulphuretted hydrogen or sulphur, forms the material for physiolo- 

 gical combustion. 



We may be sure, however, that in the plant passive oxygen is 

 drawn into metabolism, and that oxygen is not brought into the 

 active state in order to accomplish physiological oxidation. For we 

 can prove with complete certainty that at no time does any such 



