Elements of Physical Biology

2Tt is easy to strike a match, but this is merely a way of borrowing the efforts of others, those who have made the match possible, and those who have manufactured it. How many city dwellers could, by their unaided efforts, start a fire where none was before? Anyone who, in an emergency, may have been forced to attempt the feat will appreciate how among the ancients “the spark of fire was zealously guarded and soon invested with sacred attributes . . . . The chief function of the vestal virgins in Rome was to keep the perpetual fire; and in the Catholic church today with its never extinguished light we have the last survival of what was once a social custom. . . . . ” Another curious survival of this custom is quoted by R. A. Seligman (Principle of Economics, 1908, p. 69): ‘Whenever the location of gas works is changed the fire is transferred by a brand from the old to the new building. Under no consideration would a new fire be started.”’

in which they stand in the life processes of the organism, we shall consider jointly, in this chapter, the circulation of carbon on the one hand, and of oxygen on the other. The Carbon Cycle. A very particular interest attaches to the carbon cycle. Carbon is the organic element par excellence, whose absence from any chemical substance stamps this forthwith, by common if somewhat arbitrary consent, as inorganic: whose presence affords the soil and season for the growth of what might be termed the tropical jungle in the domain of chemistry. For in the compounds of carbon nature seems to have run riot, in a revel of creative versatility, as if vying to set a record unapproached elsewhere in all the realm of chemistry, for number, variety and complexity of her children. Other elements—oxygen, nitrogen, phosphorus, sulphur, iron, indeed play a significant réle in life processes; but the indispensable bond that ever links all other ingredients in organic unity is carbon. Furthermore, carbon is preéminently the energy carrier, the standard coin of the organic real, in which both the first cost of installation, of anabolic tissue building, and also the running cost of operation, of metabolism, is defrayed.

Indescribably complex—far beyond the understanding of the organic chemist of today—as are the metamorphoses that carbon undergoes in the economy of the organism, its source and its gate of entry into the organic cycle are comparatively simple. Some two and a half million million tons (2.2 X 10! kgm.) of carbon dioxide, in the air, and perhaps twenty to twenty-five times this amount contained in the waters of ocean, lakes and rivers, these constitute the store from which all life ultimately draws its supply.

Of this vast store, according to an estimate made by C. Engler, about one-thousandth part actually takes part in the cycle of life. The total carbon locked up in living organisms, which would be a measure, in a way, of the spread of life on our globe, is difficult to gage even roughly. One hardly knows how much or how little significance to attach to an estimate by A. G. Hégbom! that the total quantity of carbon in all living matter is of about the same order as that contained in the atmosphere, namely, 6 X 10" metric

* Linck, Kreislaufvorginge in der Erdgeschichte, 1912, p. 6; Engler, Uber Zerfalls-prozesse in der Natur. tons; an amount which, spread over the entire surface of the globe, would cover it with a film of carbon 1 mm. thick, or with a film of living matter about 3 inch thick. The organic carbon cycle, reduced to its simplest terms, is a closed chain of three links. Green plants, under the influence of sun light, absorb CO: from the atmosphere and convert it, with elimination of oxygen, into the many and complex compounds of the plant substance.’ Animals consume plants (directly or indirectly) as food, and in the course of the operations of their typically active lives (as compared with the typically passive, vegetative existence of the majority of plants) they reoxidize the carbon reduced in the photosynthetic plant processes, and return CO, to the atmosphere, thus completing the cycle.

In actual fact this simple fundamental cycle is complicated by a number of influences. The decay of dead animals and plants adds a comparatively small item to the discharge of CO: into the atmosphere. Plants are somewhat more resistant to complete decay than animals, and one result of this is the accumulation of notable 5 To discuss here the chemistry of photosynthesis in plants would lead us too far afield. Very important advances have been made recently in this field of biochemistry. It must suffice here to refer to the original literature of which the following articles may be mentioned: E. C. Baly, Photosynthesis, Nature, March 16, 1922, p. 344. Report of discussion on photosynthesis at the British Association Meeting, Nature, December 23, 1922, p. 856. I. M. Heilbron, The Photosynthesis of Plant Products, Nature, April 14, 1923, p. 502. O. Baudisch, On the Formation of Organic Compounds from Inorganic by the Influence of Light, Science, April 20, 1923, p. 451. O. Baudisch, The Influence of Light on Inorganic Matter and Life Processes, Jour. Industrial and Eng. Chemistry, May, 1923, p. 451. J.C. Bose, Effect of Infinitesimal Traces of Chemical Substances on Photosynthesis, Nature, July 21, 1922, p. 95. Baly, Heilbron and Parker, Photochemical Production of Formaldehyde, Nature, September 1, 1923, p. 323. J. H. Mathews, Trends in Photochemical Research, Jour. Ind. and Eng. Chem., September,

quantities of reduced carbon in the form of peat and coal. This process of fossilization is slow, and would not in itself, in any short period, materially affect the carbon cycle. It has, however, furnished the occasion for a phenomenon which, judged in a cosmic perspective, represents a purely ephemeral flare, such as must ultimately appear utterly insignificant in the geological calendar, if duration alone is considered; but which to us, the human race in the twentieth century is of altogether transcendent importance: The great industrial era is founded upon, and at the present day inexorably dependent upon, the exploitation of the fossil fuel accumulated in past geological ages.

We have every reason to be optimistic; to believe that we shall be found, ultimately, to have taken at the flood this great tide in the affairs of men; and that we shall presently be carried on the crest of the wave into a safer harbor. There we shall view with even mind the exhaustion of the fuel that took us into port, knowing that practically imperishable resources have in the meanwhile been unlocked, abundantly sufficient for all our journeys to the end of time. But whatever may be the ultimate course of events, the present is an eminently atypical epoch. Economically we are living on our capital; biologically we are changing radically the complexion of our share in the carbon cycle by throwing into the atmosphere, from coal fires and metallurgical furnaces, ten times as much carbon dioxide as in the natural biological process of breathing. How large a single item this represents will be realized when attention is drawn to the fact that these human agencies alone would, in the course of about five hundred years, double the amount of carbon dioxide in the entire atmosphere, if no compensating influences entered into play. In point of fact the percentage of carbon dioxide in the atmosphere exhibits remarkable constancy and there are several very large items, in addition to those already touched upon, both on the ingoing and outgoing side of the account. The case of man has been singled out for mention here merely because our knowledge of the human population and economy enables us to make a reasonably close estimate of his contributions. The quota supplied by the remaining animal species can hardly even be guessed at. But probably the greatest source of atmosphere CO, are volcanoes and mineral springs. Cotopaxi alone has been credited with an annual discharge of two million tons of the gas.

On the debit side there is first of all the item of consumption by plants. E. H. Cook,® from very uncertain data, computes that leaf action alone more than compensates for the production of carbon dioxide, consuming about one-hundredth of the total atmospheric oxygen ina year. Yost? computes that if the entire land area were planted with sun flowers, about 6.5 10! tons of CO, would be absorbed per annum. Forests would be considerably less efficient, and would take care of about 2.8 X 10! tons per annum, or, say in round numbers one-hundredth of the atmospheric carbon dioxide. An older estimate, by Liebig, puts the annual output of the soil of Central Europe at 2.5 tons of dry organic matter per hectar, or, say 1 ton per acre. Allowing 40 per cent carbon in such organic matter we find for the total annual production of carbon in plants 13,000 million (1.3 x 10!) tons. This is about ten times the world’s annual coal consumption, and about one-fiftieth of the total carbon in atmospheric carbon dioxide. Arrhenius points out that if all this carbon fixed by plants were deposited in peat bogs, the atmosphere would be depleted in half a century. But, of course, only a small proportion of the bodies of plants are thus “horded” and removed from the organic life cycle.

A figure of perhaps greater interest, because based upon observations of processes actually going on in a selected portion of the universe, is given by W. R. G. Atkins. From the observed change in the hydrogen ion concentration in the water of the English Channel, this author has calculated that 250 metric tons of organic carbon (figured as hexose) were produced per square kilometer between July and December. From similar observations made at Port Erin, Moore found a production of 300 metric tons per square kilometer during the six months that included the vernal maximum of diatom production.*®

An important item among the withdrawals from the atmosphere is the absorption of carbon dioxide in the weathering of rocks, with replacement of silicates by carbonates. A. G. H6égbom reckons this item as about balancing the reproduction of carbon dioxide in the combustion of coal. Its accumulated record is seen in the sedimentary rocks, which, according to F. W. Clarke,® contain 30,000 times as much as CO, as are today present in the atmosphere.!° Sterry Hunt “illustrates the effect of weathering by the statement that the production from orthoclase of a layer of kaolin (china clay) 500 meters thick and completely enveloping the globe would consume 21 times the amount of CO: now present in the atmosphere.” Chamberlin and others estimate that it would take about 10,000 years to consume the present amount of atmospheric CO, by the weathering of rocks. Loss of CO. by peat formation may be estimated at the same figure. The formation of CO: by the burning of coal would, according to these estimates, cover the loss by weathering and peat formation combined, seven times over.

Finally, there is the great reservoirgof the ocea, in equilibrium with the atmosphere, and absorbing, under present conditions, some 18 to 25 times as much CO, as it leaves in the air above it. Thus the sea acts as a vast equalizer; of every ton of CO: thrown into the atmosphere by volcanoes,’or by coal fires, for example, the ocean ultimately receives directly or indirectly, about 1900 pounds, only the balance of 100 pounds remaining in the atmosphere. It is thus seen that even extensive contributions from the lithosphere have but a slight effect upon the atmospheric store, and fluctuations are in this way ironed out and moderated. Arrhenius" points out, moreover, that at the present time the carbon dioxide content of the air over the ocean is on an average 10 per cent lower than over land. From this, and the fact that generation of CO: by coal (and probably also from volcanoes) has in late years been increasing, he concludes that the air is, at the present epoch, becoming richer in this gas.

't Loc. cit., p. 54. Arrhenius’ figure differs somewhat from the one given above. He supposes that the sea takes up five-sixths of the CO, thrown into the air, i.e., 1 ton would yield up 1667 pounds to the ocean, leaving 333 pounds in the air. As to which side of the account shows a net balance, in the carbon cycle, we have no certain knowledge. Arrhenius builds his conception of the future industrial development of our race on the expectation that the atmosphere is gaining in carbon dioxide, under the present régime of “evaporating”? our coal mines, as it were into the air. On the other hand, if our atmospheric CO, is of voleanic origin, and the balance is maintained today with the aid of discharge from the lithosphere, then the ultimate extinction of the earth’s plutonic fires would bring in its train the depletion of the atmosphere and secondarily the extinction of life. ‘‘The cessation of volcanism would signify the end of life on the globe.’”’ A similar position is taken by C. Schuchert,!? who further remarks:

We should add that if there were again as much life as there is at present, all the carbon of the atmosphere would be in the living plants and animals, and, if such a condition were possible death would come to them all Life and its abundance at any time are conditioned by the amount of this gas (CO) present in the atmosphere. This remark of Schuchert’s is suggestive as illustrating in concrete manner the relative amounts of carbon concerned in the life balance. It is, perhaps, somewhat misleading in making no mention of the equalizing influence of the ocean which has been noted above. In point of fact, if all the CO, in the air were withdrawn, a nearly equal amount would rise from the ocean to take its place.

A summary, in graphic form, of the principal relations in the carbon cycle noted in the preceding paragraphs, will be found in figure 47, which should aid in giving a comprehensive picture of the situation. The Oxygen Cycle. The organic oxygen cycle is, of course, directly related to the carbon cycle, although other features also enter into operation in regulating the oxygen balance of the atmosphere. The complementary relation between animals (essentially oxidizers of carbon) and plants (essentially reducers of carbon dioxide) is indeed a biological fact of fundamental importance at the present stage of evolution. But if we look back through the vista of ages, to the time before the advent of life, such as we know it, our curiosity is aroused as to the origin of the atmospheric carbon

2 The Evolution of the Earth and its Inhabitants, 1919, p. 52. @TIOAZ) NOMUV() FHL, ‘AUOLVN NI SINGANAIY FAL dO NOILVIOOUIYD “LP “SIA dioxide and oxygen. Various views have been upheld on this subject. F. W. Clarke remarks: It is likely that carbon dioxide has been added to the atmosphere by volcanic agency, in some such manner as this: Primitive carbon, like the graphite found in meteorites, at temperature no greater than that of molten lava, reduced the magnetite of igneous rocks to metallic iron, such as is found in many basalts, and was itself thereby oxidized. Then, discharged into the atmosphere as dioxide, it became subject to the familiar reactions which restored it to the lithosphere as coal or limestone.

Arrhenius, referring to Koehne’s reflections on this subject, points out that the atmosphere contains about 1.2 x 10'% tons of oxygen, an amount which roughly” corresponds with the mass of fossil coal in the sedimentary rocks. ‘‘The supposition appears natural, therefore, that all the oxygen of the air may have been formed at the expense of atmospheric carbon dioxide. Probably all the oxygen of the air owes its existence to plant life.” F. W. Clarke resumes the views of a number of investigators as follows :!®

C. J. Koehne assumed that the primitive atmosphere contained no free oxygen, and he has been followed by T. L. Phipson,!? J. Lemberg,!* J. Stevenson,!® and Lord Kelyin.*® Lemberg and Kelvin, however, do not go to extremes, but admit that possibly some free oxygen was present even in the earliest times. Lemberg argued that the primeval atmosphere contained chiefly hydrogen, nitrogen, volatile chlorides, and carbon compounds; the oxygen which is now free, being then united with carbon and iron. The

15 The correspondence is rather distant if we accept Engler’s estimate of the world’s coal reserves, namely, 3 X 10! tons, containing 75 per cent carbon. See Linck, loc. cit., p. 37. Engler’s estimate is probably low. The World Almanac, 1921, p. 201, gives 7.5 X 10 tons. This would correspond to 1.5 X 10'3 tons oxygen, as against 1.2 x 10!° tons in the atmosphere. It is true that these estimates of coal reserves cover,only such coal as it would pay to mine.

liberation of oxygen began with the appearance of low forms of plant life, possibly reached a maximum in Carboniferous time, and has since diminished. Stevenson’s argument is much more elaborate, and starts with an estimate of the uncombined carbon now existent in the sedimentary formations. In the deposition of that carbon, oxygen was liberated, and from data of this kind it is argued that the atmospheric supply of oxygen is steadily increasing, while that of carbon dioxide diminishes. The statement that no oxygen has been found in the gases extracted from rocks is also adduced in favor of the theory. First, an oxidized crust and no free oxygen in the air; then processes of reduction coming into play; and at last the appearance of lower forms of plants, which prepared the atmosphere to sustain animal life. The arguments are ingenious, but to my mind they exemplify the result of attaching excessive importance to one set of phenomena alone. It is not clear that due account has been taken of the checks and balances which are actually observed. At present the known losses of oxygen seem to exceed the gains. For example, C. H. Smyth”! has estimated that the oxygen withdrawn from the air by the change of ferrous to ferric compounds, and so locked up in the sedimentary rocks, is equal to 68.8 per cent of the quantity now present in the atmosphere.

G. Bunge” also supports the view that atmospheric oxygen is continually diminishing, becoming bound by the ferrous oxide resulting from the decomposition of silicates. Accumulation of carbon dioxide in the atmosphere, at any rate under present conditions, is also assumed by S. Arrhenius, as has already been remarked. If the demand becomes insistent enough, we cannot doubt that methods will be devised which will give us the desired results. To question that would be to admit that man has neared the culmination of his evolutionary career and is preparing to bequeath the mastery of the earth to his successor, whoever that may be.—G. W. Martin.

Natural Demand and Supply. The proportion of nitrogen to carbon in the human body is 1:3, in the atmosphere it is5,500:1. A human adult contains in his body about 42 pounds of nitrogen, and over 50 pounds of carbon. Over every square foot of the earth’s surface rises a column containing some 1500 pounds of nitrogen, and only about 1/4 pound of carbon. The demand and the supply of these two elements appear, therefore, at first sight, to be altogether out of all proportion favorable to nitrogen. Yet, in point of fact, the practical problem of securing an adequate supply for the substance and expansion of life is incomparably more complex in the case of nitrogen than in the case of carbon. The reason for this somewhat remarkable inversion is to be seen in the fact that nitrogen is readily accessible as food for living organisms only when it occurs in certain chemical combinations, and nitrogen thus combined is far from plentiful. It has been estimated by T. H. Norton that this available or “nomadic” nitrogen—i.e., that which takes part in the migration through the organic cycle—amounts to only about two one-millionths of the total nitrogen of the atmosphere, or, say, to about 8 x 10° tons. In fact, nitrogen is today probably the chief of those limiting factors! which, in accordance with Liebig’s law of the minimum, establish the bounds for the extreme expansion of living matter upon the earth. At the same time the circumstance of the chemical idiosyncrasies of the element nitrogen introduces a certain complexity into the nitrogen cycle, which strikes the eye at a glance in the charts, figures 48 and 49, exhibiting the essentials of the nitrogen

HIOAD NADOULIN TH], ‘TUALVN NI SLNANAIG TAL dO NOILVTAOUL) “8p “D1 ‘g2Ua74N220 /PbAGNOP 4O Sudljowsosouoy epo2/pur sysoUs UOIZsONnD gonpesd KOH $uaAQ ayo? sofos0uny apiuonz wni707 EHN sonposd-Kg Spowiuy wf z : 5 pve ouowiliy ‘SYIOM FED SPOUOZ/IA eycle. Figure 48 shows in broader outline the main feature of the cycle, while figure 49 exhibits in greater detail especially those stages in the cycle that are most intimately associated with life agencies.?

2 For details on this phase of the subject the reader must be referred to the special literature. A good summary, fairly detailed and complete, yet concise, will be found in R. Huber, Zur Stickstoff-Frage, Bern 1908 Gate of Entry into Nitrogen Cycle. The natural gateway for the entry of atmospheric, elementary nitrogen, into the organic cycle is a narrow one. So far as at present known, only a limited class of organisms possess the faculty of ‘‘fixing’”’ this element, that is, taking it in its gaseous state from the atmosphere and converting it into the condensed (liquid or solid) form, in which only it has common acceptation as coin of the organic realm. The organisms known to take part in this natural process of nitrogen fixation are three, namely certain bacteria having their habitat in the soil; certain leguminous plants (peas, beans, clover, alfalfa), working in conjunction, in symbiosis, with nitrogen-fixing bacteria lodged in tubercles upon their roots; and, thirdly, the wheat plant has recently been shown to possess the independent faculty of assimilating nitrogen from the air. This recent demonstration,’ of course, suggests the ready question whether after all, a number of other plants may not be similarly endowed. ‘This remains as a matter for further investigation, but it is improbable that we shall have occasion to change materially our present impression, namely, that the natural avenues by which elementary nitrogen gains admission from the atmosphere into the cycle of life are rather narrowly restricted. As to other avenues opened up by the man, these will be considered presently.

Leak of Nitrogen out of Circulation. While there is thus a narrowly restricted class of vegetable organism through which nitrogen trickles in a thin stream from the elementary supply in the atmosphere into the life cycle, the majority of plants derive the supply for their biological needs from ready formed nomadic nitrogen in the soil, that is to say nitrogen combined in the form of ammonium salts, nitrites and nitrates. These substances are subject to oxidation and reduction in the soil under the influence of various bacteria, as indicated in the chart figure 49. The result of these changes is that a certain fraction of the nomadic nitrogen is continually leaking out of the circulation and joins the general reservoir of free nitrogen in the atmosphere. This is only one of a number of items on the losing side of the balance sheet. Other items will be found in following up the details of the two charts already

* C. B. Lipman and J. K. Taylor, Science, November 24, 1922, p.605. These authors report that in their experiments wheat plants assimilated 13 to 21 per cent of their nitrogen content from the air. referred to. There is loss in the autumnal leaf fall of deciduous plants; in the decay of dead plants, or in their fossilization as peat, lignite and coal. Forest fires, and the burning of wood and coal; the distillation of coal for illuminating gas; the oxidation of coal in metallurgical furnaces, the coking of coal in ovens of the so-called beehive type; all these are operations in which a greater or less proportion of the combined nitrogen in the coal is liberated into the air in the free, unavailable form. In view of the great loss which this represents in the economy of our food resources, the highest importance attaches to the modern drift away from the beehive coke oven to the by-product oven, in which the major part of the nitrogen in the coal is recovered as ammonia.

Fraction, of total output of coke in the United States, produced in by-product ovens These by-product ovens were introduced in 1893. It is estimated that during the period from 1893 to 1910 alone, through the continued use of the old beehive type coke oven, over 9,300,000 tons of ammonium sulphate were wasted, representing, at the prices then prevailing, a value of 558 million dollars. In addition to this must be reckoned a further loss in the resulting field crops. Had all the nitrogen wasted in the beehive ovens been spread as fertilizer on the field, this would have increased the crops some 20 per cent.* An idea of the extent and significance of the healthy modern drift towards replacement of the beehive by the recovery coke oven may be gathered from table 22, reproduced from an article by Grinnell Jones in the Quarterly Journal of Economics, 1920, vol. 34, p. 402. H. E. Fischer, writing in the Journal of the Franklin Institute, 1920, vol. 190, p. 191, remarks that if all the coal in the United States were used as coke, and the ammonia recovered in the process, this alone would

‘This and other data regarding nitrogen losses here set forth are drawn, largely, from an article by J. D. Pennock in the Journal of Industrial and Engineering Chemistry, 1911. furnish one million tons of ammonia, which corresponds to about one-half the world’s total production of nitrogen compounds. Returning to the chart (fig. 48) and continuing to trace the progress of nitrogen in the organic cycle, we note next that combined nitrogen is absorbed from plants by animals in their food. It is rejected from the animal economy in part as excretory matter (manure, etc.), in part in the bodies of dead animals, in so far as these are not themselves consumed as food. A large item here, in the economics of the human community, is the refuse from slaughter houses. Certain portions of this are recovered for various uses (glue, leather, etc.). Some is made into fertilizer. Much of it goes to waste, and thus gives opportunity for another leak of nitrogen out of the life cycle to the elementary form in the atmosphere. It is difficult to form any estimate of the extent of this loss, but there can be no doubt that it represents a waste of hundreds of tons of nitrogen daily. Much also is lost from the other item of animal waste materials, not a little of the loss being occasioned by modern methods of sewage disposal in large cities. Such methods represent, from the standpoint of agricultural economy, a luxury, which however, will be thought worth the price if the means are at hand to make good the loss from other sources. Those portions of animal refuse which are placed on the soil of crop-bearing fields and pastures return, at least in part, into the organic cycle.

Accessory Sources of Combined Nitrogen. It is of course absolutely essential for the continuance of the life cycle that the losses of combined nitrogen which have been noted should in some way be compensated by equal or greater accessions to the total amount of nomadic nitrogen. One source of such compensating revenue has already been noted, namely the direct assimilation of elementary nitrogen from the atmosphere by a narrowly restricted class of plants. There are two other natural sources. Volcanoes and fumaroles belch notable quantities of ammonium chloride into the air ; nitric acid is also formed by the action of lightning, while ammonia is produced by the passage of the silent electric discharge (aurora) through the atmosphere. Arrhenius® estimates that the amount of nitrogen annually bound in this Way amounts to about 1.4 « 109

metric tons, or one part in 3 millions of the total atmospheric nitrogen. The products are washed into the soil by the descending rain, together with more or less of the same substances that have Sooo into the atmosphere from the ground and are thus restored to the soil. Estimates which have been made of the quantities involved are somewhat conflicting, but on the whole the gains of the soil in this way are held to exceed its losses.° The stages and agencies so far reviewed may be collectively designated as those constituting the “natural nitrogen cycle,” as distinguished from a group now to be considered, which are charac-