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Volume 42, 1909
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Art. XXXII.—A Possible Relation between Atmospheric Carbon-dioxide and Leaf-development.

[Read before the Philosophical Institute of Canterbury, 4th August, 1909.]

The existence of all green plants is dependent upon atmospheric carbondioxide, and this is relatively so small in amount that the annual growth of plants the world over absorbs each year a very appreciable proportion of the whole supply. Liebig calculated that in Central Europe one acre of vegetation removes three-fifths of a ton of carbon per year; and Arrhenius,* using this estimate, finds the annual carbon-production of plants the world over to be 13,000 million tons—not less than one-fiftieth of the whole CO2 of the atmosphere. Although the greater part of this, by decay, goes back into the air each year, Arrhenius estimates that the carbon annually obtained from the air and stored up as peat would use up all the atmospheric CO2 in ten thousand years.

Existing peat and fossil carbon generally, corresponds approximately in amount to the oxygen of the air—that is, would require the whole of the atmospheric oxygen to burn it up completely. Hence it was suggested by Keohne, of Brussels, in 1856, that all the oxygen of the air—1,216 billion tons—may have been produced from CO2 by plants.

It is clear, then, that the relation between plants and the world's supply of CO2 is a very intimate one, and that any considerable fluctuations in the aerial proportion of this essential gas is likely to be of supreme importance to plant life. That fluctuations have occurred—that at one time in the earth's history the amount of CO2 in the atmosphere was very different from and very much greater than at present—there is a good deal of evidence to show. There can be no doubt that at one time the crust of the earth was at a high temperature, above its melting-point, and far above the ignition-point of carbon in air. Hence any free carbon whatever which reached the surface of the fluid earth during this period—a very long period—must have been burnt into CO2. Not only so, but the free carbon below the surface, protected from air, could scarcely escape oxidation by the water and metallic oxides of the molten mass.

Pratically all metallic oxides are now known to be reducible by carbon at very high temperatures, and many igneous rocks, old and new, undoubtedly contained considerable water while in fluid or semi-fluid condition.

It has been suggested by Mendelèef and others that at this stage the carbon escaped oxidation by forming metallic carbides similar to the ironcarbides of cast iron and the now well-known calcium-carbide used for generating acetylene. But carbides are readily oxidized into CO2 by water, air, or metallic oxides. Examples of such action are found in the Bessemer converter and in the production of steel from cast iron by burning out the contained carbon with oxides of iron. The traces of CO2 now found occasionally in rocks in the liquid form may possibly have their origin in some such deep-seated oxidation of carbon or carbides.

The weight of evidence, then, appears against the supposition that the bulk of the earth's carbon was retained in the crust during the hot period,

[Footnote] * “Worlds in the Making.”

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and tends to show that it was burnt into CO2 and found its way into the atmosphere.

Let us compare, then, the quantities of carbon at present existing in the earth's crust and in the air respectively, in order to see how far the aerial supply would be increased if all known carbon existed as CO2

Percentage of CO2 in air = by volume .04

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Percentage of carbon in air = .04*3/11 = .011 (nearly).

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Atmospheric carbon in pounds per square inch of earth's surface = .011/100 *15 = .0016.

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Percentage of C in earth's crust = .21 (F. W. Clark).

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Density of crust = say, 3.

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Terrestrial carbon in pounds per cubic inch of crust =62.5 * 3/1728 * .21/100 = .00022.

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Atmospheric carbon per square inch of crust/Terrestrial carbon per cubic inch of crust = .0016/.00022 = 8 (approximate).

Hence, a column of the earth's crust 8 in. deep contains as much carbon as a similar column of air of the full depth.

Assuming the earth's crust to be fifty miles deep, if 1 per cent. of its carbon was burnt and thrown into the air, the atmospheric CO2 would be increased more than three thousand five hundred times.

The problem as to the amount of CO2 formerly in the air may be considered also from another point of view.

Plants are not and never have been the only absorbers of CO2. The weathering of rocks in ceaseless. Many igneous rocks consist largely of silicates of potassium, sodium, calcium, magnesium, and iron. CO2 dissolved in rain-water converts these into carbonates. Hence the probability of all the CO2 found as carbonates in the sedimentary rocks being obtained from the air.

Hogbom estimates that the earth's limestones and dolomites contain twenty-five thousand times as much CO2 as the air.

Chamberlin (American geologist), omitting the pre-Cambrian (pre-plant) limestones, estimates that those formed since life appeard contain twenty to thirty thousand times as much CO2 as the air.

It is absolutely clear, therefore, that the air was once enormously richer in CO2 than now. Was this so when plants first appeared ? Carbon separated from the air and now existing as fossil plants, coal, peat, &c., contains hundreds of times as much carbon as is now in the air; also, the limestones of fossiliferous periods contain twenty to thirty thousand times as much CO2 as the air.

Unless, therefore, there has been a regular supply of CO2 from volcanic vents or otherwise to balance this enormous removal of carbon, the atmosphere at the period when plant-life began must have been excessively rich in CO2. Also, during the plant period the atmospheric CO2 has been reduced apparently in a steady continuous progressive manner from a very large to its present very small proportion.

One need not be a biologist or botanist in these days of popular scientific literature and popular scientific lectures to be thoroughly familiar with the fact that environment has a great deal to do with the from, character, and habits of plants. Relatively small differences in temperature, in dryness of air or of soil, in wind, in altitude, in presence of insects or herbivorous

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animals, &c., produce in time great changes, and are largely responsible for the diverse plant forms now existing. And in the light of the present knowledge as to the effect of environmental changes, it is impossible to conceive that a change—huge, continuous, progressive—in a most vital condition, extending for thousands or millions of years, could be without a corresponding change in the plant forms dependent upon it.

The collection of the carbon essential to its existence is confined to the surface of a plant. The surface is the part most directly concerned. Hence, if any change occurs to balance this continuous impoverishment of the air, it should be in the direction of a modification of the surface.

Two distinct kinds of modification appear possible—(1) An increase in the effectiveness of the original surface; (2) an increase in the area of this surface.

For our own present purpose the first may be neglected, as it seems hardly possible to discover much about it.

As to the second, palæontological records show that the earliest plants, other than ferns, were practically leafless, and little branched. The ratio of surface to mass was comparatively small.

The lepidodendrons and calamites of the Devonian period were massive plants showing quite a small surface. Later plants showed more branching, more surface area; but it was not until late in the Carboniferous era, by which time immense amounts of CO2 had been used up, that leaves were common.

An examination of plant-remains shows that since Carboniferous times a fairly continuous increase in the ratio of surface to mass in the dominant plants has taken place, until now the average flowering-plant shows an abundance of leaf, a ratio of surface to mass that has not been exceeded at any previous period.

It seems a fair inference, then, that the evolution of plant surface—in other words, the evolution of the leaf—has been primarily due to the necessity for more efficient CO2 collection, and has been one of the results of the progressive diminution of atmospheric CO2.

Arrhenius* points out that the actual amount of CO2 in the air is so small that in 1904 the coal burnt produced CO2 equal to one-seventieth of that of the air. The atmospheric CO2 is consequently again increasing or likely to increase.

One objection to the suggestion that decrease of CO2 has developed leaf surface is that ferns were amongst the earliest plants, and these exposed a comparatively large surface from the beginning. This was probably due to an entirely different cause.

Consider the factors in the development of plant surface. They appear to be of two kinds, respectively in favour of and against such development: In favour—(1) Need of CO2; (2) need of sunlight. Against—(1) Drought; (2) wind (mechanical effect); (3) excessive sunlight.

The early forests appear to have been dense, and then, as now, ferns appear to have been forest-floor plants, living in partial shade. Forest-floor plants would be likely to have CO2 in plenty, but little light. Light, however, is absolutely essential, and a plant growing in the shade can increase its light-collecting power only by spreading itself out as much as possible.

While there are many drawbacks to a great extension of surface in an exposed plant, on the forest-floor there is no wind to damage, no drought to shrivel, no sun to scorch.

[Footnote] * “Worlds in the Making.”

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The problem arose much sooner, was much easier than in the case of exposed plants, and apparently was solved much earlier. I believe that at the present time these advantages still show, ferns are able to extend themselves recklessly—there are probably few exposed plants which rival the ferns in ratio of surface to mass.

The New Zealand kidney-fern possesses an extraordinary extension of surface in proportion to its mass; a young and vigorous New Zealand cabbage-tree also shows very considerable surface: but if cut and exposed to a drying wind the fern shrivels in a few minutes, the cabbage-tree scarcely in a week. The fern in gaining large surface has become excessively open to damage by light, by wind, and by drought; the cabbage-tree in gaining a great extension of surface has retained its power of resisting all three.

It seems, therefore, reasonable enough to assume that even if the necessity for increased surface arose at the same time in ferns and exposed flowering-plants, the latter, faced by such difficulties, would take much greater time to develop it safely.