October i, 190S] 



NA TURE 



559 



effect the hydrolysis, oxidation, reduction, or splitting of 

 some definite organic compound or group of compounds 

 containing similar radicals. 



Innumerable enzymes have in late years been isolated 

 from the plant-body, so that it would seem that there is 

 one present catalytically to accelerate each of the slow 

 single changes that in the aggregate make up the complex 

 metabolisin of the plant. 



The law of mass applies with equal cogency to catalytic 

 reactions. If twice the amount of acid is added to a solu- 

 tion of cane-sugar (or twice the amount of enzyme) then 

 the reaction-velocity is doubled, and hydrolysis proceeds 

 twice as fast. As the catalyst is not destroyed by its 

 action, but is continually being set free again, the con- 

 centration of the catalyst remains the same throughout the 

 reaction : while, on the contrary, the ainount of cane-sugar 

 continually decreases. 



If the catalyst be present in great excess the amount 

 of hydrolysis will be limited by the amount of cane-sugar 

 present, and as this is used up so the reaction will progress 

 by a logarithmic curve as in Fig. i, a. In this case B 

 may represent the amount of catalyst. If, on the contrary, 

 there is a large amount of sugar and very little acid or 

 enzyme present, so that the catalyst becomes the limit- 

 ing factor, then we happen upon a novel state of things ; 

 for by the law of mass the rate of hydrolysis will now 

 remain constant for some time until the excess of sugar 

 is so far reduced that it in turn becomes a limiting factor 

 to the rate of change. In this case the velocity curve 

 would consist of a first phase with a straight horizontal 

 line of uniform reaction-velocitv leading into the second 

 phase of a typical falling logarithmic curve (see Fig. i, c). 

 These conditions have been experimentallv examined bv 

 Horace Brown and Glendinning, and fully explained and 

 expounded by E. F. Armstrong in part ii. of the critical 

 "Studies in Enzyme Action."' 



Having now outlined the three fundamental principles of 

 reaction-velocity, the law of mass, and the catalvtic ac- 

 celeration of reaction-velocity, we are in a position to con- 

 sider the broad phenomena of metabolism or chemical 

 change in the living organism from the point of view of 

 these principles of chemical mechanics. 



Tur Mf.tabolism of the Plant considered as a Catalytic 

 Reaction. 



Plants of all grades of morphological coinplexity, from 

 bacteria to dicotyledons, have this in common, that through- 

 out their active life thev are continually growing. Putting 

 aside the qualitative distribution of growth that determines 

 the morphological form, as a stratum of phenomena above 

 the fundamental one that we are about to discuss, we find 

 that this growth consists in the assimilation of dead food- 

 constituents by the protoplasm with a resulting increase in 

 the living protoplasm accompanied with the continual new 

 formation of dead constituents, gaseous CO,, liquid water, 

 solid cellulose, and what not. This continual flux of ana- 

 bolism and katabolism is the essential character of meta- 

 bolism, but withal the protoplasm increases in amount by 

 the excess of anabolism over katabolism. 



Protoplasm has essentially the same chemical composition 

 everywhere, and in the whole range of green plants the 

 same food-materials seem to be required ; the six elements 

 of which proteids are built are obviously essential in quan- 

 tity as building material, but in addition small amounts of 

 Fe, Ca, K, Mg, Na, CI, and Si are in some other way 

 equally essential. What part these secondary elements play 

 is still largely a matter of hypothesis. 



Regarding metabolism thus crudely as if it were merely 

 a congeries of slow chemical reactions, let us see how far 

 it conforms to the laws of chemical mechanics we have 

 outlined. 



If the supply of any one of these essential elements comes 

 to an end, growth simply ceases and the plant remains 

 stationary, half-developed. If a Tropseolum in a pot be 

 watered with dilute salt-solution, its stomata soon close 

 permanently, and no CO, can diffuse in to supplv the 

 carbon for further growth of the plant. In such a con- 

 dition the plant may remain for weeks looking quite 

 healthy, but its growth may be quite in abeyance. 



^ Proc. Roy. Foe, vol. Ixxiii. 

 NO. 2031, VOL. 78] 



T9C4, p. 511. 



In agricultural experience, in inanuring the soil with 

 nitrogen and the essential secondary elements, the same 

 phenomenon is observed when there is a shortage of any 

 single element. If a continuous though inadequate supply 

 of some one element is available, then the crop develop- 

 ment is limited to the amount of growth correspondmg 

 to this supplv. Agriculturists have formulated the " law 

 of the minin'ium," which states that the crop developed 

 is limited bv the element which is minimal, i.e., most in 

 delicit. Development arrested by " nitrogen-hunger " is 

 perhaps the commonest form of this. All this is of course 

 in accordance with expectation on physical-chemical prin- 

 ciples. The quantity of anabolic reaction taking pl.ace 

 should be proportional to the amount of actively reacting 

 substances present, and if any one essential substance is 

 quite absent the whole reaction must cease. It therefore 

 seems clouding a simple issue and misleading to say of 

 a plant which, from the arrested development of nitrogen- 

 hunger, starts' growth again when newly supplied with 

 nitrogen that this new growth is a response to a " inlrogcn 

 stimulus." It would appear rather to be only the removal 

 of a limiting condition. 



Let us now move on a stage. Suppose a growing plant 

 be liberally supplied with all the thirteen elements that 

 it requires,' what, then, will limit its rate of growth? Fairy 

 bean-stalks that grow to the heavens in a night elude the 

 modern investigator, though some hope soon to bring back 

 that golden age with overhead electric wires and under- 

 ground bacterial inoculations. If everything is supplied 

 the metabolism should now go on at its highest level, and 

 quantities of carbon, nitrogen, hydrogen, and oxygen sup- 

 plied as CO,,, nitrates, and water will interact so that 

 these element's become converted into proteid, cellulose, &-c. 

 Now- this complex reaction of metabolism only takes place 

 in the presence of protoplasm, and a small amount of 

 protoplasm is capable of carrying out a considerable aniount 

 of metabolic change, remaining itself undestroyed. \\ e are 

 thus led to formulate the idea that metabolism is essentially 

 a catalytic process. In support of this we know that many 

 of the "inherent parts of the protoplasmic complex are cata- 

 lytic enzymes, for these can be separated out of the proto- 

 plasm often simply bv high mechanical pressure. We 

 know' too, nowadays that the same enzymes that accel- 

 erate katabolic processes also accelerate the reverse anabolic 

 processes. .,, , ., . . ^ 



In time a small mass of protoplasm will, while remaining 

 itself unchanged, convert manv times its own weight of 

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

 photosynthesis to the carbon dioxide (CO,) of respiration 



If metabolism is a complex of up-grade and down-grade 

 changes catalysed by protoplasm we must expect the 

 amoilnt 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. \Vhen 

 all are in excess, then the amount of the catalyst present 

 becomes in its turn the limiting factor. Transferrma this 

 point of view to the growing plant, we expect to hnd the 

 limited mass of protoplasm and its constituent catalysts 

 settintf 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 accel- 

 erate the rate of growth and metabolism beyond the limit 

 set bv the mass of protoplasm. This, of course, is in ac- 

 cordance with common experience. The clearest experi- 

 mental evidence is in connection with respiration and the 

 supplv of carbohydrates— this, no doubt, because the carbo- 

 hydrate material' oxidised in respiration 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 absorp- 

 tion and translocation to obscure the issue, especially in 

 the case of a higher plant. , . j » i 



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 reserws are steadily decreasing. Pal- 

 ladine' has investigated germinating wheat by analysing 

 1 " Kevue gen. de Botaniqne," tome viii., 1896. 



