Popular Science Monthly/Volume 23/October 1883/Symbiosis and Vegetating Animals
|SYMBIOSIS AND "VEGETATING ANIMALS."|
By W. T. SEDGWICK, Ph. D.,
ASSOCIATE IN BIOLOGY, JOHNS HOPKINS UNIVERSITY, BALTIMORE.
SINCE the publication of the interesting observations and speculations of Dr. Karl Brandt concerning the occurrence of chlorophyl in animals, of which a summary account was given in a recent number of this periodical, under the heading "A Partnership of Plant and Animal Life," at least two important contributions to the same subject have made their appearance. One of these, written by Mr. P. Geddes, of Edinburgh, may be found in "Nature" of January 26, 1882. The other, from the pen of Professor E. Ray Lankester, was published in the April number of the "Quarterly Journal of Microscopical Science."
The spectroscopic examination of animal pigments, a line of investigation in which Professor Lankester was one of the earliest observers, has shed much light on these previously little known compounds, and has opened a wide field for further research. Especially fruitful in this direction was the announcement, perhaps a dozen years ago, that various animal greens yield a spectrum identical or nearly so with that furnished by chlorophyl, the common green coloring-matter of plants. At once a host of interesting questions sprang up as to the occurrence structure, and properties of "animal chlorophyl"—all of which, however, culminated in this: Is the color "accidental" and unimportant in animals, or has it rather some profound significance, such as attaches to it when it is found in vegetable protoplasm?
It may well be considered as one of the fundamental truths of biology that chlorophyl in vegetable substance is no insignificant intruder. Though the exact method of its working is just now a matter of much dispute, the broad fact of its usefulness is freely admitted; in one way or in another, chlorophyl aids in the economy of the vegetable cell so that it feeds greedily upon the carbonic acid of the surrounding air or water, and tears to pieces, for building up afresh its own substance, compounds which, without chlorophyl, it would be powerless to utilize, or which if utilized would be speedily sacrificed. A long series of experiments, reaching from the time of Priestley to the present day, uphold and strengthen this fundamental fact; but, among them all, the simplest is the most unique and the most pronounced. If a piece of a green plant be immersed in water and exposed to sunlight, bubbles of oxygen are given off, and analysis will prove that carbonic acid has simultaneously disappeared from the water. With plants not green, or with animals, the facts are reversed—oxygen disappears and is replaced by carbonic acid.
The green plants, then, exhibit a peculiar power, viz., that of splitting up carbonic acid, and of availing themselves for the manufacture of starch, etc., of the carbon thus gained; at the same time procuring such a large supply of oxygen that they are able to reject a vast amount over and above their own needs. In this respect they differ from colorless plants and animals, and for this power they depend exclusively upon the agency of chlorophyl. We have no evidence whatever that the chlorophyl of plants has ordinarily any other function.
Very considerable interest has consequently been felt in the solution of the question, What significance (if any) has chlorophyl when found in animals? And since the only known use to which it could in general be put (inferring from plants) is in decomposing carbonic acid, and thus aiding alimentation, it has become an important question to discover if this same physiological habit belongs to animal chlorophyl. If it can be proved that animal substance endowed with chlorophyl can, just as well as plant protoplasm similarly colored, break down carbonic acid and utilize its elements, then we remove the last formidable barrier between plant and animal protoplasm. If it should be shown, on the other hand, that when found in animals it is always functionally incapable of splitting up carbonic acid, it is true that we preserve a point of difference between plants and animals, but a result so negative would land us in a new difficulty, viz., that of explaining its presence by attributing to it some other function, and one unknown to plant chlorophyl. No sufficient explanation of this kind has hitherto been offered. In some cases a "protective resemblance" to vegetation would explain its occurrence; but in most cases this is out of the question.
Now, up to 1878 no evidence of value had ever been advanced to show that animal chlorophyl does enable its host to split up carbonic acid and give off the excess of oxygen resulting. In that year, however, Mr. Geddes, whose later work is referred to above, visited Roscoff and found there quantities of "the grass-green planarian (worm) Convoluta Schultzii, of which multitudes are to be found in certain localities on the coast, lying on the sand covered only by an inch or two of water, and apparently basking in the sun. It was only necessary to expose a quantity of these animals to direct sunlight to observe the rapid evolution of bubbles of gas, which, when collected and analyzed, yielded from forty-five to fifty-five per cent of oxygen."
Having obtained so much of valuable evidence, Mr. Geddes followed up his discovery by examining these green worms to see whether or not the animal protoplasm derives the same kind of benefit from its work as does plant protoplasm which is known, as a result of the co-operation of chlorophyl, to build up starch or starchy compounds.
This, indeed, proved to be the case, for he adds: "Both chemical and histological observations showed the abundant presence of starch in the green cells; and thus these planarians, and presumably also Hydra, Spongilla, etc., were proved to be truly 'vegetating animals.'"
The only link here needed is the full proof that the "grass-green planarian" owed his color to veritable chlorophyl. There is little or no reason to doubt it; yet, when we are told by Professor Lankester that in Spongilla alone among animals has spectroscopic investigation really proved the presence of that pigment, we can not help wishing that this confirmatory evidence had been obtained by Mr. Geddes.
Meantime slow progress had been making in a kindred subject. Before speaking of this, however, it must be said that chlorophyl is now known to be by no means a simple substance, but is rather made up of several simpler compounds not well understood. Nor is it always green, though in that form it occurs most familiarly. Sometimes the true chlorophyl is obscured by some other color, and is nevertheless perfectly functional, physiologically speaking; for instance, a yellow color is particularly common in the lower forms of plants—as in the algæ.
In 1871 Cienkowski boldly announced his conviction that certain yellow cells, which, first pointed out by Huxley, had for some time attracted attention in the substance of radiolarians, were really no part of the animals themselves, but rather veritable algæ living in the animals. Haeckel had previously called them "liver-cells," and when starch was found in them he believed his view confirmed, as it is in the liver of the higher animals that glycogen—a form of starch—is constantly present. The views of Cienkowski made little progress, however, till 1879, when a distinguished morphologist, R. Hertwig, of Jena, who had previously taken sides with Haeckel, adduced reasons which inclined him to the belief that the yellow cells were "parasitic," as Cienkowski had considered them to be.
In the same year (1879) the brothers Hertwig concluded that the so-called pigment-bodies in the tentacles of certain sea-anemones are true algæ—plants, multiplying bydivision. Then followed quickly the paper by Dr. Brandt, referred to at the beginning of this review. His work was extensive, and resulted in a complete confirmation of the observations of Cienkowski and the Hertwigs. He fully believes that the yellow cells are true algae, and was able to prove his points to his own satisfaction. He went, however, a step further, and announced his conviction that all animal chlorophyl is to be considered as located in associated vegetable organisms, which, together with the animal, make up "a partnership of plant and animal life." He unhesitatingly puts Hydra viridis and Spongilla (green variety) in this position, and thus disposes of all "vegetating animals," or animals living like plants endowed with chlorophyl.
In October, 1881, Mr. Geddes visited Naples for the sake of making further studies upon this subject, and in the paper in "Nature," referred to above (and which has been freely drawn upon in preparing this review), he gives a summary account of his work.
He devoted his attention at first to the yellow cells of Radiolaria, and was completely successful in demonstrating in them not only a cell-wall of cellulose and contents made up of protoplasm and nucleus, but he was also able to watch their growth both before and after the death of the animal; and, what was of special interest, he obtained a fair amount of evidence that certain tiny bubbles which in sunlight studded the radiolarians were really made up, in part at any rate, of oxygen. Besides this, he pronounces starch to be invariably present, and completely confirms the observations of Cienkowski and Brandt as to the survival and growth of the yellow cells long after the animal has perished. The yellow pigment, he says, is identical with that of diatoms, and gives the same chemical reactions.
Turning his attention next to the sea-anemones, medusa?, etc., he was equally successful. He is convinced that the pigment-bodies are true algæ and he records this remarkable experiment: "The exposure of a shoal of the beautiful blue pelagic Siphonophore, Velella, for a few hours, enabled me to collect a large quantity of gas which yielded from twenty-four to twenty-five per cent of oxygen. . . . But the most startling result was obtained by the exposure of the common Anthea cereus [sea-anemone], which yielded great quantities of gas, containing on an average from thirty-two to thirty-eight per cent of oxygen." He was able also to prove that this gas came from the associated algæ—specimens destitute of algæ giving off no oxygen whatever.
It is, therefore, now very certain that the yellow cells of radiolarians and the pigment-bodies of cœlenterates are in many cases true alga; living in the animal substance. Geddes's work, when added to that of Cienkowski, the Hertwigs, and Brandt, makes this so clear that we are justified in fully accepting their theory, and in hereafter considering the association of certain chlorophyl-bearing plants with certain animals as an established fact. It has been proposed to apply to this association the term symbiosis, and to designate animals which are thus supplied with algoid messmates as "symbiotic." So much for the "yellow cells" and pigment-bodies of radiolarians and coelenterates. Dr. Brandt, it will be remembered, has expressed his belief that the green-colored Hydra and Spongilla are also symbiotic. In Mr. Geddes's paper nothing is said upon this subject, although, from a remark let fall near the close, it is plain that when the paper was written he did not accept Dr. Brandt's view, but would reserve Hydra, Spongilla, and Convoluta, for a special group of "vegetating animals" distinctly unlike those which are symbiotic.
Professor Lankester, however, while inclined to accept all that has been shown for symbiosis in the other cases, refuses most emphatically to apply that doctrine to Hydra and Spongilla. His dissent is all the more important, because he has paid much attention to the study of animal and plant pigments (especially with the spectroscope); and, because he was the first to establish the presence of chlorophyl in Hydra and Spongilla, he has a special right to be heard in this case. In the paper quoted above he attacks the subject with great vigor, and describes several important experiments tending to show that the green color of Hydra and Spongilla is due to chlorophyl bodies analogous rather to those structures in plants than to any algoid messmates. He fails to confirm Dr. Brandt's observations, and questions the virtue of his inferences so generally, that the two authorities are practically in diametrical opposition. It is obvious that still further studies must be made upon these so-called "vegetating animals," and that at present it would be highly unsafe to consider them as symbiotic. Professor Lankester's paper is full of entertaining facts of his own discovery, but a number of experiments made by Mr. Blomfield, of Oxford, and University College, London, and quoted by Professor Lankester, are of special interest to us in this connection, as they furnish some evidence that the green Hydra does, like Convoluta, evolve oxygen in the sunlight. The observations are incomplete, but nevertheless of much value as they go to establish a second case in which animals destitute of yellow cells and pigment-bodies, but endowed presumably with true chlorophyl, do actually give off oxygen.
Meantime the reader can not have failed to perceive that the question as to the evolution of oxygen has become of secondary importance. It is nothing strange if algæ living in animals give off oxygen by virtue of their chlorophyl. In any special case we must now first inquire—Are the colored parts mere plants dwelling within the animal, or are they not? If not, then we must, if possible, apply the spectroscope for the study of the pigment (the chlorophyl group giving rather characteristic spectra), and then, if chlorophyl is present, test, if we can, for oxygen elimination. It is tolerably clear that the occurrence of native chlorophyl in animal protoplasm is not so wide-spread as was suspected before symbiosis was detected; yet the cases of Hydra, Spongilla, and Convoluta are still unsettled, and others may be added to their number: it must be granted, however, that the indications seem to be that in some cases animals may possess veritable chlorophyl arranged as in plants, giving the same spectrum and having the same power over carbonic acid.
At present it will be far more profitable to consider the significance of symbiosis than to speculate upon the result of observations belonging to the future. Professor Semper, in his work entitled "Animal Life," reminds us that, if it should come to pass [as it has] that we must consider much of the chlorophyl found in animals to be borne by vegetable messmates, we need not be surprised; since lichens—formerly supposed to be simple vegetables—have now been shown to be associated organisms—a fungus parasitic upon algæ. There is, indeed, much superficial resemblance between the two phenomena, and it is said to have been from the literature of lichens that the expressive word, symbiosis, was borrowed. In truth, there is really less analogy than at first appears; and, as there is no reason for considering the lichens as other than interesting and complicated cases of parasitism, we may hereafter, I think, reserve the word symbiosis for the description of that very different association of algæ with animals which it has been the purpose of the writer to elucidate. The word zoöphyte might, indeed, be used here with an accurate meaning had it not already a very definite (though utterly senseless) use in pseudoscientific books and minds.
Mr. Geddes pictures at considerable length the probable physiological relationship between the organisms associated in symbiosis; and in what comes hereafter we must to a great extent follow his lead. The algae which are found in animal substance have been referred to above as "parasitic," but it is chiefly to avoid the use of this term that the more accurate one (symbiosis) has been employed. A closer analogy than that offered by the lichens would be, it seems to me, afforded by any perfect plant—an oak, for example. Here the colorless cells—of the root, let us say—are bound to live at the expense of the green cells in the stem and leaves. Yet we do not think of this as a parasitic event. The root-cell is rather a unit in a vast colony of units (cells) associated for mutual benefit. The green cell gets quite as much good from the root-cell as the latter gets from the green cell; water and salts are exchanged by the root-cells for sugary matters and other things readily made use of by any cell, and no harm (as would be the case in parasitism), but rather much good, is done by the exchange. Evidently the oxygen thrown off by the alga is precisely what the plant needs, and the carbonic acid and nitrogenous waste eliminated by the animal is most useful to the alga. Moreover, the algae gain the advantage of ready locomotion with their host, and the animal can go further into unfavorable media when stocked with algae ready to build up starches and sugary matters from carbonic acid and water. If either dies, the other is the gainer; since the algae can thrive on the products of animal decomposition, and algæ—digestible, i. e., dead algæ—are much esteemed by most animals.
The whole burden of the physiological history of symbiosis is forcibly summed up by Mr. Geddes as follows: "Thus, then, for a vegetable cell no more ideal existence can be imagined than that within the body of an animal cell of sufficient vital activity to manure it with carbonic acid and nitrogen waste, yet of sufficient transparency to allow the free entrance of the necessary light. And, conversely, for an animal cell there can be no more ideal existence than to contain a vegetable cell, constantly removing its waste products, supplying it with oxygen and starch, and being digestible after death. . . . In short, we have here the relation of the animal and the vegetable world reduced to the simplest and closest conceivable form.
"It must be by this time sufficiently obvious that this remarkable association of plant and animal is by no means to be termed a case of parasitism. If so, the animals so infested would be weakened, whereas their exceptional success in the struggle for existence is evident. Anthea cereus, which contains most algae, probably far outnumbers all the other species of sea-anemones put together, and the radiolarians, which contain yellow cells, are far more abundant than those which are destitute of them. . . . Such an association is far more complex than that of the fungus and alga in the lichen, and indeed stands unique in physiology as the highest development, not of parasitism, but of the reciprocity between the animal and vegetable kingdoms."
The foregoing paper was written several months ago, and since that time important contributions to the subject have been made by Geddes, Hamann, Jickeli, Entz, and especially by Brandt. The latter has published a summary of the whole matter, enriching it by observations of his own made at the zoölogical station at Naples. Brandt attaches less physiological significance to symbiosis than does Geddes, from whom he still differs in considering even Hydra and Spongilla as symbiotic and not "vegetating" animals. At present the views of Brandt, as laid down in his last paper (see "Mitt, aus zool. Stat. Neapel," iv, ii, 1883), seem likely to prevail; and in that article the student will find a complete literature and a full discussion of the whole subject.