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THE AMERICAN NATURALIST [Vol. XLII 



elements become converted into proteid, cellulose, etc. 

 Now this complex reaction of metabolism only takes place 

 in the presence of protoplasm, and a small amount of pro- 

 toplasm is capable of carrying out a considerable amount 

 of metabolic change, remaining itself undestroyed. We 

 are thus led to formulate the idea that metabolism is es- 

 sentially a catalytic process. In support of this we know 

 that many of the inherent parts of the protoplasmic com- 

 plex are catalytic enzymes, for these can be separated out 

 of the protoplasm, often simply by high mechanical pres- 

 sure. We know, too, nowadays that the same enzymes 

 that accelerate katabolic processes also accelerate the re- 

 verse anabolic processes. 



In time a small mass of protoplasm will, while remain- 

 ing itself unchanged, convert many times its own weight 

 of carbon from, let us say, the formaldehyde (HCHO) 

 of photosynthesis to the carbon dioxide (C0 2 ) of respira- 

 tion. 



If metabolism is a complex of upgrade and downgrade 

 changes catalyzed by protoplasm we must expect the 

 amount of metabolism to obey the law of mass and to be 

 proportional to the masses of substances entering into the 

 reaction. The case when any one essential element is a 

 limiting factor we have already considered. When all are 

 in excess, then the amount of the catalyst present becomes 

 in its turn the limiting factor. Transferring this point 

 of view to the growing plant, we expect to find the limited 

 mass of protoplasm and its constituent catalysts setting 

 a limit to the rate of metabolic change in the extreme case 

 where all the materials entering into the reaction are in 

 excess. When once this supply is available further in- 

 crease in supplies can not be expected to accelerate the 

 rate of growth and metabolism beyond the limit set by the 

 mass of protoplasm. This, of course, is in accordance with 

 common experience. The clearest experimental evidence 

 is in connection with respiration and the supply of carbo- 

 hydrates—this, no doubt, because the carbohydrate ma- 

 terial oxidized in respiration is normally stored inside 

 plant-cells in quantity and can be estimated. When the 



