PRESIDENTIAL ADDRESS. 801 



that the same enzymes that accelerate katabolic processes also accelerate the 

 reverse auabolic processes. 



In time a small mass of protoplasm will, while remaining itself unchanged, 

 convert many times its own weight of carbon from, let.usjsay,? the formaldehyde 

 (HCHO) of photosynthesis to the carbon dioxide (COo) of respiration. 



If metabolism is a complex of up-grade and down-grade changes catalysed 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 increase in supplies cannot 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 carbohydrates — this, no doubt, because the carbohydrate 

 material oxidised in respiration is normally stored inside plant-cells in quantity 

 and can be estimated. AVhen the supplies for an internal process have to be 

 obtained from outside, then we have the complications of absorption and 

 translocation to obscure the issue, especially in the case of a higher plant. 



Let us first take a case where the carbohydrate supply is in excess and the 

 amount of catalytic protoplasm is small and increasing. Thus it is in seeds 

 germinating in the dark : respiration increases day by day for a time, though 

 carbohydrate reserves are steadily decreasing. Palladine ^ has investigated 

 germinating wheat by analysing the seedlings and determining the increase of 

 the essential (non-digestible) 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 approximately constant ratio between the amount of 

 protoplasm at any stage and the respiration. 



As germination progresses in the dark the supplies of reserve carbohydrate pre- 

 sently 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 CO^ 

 production is limited by the shortage of respirable material. 



This second type of case was more completely investigated by Miss Matthsei 

 and myself in working on the respiration of cut leaves of cherry-laurel kept 

 starved in the dark. For a time the C0„ production of these non-growing struc- 

 tures remains uniform, and then it begins to fall ofl'in a logarithmic curve, so that 

 the course of respiration is just like c in fig. 1. We interpret both phenomena in 

 the same way : in the initial level phase the respirable material in the leaf is in 

 excess, and the amount of catalytic protoplasm limits the respiration to the 

 normal biological level ; in the second falling phase some supply of material is 

 being exhausted, and we get a logarithmic curve controlled by the law of mass, 

 as much, it would seem, as when cane-sugar is hydrolysed in aqueous solution. 



After these two illustrations of the action of the \a,\j of mass from the more 

 simple case of respiration we return to the consideration of the totality of 

 metabolic reactions as exemplified in growth. 



What should we expect to be the ideal course of growth, that is, the increase 

 of the mass of the plant regarded as a complex of reactions catalysed by proto- 

 plasm ? Let us consider, first, the simplest possible case, that ox a bacterium 

 growing normally in a rich culture solution. When its masa hai increased by 

 anabolism of the food material of the culture medium to a certain amount it 

 divides into two. As all the individuals are alike, counting them would take 

 the place of weighing their mass. The simplest expectation would be that, 

 under uniform conditions, growth and division would succeed each other with 



' Revue gen. de hotanitiue^ tome viii. 1896. 



