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Popular Science Monthly/Volume 6/November 1874/The Respiration of Plants

< Popular Science Monthly‎ | Volume 6‎ | November 1874

THE RESPIRATION OF PLANTS.
By ÉMILE ALGLAVE.

TRANSLATED FROM THE REVUE SCIENTIFIQUE, BY J. FITZGERALD, A. M.

I.

THE functional contrast between the two organic worlds of plants and animals was, till lately, the groundwork of all scientific speculations. The labors of the most illustrious men of science had confirmed this theory; and then, too, it was in accord with all the known facts.

Plants, it was held, grow in order to supply animals with food, and to make life possible for them; the activities of vegetal life produced the immediate principles of food, and animal life destroyed them; the various excretions of animal life were the natural ferment of vegetal life, and the latter purified the air, contaminated by animal emanations; finally, that function of the organism which is most continuous, namely, respiration, consisted, in animals, in the absorption of oxygen, followed by exhalation of carbonic acid, while in plants it consisted in the absorption of carbonic acid, followed by exhalation of oxygen. In this way the respiration of plants would decompose the carbonic acid produced by the respiration of animals, thus preserving the normal constitution of the atmosphere.

The famous experiments of Claude Bernard on the glycogenic function of the liver, revealing, as they did, the formation in the liver of animals of one of the most important of these immediate principles, to wit, sugar, delivered on this apparently philosophic and well-established theory a severe blow, from which it could not recover. Soon, a very different theory, one no less philosophic in its general form, was proposed; and this theory was so bound up with the tendencies of modern science and philosophy, that its success was assured from the outset. In place of the harmonic contrast of the two kingdoms, we have now the functional unity of living Nature. Our readers have not forgotten the lectures delivered during several years by Claude Bernard, at the Paris Museum of Natural History, in which he has developed this grand conception. Instead of comparing together animals and plants, pointing out their differences, as the usual course has been, Bernard enumerates their resemblances; and this simple change in the point of view at once gives to the ensemble of the facts a very different meaning.

But, still there appear to remain some fundamental differences, chiefly with regard to respiration.

Since the date of the early researches, which made out the respiration of plants to be an exhalation of oxygen resulting from the decomposition of the carbonic acid of the atmosphere, sundry not very recent experiments have to a certain extent limited the bearings of the original conclusions. It was soon discovered that this mode of respiration is subordinated to the action of solar light, and that it occurs only in the leaves and in the green portions of the plant, the coloration being due to the presence of a special principle called chlorophyll; and thus the chlorophyll came to be regarded as the organ, the essential agent of plant-respiration. Next, the discovery was made that the flowers of a color different from green, and even the green portions themselves when placed in the dark, not only do not absorb carbonic acid out of the atmosphere, and then exhale the oxygen of that acid; they go further, and do the very opposite, absorbing oxygen and giving up carbonic acid, just as animals do.

Hence the assignment to plants of a second mode of respiration, known as nocturnal, as opposed to the other mode, called the diurnal respiration. But, notwithstanding the discovery of a number of facts which tended to enlarge the province of this so-called nocturnal respiration, it was far from attaining the importance of the diurnal, which all the botanists held to be the true respiration of plants, and which, as compared with the other mode, clearly deserved this distinction, owing to the number, the duration, and the extension of the phenomena which it represented.

One might wonder at this strange duality of respirations in a single being—respirations that were antagonistic in their very essence; especially might one ask how plants could be deprived, during one-half of their life, of that physiological function, the unceasing performance of which would seem to be the most indispensable of all, to wit, the respiratory function—for this was held to be identical with the diurnal respiration; it might even be observed that certain plants, grown in the dark, perform this function very seldom; but, for all that, the facts all seemed to require the acceptance of the current theory.

These preliminary remarks will enable the reader fully to see the importance of the researches recently explained to the Lille Society of Sciences, by M. Corenwinder, of whose paper we propose to give a summary. The author, who has for twenty years pursued in one direction his studies of vegetal physiology, has proved that the nocturnal respiration of plants, though supposed to be exceptional, is in fact perfectly continuous, and constitutes their only true respiration. What hitherto has been called diurnal respiration, viz., the absorption of carbonic acid, the seat of which is the chlorophyll, instead of being the true respiratory phenomenon, is a phenomenon of assimilation and digestion, as pointed out by Claude Bernard. Plants and animals respire both in the same way. This is the grand fact, the proofs of which are given by Corenwinder.

 
II.

Buds, young shoots, and nascent leaves, discharge a function hitherto insufficiently considered, but yet this function is of such a nature as to elucidate the most important laws of vegetal physiology. It may be readily shown by very simple experiments that, in this first period, and for a certain length of time, plants absorb oxygen unmistakably and uninterruptedly, exhaling carbonic acid. Nor is it only in the dark that they discharge this function; indeed, it is not very apparent during the night, when the weather is cold, as is often the case in spring. It is during the day, and when the sunlight is strongest, that this function becomes characteristic, and especially when the temperature is rising.

This is easily shown by placing delicate plants, gathered in the early stages of their growth, under a close bell-glass, connected with a receiver holding concentrated baryta-water, the receiver in turn being connected with an aspirator, which causes the air in contact with the plant to pass gently over the baryta solution. For instance, take a freshly-opened bud of the chestnut, and presently, or at least after a very little while, there is seen to form, in the daylight, a deposit of carbonate of baryta, and this increases very rapidly. Of course, care must be taken to deprive the air of its carbonic acid before it is admitted into the bell-glass.

A very simple experiment will make it plain that, in the course of this first period, the nascent leaves absorb to an appreciable extent the oxygen of the air both day and night. We have only to place the plant in a small bell-glass containing common air, the mouth of the vessel being stopped by means of a solution of caustic potash in a saucer. Soon we observe the solution rising in the bell-glass, and standing still at a certain point, which it never goes beyond. (Care must be taken not to allow the alkaline liquid to touch the petioles of the leaves.) If we now examine the elastic fluid which remains unabsorbed, we find that it contains nothing but nitrogen. In this operation the oxygen is inhaled by the leaves, which transform that gas into carbonic acid; this they expire in variable proportions according to their age, and it is absorbed by the caustic-potash solution.

But this power of absorbing oxygen and of exhaling carbonic acid in the daytime, while very evident at the instant of the opening out of the buds, becomes sensibly less pronounced, according as the leaves grow, and, as a general rule, this phenomenon ceases to be presented after these organs have attained their normal development. Hence, it is certain that plants, in their earlier stages, respire after the manner of animals, absorbing oxygen and exhaling carbonic acid. These physiological facts were demonstrated by M. Corenwinder, in a memoir published in 1866 by the Société des Sciences of Lille.

 
III.

It is not alone young plants just produced from the seed or from the bud in the spring that offer these characters: all foliaceous organs, while young, tender, and injected with nitrogenized materials, and just beginning to derive their nourishment from the carbon of the atmosphere, sensibly exhale carbonic acid in the daytime. If we observe the young branches which, during summer, grow on trees of persistent foliage, the Laurocerasus, for example, we find that, in these new growths, the phenomenon of respiration predominates: they exhale sensibly carbonic acid in the daytime.

But, if we place under a closed bell-glass an entire branch bearing leaves of the current and of the preceding year, collecting the air that has been in contact with them in a receiver holding baryta-water, and provided with an aspirator, we find that the result varies according to the relative quantities of new and old leaves. If the latter are in excess, they absorb the carbonic acid exhaled by the former, and the baryta-water remains clear, but it grows turbid when the new leaves predominate.

If the experiment be made at the period when all the leaves of the current year have attained their adult age, the branch of Laurocerasus gives out no carbonic acid while exposed to light, provided the light is not very feeble.

The point at which plants cease perceptibly to give out carbonic acid in the daytime varies widely according to the species. Corenwinder has found some which exhibit this property for a long time, while others lose it very early. In the first category we may class a perennial plant common in our gardens, Diclitra spectabilis, and in the second the young leaves of the beet.

The cause of this peculiarity cannot at present be assigned; certain it is, however, that it largely depends on external circumstances, heat, for instance, which quickens all the chemical actions of oxygen, or the intensity of the light which promotes the assimilation of the carbon. But the special nature of the plant also plays a part. Hence we must not jump at conclusions after one of these experiments, if we would avoid setting up artificial laws with many exceptions.

It was at first difficult to account a priori for the fact of this property of nascent plants constantly exhaling carbonic acid, being at the outset very patent, and then diminishing in intensity as they grow, and finally disappearing. But experiments of another kind, described eight years ago by M. Corenwinder, put him on the track of this phenomenon, and gave him a plausible explanation of it.

Adopting the same processes which enabled Bonnet, Ingenhousz, and Sennebier, to lay the foundations of plant physiology, he placed buds and young stems bearing new leaves in bell-glasses filled with spring-water containing bicarbonate of lime or in water charged with carbonic acid, and then exposed them to the sun. As was to be expected, the leaves were soon covered with bubbles, and gave off oxygen; and this is the case even with leaves whose evolution is not yet complete. Hence it is plain that, from the earliest period of their life, plants decompose the carbonic acid of the atmosphere and assimilate its carbon.

Thus the foregoing experiments prove two facts which seem to be contradictory, and which, nevertheless, are simultaneous: I. Inhalation of oxygen, accompanied with emission of carbonic acid; 2. Absorption of carbonic acid, leading to a discharge of oxygen. Hence, in young plants, there is simultaneity of the two modes of respiration commonly attributed to older plants; but, in the latter, these two modes have different conditions or different organs. This was the starting-point, and it had to be made clear by means of accurate research.

 
IV.

As we now see, the plant begins, in the early stages of its life, to respire as the animal does, absorbing oxygen, and exhaling carbonic acid. But we have still to inquire why it is that the exhalation of carbonic acid gradually diminishes as the leaves grow in size. This is the great point to settle. Inasmuch as the respiratory organ grows in vitality and in size, it looks as though the respiration ought to become more active, and consequently augment the exhalation of carbonic acid, if this latter process is the respiration.

In order to solve this problem, M. Corenwinder judged it necessary to investigate very closely the variations occurring in the chemical composition of leaves during their vegetation. For this purpose he has made numerous researches, whereof we will describe two experiments made during the summer of 1873, one upon a white lilac, the other upon a maple with fine green leaves. These occupy a good, airy site in the author's garden, near the city of Lille.

M. Corenwinder gathered leaves of these plants at suitable intervals, from April 15th till October 31st, analyzing them afterward to determine the amount of water, nitrogenized substances, ash, and ternary compounds, they respectively contained. In sundry cases, at the most characteristic periods, the proportion of phosphoric acid contained in the ash was accurately weighed.

As every one knows, water forms a considerable portion of the substance of leaves, as much as four-fifths. As a rule, this proportion becomes less as the season advances, and the leaves grow older, but the diminution is not regular. As M. Corenwinder has shown, it needs but to rain for a little while to very sensibly raise again the proportion of water in leaves. These variations in the water of vegetation of leaves make it difficult to compare the other elements which they contain, and hide the relative increase or decrease of each of these elements. De Saussure evaded this difficulty by calculating the leaves in the dry state, and then determining what would be the relative proportions per cent, of the various elements in each leaf, if really deprived of all its water. M. Corenwinder adopts the same course. Having given in full the results of his two series of experiments, he condenses them in the following tables, which enable us easily to follow the evolution of each of the groups of elements in the leaf:

LEAVES OF COMMON LILAC.
1873. remarks. Nitroge-
nized
Matter.
Carbona-
ceous
Matter.
Ash.
April 15 Leaves small 27.87 67.71 4.42
"18 "larger flower-buds just appearing 23.36 71.45 5.19
"21 "still larger, flower-buds developed 18.00 71.45 4.96
May 12 "normal; flowers expanded 17.86 77.68 4.46
June 6 ""flowers withered 14.75 78.35 6.90
July 1 "" 12.62 79.04 3.34
Aug. 2 "" 10.81 80.79 8.40
Sept. 2 "" 10.31 81.77 8.52
Oct. 1 "still green 11.19 80.61 8.20
"31 "withered 8.87 83.13 8.00
 
PHOSPHORIC ACID IN THE ASH.
DATE. In 100 Parts of Dried Leaves. In 100 Parts of Ash.
April 15. 1.400 31.67
June 6. 0.770 11.16
Oct. 1 0.460 5.61
Oct 31 0.256 3.20
 
LEAVES OF THE MAPLE.
DATE. REMARKS. Nitro-
genized
Matter.
Carbon-
aceous
Matter.
Ash.
May 1 Leaves small 40.94 53.06 6.00
" 7 "small, expanded 38.56 54.54 6.90
" 20 "normal 26.25 65.86 7.89
June 13 "larger 22.87 67.73 9.40
July 12 "" 19.59 68.13 12.28
Aug. 4 "" 20.19 68.17 11.64
Sept. 3 "" 20.62 65.88 13.50
Oct. 3 "fading 20.00 65.25 14.75
" 14 "fallen 14.80 69.00 16.20
 
PHOSPHORIC ACID OF THE ASH.
DATE. In 10 Parts of Dried Leaves. In 100 Parts of Ash.
May 1 2.797 49.62
June 13. 0.957 10.18
Oct. 3. 0.119 0.73

It will be at once observed that the absolute amount of nitrogenized substances differs widely as between the leaves of the two plants: there is at first far more in those of the maple than in those of the lilac, and this superiority is maintained during the whole period of vegetation.

Probably were we to examine from this point of view a large number of plants, we should find differences as great as these. Even between trees of the same species similar differences occur, according to their age and vigor, and more particularly according to the surroundings. Thus, on July 12, 1873, M. Corenwinder collected leaves of the common lilac in a garden situated in the open country near Saint-Quentin, and in them found:

 
Nitrogenized matters. 18.56
Carbonaceous matters 72.90
Ash 8.54
———
100.00

Comparing this analysis with that of the lilac-leaves which were gathered at the same time, but in a city-garden, we see that those which had plenty of air, growing in the country, remote from aggregations of human beings, are the richer in nitrogenized substances. They also grow thicker and larger, the activity of respiration developing their vitality, and promoting the growth of the organs which discharge that function. Hence it would appear that, for plants, as for animals, an abundant absorption of pure air, rich in oxygen, is the essential condition of a strong, vigorous constitution.

But these individual or specific differences have no importance as regards our problem. The point for us to consider is, not the absolute proportion of a given element, but rather the relative modifications which the initial proportion undergoes during the life of the leaf. Let us see what the tables have to say on this point:

1. During the growth of the leaves, the relative proportion of nitrogenized matter in their tissue grows rapidly less. It is at the maximum just when these organs are breaking out of the bud, and it goes on decreasing thenceforward till about the beginning of July, when the fruit of the lilac has been formed. From that time on, the quantity of nitrogenized matters varies but little, though it seems to gain a little in leaves approaching maturity. Finally, it is at the minimum when the process of vegetation is complete.

At the moment of their falling, we find in lilac-leaves only about one-third the amount of nitrogenized matters they contained at the outset. In maple-leaves the amount is relatively greater, but the difference is not very important.

2. If, now, we look at these analyses with respect to the amount of carbonaceous matters, we find that the latter rapidly increases from the moment when the leaves start from the bud, down to the time when they have attained their greatest size, i. e., when they have reached the adult age. As regards the lilac, this is the case when the flowers are ready to expand. The carbonaceous matters thenceforth gain less notably till September; but then we perceive a sensible decrease, especially as regards the maple. Finally, they attain the maximum at the time of falling from the tree, this rise being due to the disappearance of a notable amount of nitrogenized substances.

3. The ash, too, increases rapidly down to June, but then it grows less pronounced. There is relatively a greater amount of mineral matter in the faded leaves of the maple than in those of the lilac. The latter, at the close of their life, show a slight diminution, which is perhaps accidental, in mineral salts: being more tender than maple-leaves, they probably lose a little of their soluble salts under the action of rains.

We have only to compare the mature leaves of the maple with those of the lilac, in order to see that the former must contain more fixed salts than the latter: the fibres which traverse them are thicker, stronger, and more numerous, than in the leaves of the lilac, and hence they are richer in silica and salts of lime.

The ashes of these two trees differ very widely from one another. Even in the same species the quantity of the ash, like that of the nitrogenized matter, differs according to the surroundings, the age of the plant, the humidity of the soil, and the heat to which it is exposed. It was proved by De Saussure that the quantity of ash is much less in nascent leaves than in those which have attained the term of their existence.

4. De Saussure also proved that in the ash of buds and nascent leaves there is more phosphoric acid than is found at any later stage. Since his time this fact has been confirmed by Garreau, of Lille, and by Corenwinder. The present series of experiments furnishes a new demonstration of this important phenomenon.

In the tables we have given the reader will observe that the proportion of phosphoric acid, which at the outset was considerable, especially in the maple-leaves, rapidly grows less, and when the process of vegetation is at an end it is very small indeed. Thus, when coming from the bud, they contain (in the dry state) about .028 phosphoric acid; but at last they contain only about .001. It was long ago proved by M. Corenwinder that the phosphorus contained in plants is an essentially variable quantity. It almost entirely disappears from the tissue of annual plants at the end of their growth, being condensed in the seeds, and ultimately serving to perpetuate the species. In perennial plants, the phosphorus does not go into the seeds merely—it is also diffused through the trunk and the branches; further, it hibernates in the buds, which contain the essential elements of the seed, and which perform the same functions as the latter in the evolution of leaves.

 
V.

Having now made the experiments tell their story, and described the comparative evolution of the various elements of the leaves during their annual life, let us next see whether these variations in chemical constitution may be coupled together under a theory which shall explain the modifications undergone by the gaseous exhalations of plants at the various stages of their life.

When we study closely the figures relating, for instance, to the maple, we find that, in the first stage of growth the nitrogenized matters are very considerable. Probably they have an organization of their own, and exist independently of the vegetal cells; at all events, they discharge functions which may be called animal—they respire, and in this early stage respiration is the dominant function. The carbonic acid resulting from this operation is at first only in part retained in the plant by the reducing action of the chlorophyll. The young plant, when exposed to the light and placed in atmospheric air, exhales an excess of carbonic acid.

In the second period, the relative proportion of nitrogenized matters grows less, while, on the other hand, the carbonaceous matters increase. The plant now exhales only a small amount of carbonic acid, the latter being almost entirely retained by the chlorophyll, which decomposes it, and fixes its carbon.

Later, the disengagement of carbonic acid ceases, that gas being instantly absorbed by the chlorophyll as soon as it is produced in respiration. The plant has now entered the adult stage. It freely absorbs the carbonic acid of the air, under the influence of the sun's rays, and gives off oxygen. The phenomenon of respiration is at this period completely masked, and cannot be shown to exist except by indirect processes, as we shall soon see.

On reading the column of figures headed "Carbonaceous Matters," we find that, in September, they grow rapidly less. Further, M. Corenwinder has discovered that, toward the beginning of October, the leaves exhaled a little carbonic acid in the daytime. Here the phenomenon is not of the same nature as at the beginning of vegetation: the yellow leaves are dying, and lose their carbon, like all decaying or dead organic matters exposed to the air. When vegetation is at an end, the proportion of carbonaceous matters seems to increase, owing to the rapid disappearance of nitrogenous matters.

From the facts established in the physiological and chemical experiments we have described we derive a very probable explanation of two phenomena, which at first view would seem to be mutually incompatible, viz., exhalation of oxygen and exhalation of carbonic acid. The latter of these has its seat in the nitrogenized matters, and constitutes the respiration of plants, which is henceforth to be esteemed the same as the respiration of animals. The other phenomenon has its seat in the chlorophyll. It has been wrongly held to be a respiratory act: it is, in fact, a true digestion of carbon.

 
VI.

From what has been said, we may unhesitatingly conclude that leaves in their earliest stage simultaneously perform two physiological functions: 1. They respire by means of their nitrogenous constituents; 2. They assimilate carbon by means of the carbonaceous matters organized in their tissues, i. e., the chlorophyll.

This act of respiration, as we have seen, becomes less apparent as the plant begins to assimilate carbon, but, in reality, it goes on uninterruptedly, being only masked by the increased activity of the other function. That this is the case may be proved experimentally.

In the first place, we know that plants cannot live in an atmosphere without oxygen. When they are placed in close vessels containing hydrogen and nitrogen, they live for a while, owing to the small quantity of atmospheric air in their cells; but yet, though outwardly they may give no sign of decay, on being taken out of the vessels they are found to be dead. The development of the buds ceases utterly in an oxygenless atmosphere. On March 29, 1872, M. Corenwinder placed a chestnut-bud in a test-tube containing atmospheric air, the mouth of the tube being immersed in a solution of caustic potash. The bud formed carbonic acid rapidly, inhaling the oxygen of the air in the tube. The acid was absorbed, as soon as formed, by the alkaline solution, and the latter rose in the tube until there was no gas left except nitrogen.

The bud was kept in the nitrogen till May 2d; it then began to give signs of decay. During all this time it never gained in size, and retained its original form. The conclusion is, that bud development cannot go on in an atmosphere deprived of oxygen.

We know, further, from the observations of Th. de Saussure, that germination is impossible when the embryo, in process of growth, does not find, in the atmosphere in which it lives, the amount of oxygen needed for its life. Hence Corenwinder's experiment gives us a fresh instance of the resemblance between germination and the evolution of leaf-axes.

Th. de Saussure also examined several plants placed in an atmosphere of nitrogen. According to their behavior under these circumstances, he divides them into two categories, viz., those which vegetate in such an atmosphere only for a few days, and those which live and even flourish there for a certain length of time. Plants of the latter class are chiefly those which inhabit marshy situations, such as Lythrum salicaria, Epilobium hirsutum, Polygonum amphibium, etc. He has expressed the opinion that plants possessed of this latter property consume less oxygen, vegetating in atmospheric air without much light.

If, in M. Corenwinder's experiments, plants wither rapidly, the reason is, that in the morning he drew off the carbonic acid formed during the night by the agency of the oxygen contained in the cells. When this is not done, the leaves may decompose the acid in the daytime, give out oxygen, and so live for a long time, the oxygen being inhaled and exhaled over and over again.

Finally, if the leaves be kept in absolute darkness, the reducing action is null, and then the act of respiration, which, of necessity, is never completely suspended, alone appears, and the plant disengages only carbonic acid. This function, however, is curiously affected by temperature, so that, at 32° Fahr., leaves usually exhale but little carbonic acid.

These early observations would of themselves suffice to show the existence in plants, at every stage of growth, of a respiratory action, like that of animals, viz., an absorption of oxygen.

What in the books is called the diurnal respiration of plants, is in reality an assimilation of carbon; in other words, it is the act whereby the leaf-organs decompose the carbonic acid of the air, and give out its oxygen. This act depends essentially on the influence of light. It is at the maximum intensity when the plant is under the direct action of the sun's rays, and gradually diminishes in importance in proportion as the light grows feebler; for instance, when the sky is overcast with clouds, and when the weather is thick and rainy. This was demonstrated in a memoir by Corenwinder, published in 1858.

Still, with full-grown leaves in the open air, and with abundance of light, we but rarely find them exhaling even a very small amount of carbonic acid; though—as was shown by M. Corenwinder ten years ago—if we transfer them to a room lighted only by side-windows, and to which the direct rays of the sun do not penetrate, they generally, under these conditions, exhale carbonic acid in the daytime, the proportion varying according to the nature of the plants, the weakness of the diffused light, and the temperature. Of all the plants experimented on in this way by M. Corenwinder, the common nettle appeared to give out the largest amount of carbonic acid, when kept in a room.

These facts M. Corenwinder announced ten years ago, but he did not then venture an interpretation of them, as he does now.

M. Boussingault has shown that leaves placed in a bell-glass containing pure hydrogen mixed with a little carbonic acid, and kept in an ill-lighted room, give off traces of oxygen. This proves that even under the most unfavorable conditions the assimilation of carbon does not entirely cease: this act is completely suspended only in total darkness.

Now, as it is likewise certain that, under the same conditions as to light, leaves kept under a bell-glass filled with air give out carbonic acid, and inhale oxygen, it follows inevitably, from these two classes of observations, that the functions inherent in plants, respiration and assimilation of carbon, are simultaneous; the latter function is, however, reduced to such a degree that it no longer completely masks the effect of the former; in other words, the chlorophyll can no longer absorb all the carbonic acid produced by respiration.

Here, then, we have a fact analogous to that which we see in the earliest stage of vegetation, with this difference, that in the case of the bud the reducing action is insufficient, owing to the relative inferiority of its carbonaceous matters, while in the other case the insufficiency results from the reduction of their power.

These facts are undoubtedly very strong arguments for the theory of two simultaneous functions in leaves. M. Corenwinder still further confirms it with sundry observations, which are easily made.

Place perfectly green, full-grown maize-leaves under the bell-glass of the apparatus already described, and it will be seen that in the daytime they do not exhale the slightest trace of carbonic acid. If we could deprive these leaves of their green matter, which to all intents and purposes performs the assimilative function, we could doubtless discover the other function which this conceals, viz., respiration. Now, this very experiment Nature makes easy for us. As all are aware, there is a species of maize with striped leaves, which often bears white leaves without a trace of green. If we examine closely the striped leaves, we shall see that their white portions have absolutely no chlorophyll within. This is not the case with the leaves of a purple or of a black color; these, in addition to the coloring matter characteristic of them, contain always more or less green matter, masked. They also possess the property, which the white leaves do not, of absorbing carbonic acid, and exhaling oxygen in a perceptible degree, when exposed to the sun. We lay stress on these phenomena, since it is for want of having understood them that very recent authors describe colored leaves as being usually deprived of the function of assimilation.

If we make the experiment already described with white leaves, we shall find that in the daytime they exhale a perceptible amount of carbonic acid.

Sennebier had observed that the yellow and red stripes of the Amarantus tricolor do not give off oxygen when exposed to the sun, but that the leaves of the Amarantus ruber, on the contrary, possess this property. So, too, leaves naturally green, which change color at the close of their life, entirely cease from absorbing carbonic acid and exhaling oxygen. Corenwinder has shown that faded leaves that are on the point of falling constantly give out carbonic acid. The fact seems to be universal. Here, however, it is not a phenomenon of vitality that appears, but an act of decrepitude, which goes on and increases after the leaf has fallen.

We observe the same phenomena in other plants, some of whose leaves contain no green matter whatever, especially in the striped maple, which is such an ornament of our gardens in summer. In August, 1868, M. Corenwinder gathered off one and the same maple some leaves that were perfectly white, and others that were perfectly green, and analyzed them to determine the amount of nitrogen they respectively contained, with the following results:

WHITE LEAVES.
Nitrogenized matters in 100 grammes, dried at 212° Fahr. 17.06 gr.
GREEN LEAVES.
Nitrogenized matters in 100 grammes, dried at 212° Fahr. 13.75 gr.

Thus we find a much larger amount of nitrogenous elements in the white leaves than in those which contain chlorophyll; on the other hand, the latter are richer in carbonaceous substances. These two observations clearly confirm M. Corenwinder's theory.

Finally, we may conclude, from all the analyses and experiments we have here detailed, that there exist in plants, at every stage of their life, two distinct functions having different centres of action. The one is respiration, which depends upon the nitrogenous organic bodies. The other is assimilation of carbon, which has its seat in special organisms, formed principally, if not exclusively, of ternary elements.

This theory gives a natural explanation of all observations upon the physiology of leaves. M. Corenwinder hopes soon to make an application of it, and will show what it is worth, by explaining, with its aid, the origin of carbon in plants.