630 



SCIENCE 



[N. S. Vol. XXVIII. No. 723 



everything is supplied, the metabolism 

 should now go on at its highest level, and 

 quantities of carbon, nitrogen, hydrogen 

 and oxygen supplied as CO,, nitrates and 

 water will interact so that these elements 

 become converted into proteid, cellulose, 

 etc. Now this complex reaction of metab- 

 olism only takes place in the presence of 

 protoplasm, and a small amount of proto- 

 plasm is capable of carrying out a con- 

 siderable amount of metabolic change, re- 

 maining itself undestroyed. We are thus 

 led to formulate the idea that metabolism 

 is essentially a catalytic process. In sup- 

 port of this we know that many of the 

 inherent parts of the protoplasmic complex 

 are catalytic enzymes, for these can be 

 separated out of the protoplasm, often 

 simply by high mechanical pressure. "We 

 know, too, nowadays that the same enzymes 

 that accelerate katabolic processes also ac- 

 celerate the reverse anabolic processes. 



In time a small mass of protoplasm will, 

 while remainiug itself unchanged, convert 

 many times its own weight of carbon from, 

 let us say, the formaldehyde (HCHO) of 

 photosynthesis to the carbon dioxide (COj) 

 of respiration. 



If metabolism is a complex of up-grade 

 and down-grade changes catalyzed by pro- 

 toplasm 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 

 ease where all the materials entering into 

 the reaction are in excess. When once this 

 supply is available further increase in sup- 



plies 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 com- 

 mon experience. The clearest experi- 

 mental evidence is in connection with 

 respiration and the supply of carbo- 

 hydrates—this, no doubt, because the 

 carbohydrate material oxidized in respira- 

 tion is normally stored inside plant-cells in 

 quantity and can be estimated. When the 

 supplies for an internal process have to be 

 obtained from outside, then we have the 

 complications of absorption and transloca- 

 tion to obscure the issue, especially in the 

 ease of a higher plant. 



Let us first take a case where the carbo- 

 hydrate supply is in excess and the amount 

 of catalytic protoplasm is small and in- 

 creasing. Thus it is in seeds germinating 

 in the dark: respiration increases day by 

 day for a time, though carbohydrate re- 

 serves are steadily decreasing. Palladine^ 

 has investigated germinating wheat by 

 analyzing the seedlings and determining 

 the increase of the essential (non-digest- 

 ible) proteids day by day. The amount of 

 these proteids he regards as a measure of 

 the amount of actual protoplasm present. 

 Assuming this to be so, he finds an approx- 

 imately constant ratio between the amount 

 of protoplasm at any stage and the respira- 

 tion. 



As germination progresses in the dark 

 the supplies of reserve carbohydrate pres- 

 ently fail, and then the respiration no 

 longer increases in spite of the abundant 

 protoplasm. According to our thesis the 

 catalyst is now in excess and the COj pro- 

 duction is limited by the shortage of 

 respirable material. 



This second type was more com- 

 pletely investigated by Miss Matthsei and 

 myself in working on the respiration of 

 cut leaves of cherry-laurel kept starved in 



-Revue gen. de botanique, Tome VIII., 1896. 



