890 TRANSACTIONS OF SECTION K. 



The Metabolism of the Plant consideeed as a Catalytic Eeaction". 



Plants of all grades of morphological complexity, from bacteria to dicotyledons, 

 have this in common, that throughout tlieir active life they are continually 

 growing. Putting aside the qualitative di?tribulion 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 grvjwth 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 anabolism 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 arc built are obviously essential in quantity as 

 building material, but in addition small amounts ofFe, Ca, K, Mg, Na, Cl,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 Tropccohnn 

 in a pot be watered with dilute salt-solution, its stomata soon close permanently, 

 and no CO., can diffuse in to supply the carbon for further growth of the plant. 

 In such a condition the plant may remain for weeks looking quite healthy, but 

 its gi'owth may be quite in abeyance. 



In agricultural experience, in manuring 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 development is limited to the amount of 

 growth corresponding to this supply. Agriculturalists have formulated the ' law 

 of the minimum,' which states that the crop developed is limited by the element 

 which is minimal, i.e., most in deficit. Development arrested by ' nitrogen- 

 hunger ' is perhaps the commonest form of this. All this is of course in accord- 

 ance with expectation on ])hysical-chemical principles. The quantity of anabolic 

 reaction taking place 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 Avhich, from the arrested development of nitrogen-hunger, starts 

 growth again when newly supplied Avith nitrogen tliat this new growth is a 

 response to a ' nitio;/en 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 

 jiTowth ? 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 over- 

 head electric wires and underground bacterial inoculations. If 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, &c. 

 ^ow 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 con- 

 siderable amount of metabolic change, remaining itself undestroyed. We 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 catalytic enzymes for these can be separated out of the 

 protoplasm, often simply by high mechanical pressure. We know, too, nowadays 



