Thus we have again found the five moments in the vegetable kingdom in which, as in the animal kingdom, we perceived the effects of the Unconscious in bodily life. Accordingly, we are no longer warranted in refusing to the plant unconscious will and unconscious presentation. That we perceive no higher mental phenomena in the plant need not surprise us, since, indeed, the purpose of the vegetable kingdom is altogether only this—to prepare the ground, the food, and the atmosphere for the animal kingdom, although we must also admit that at the same time the creative principle works itself out independently in the vegetable kingdom in its own fashion.
2. Consciousness in the Plant. —The result thus far was not difficult to foresee, and needed no great penetration. More difficult, however, is the question whether in the plant there also dwells a consciousness.
Old as Natural Science is the dispute concerning the vegetable or animal nature of certain organisms, and even at the present day it is as little capable of decision as in the time of Aristotle, for the simple reason that as an alternative it does not admit of decision.
Plants and animals have, as organic beings, certain attributes in common; by other attributes they are distinguished according to their different offices in the household of Nature. But now, if all vital phenomena are reducible to so simple a form, that those distinguishing qualities more or less disappear, and essentially only those common to the two kingdoms remain, the differences between plant and animal must also disappear, and it is foolish to maintain a dispute which in its nature must be without result. Microscopic observation is so far advanced that, if there were criteria for the vegetable or the animal nature, they could certainly not escape the investigator, and the dispute would have ended long ago; that there are, in fact, no criteria mutually admitted by the two contending parties, is a proof that there is no distinct agreement as to the point of dispute itself. Were the facts accepted without prejudice, the inference could only be that one had hitherto narrowed too much the sphere of the qualities common to the two kingdoms, that there are far fewer differences between animal and plant than one had previously supposed, and that these differences became only in their extreme forms so striking that nobody can mistake them. Quite recently this way of looking at the matter has also gained ground in scientific circles, the strictest development of the same being the attempt of Häckel to set up a third kingdom of Protista before the vegetable and animal kingdom, although he may perhaps have extended its borders too far, and his criterion of non-sexual reproduction may turn out to be untenable, because the possession of sexual generation alike by animal and plant points to a common origin, i.e., to its existence already in the realm of Protista. Altogether the attempt to give fixed definition to the naturally fluctuating boundaries between the Protist kingdom on the one hand, and the animal and vegetable kingdom on the other, must be just as vain as the earlier endeavour in respect to the two latter kingdoms.
This mode of regarding the matter is also the only one which can be approved by geology. Whilst the terrestrial creation now subsists by the equilibrium of the productions of the animal and vegetable kingdom, manifestly the first foundation-stone of organic nature could only be laid by such beings as contained this equilibrium in themselves, and accordingly stood still at the point of indifference between animal and vegetable. One of the most important of these wonderful beings, to which the history of this earth appears to owe the whole chalk formation, has been dragged to light by recent explorations of the sea-depths, and been called Bathybius. In what way this slimy gelatinous net with interspersed protoplasmic nuclei filling up the sea-bottom, and secreting little heaps of microscopic calcareous shells (Coccoliths) is fed and thrives in the absence of every ray of light, is up to this time a riddle. Only from such an insignificant commencement could the development begin on different sides, in that marine animals arose, which lived on these undifferentiated Protista (Polyps, &c.), and as their counterpoise the first stages of decided vegetable structures became possible. The more the two kingdoms became peopled, the more were means of subsistence placed at the disposal of the higher animal classes, the more higher classes of vegetable could again exist on the living and dead products of these animals, and the evolution thus kept pace in the two kingdoms, as geology teaches, whilst within each kingdom the lower grades generally always preceded the higher. From this one should, however, draw the conclusion that vegetable kingdom and animal kingdom are not subordinate, but co-ordinate departments of creation, and that the animal kingdom, when it holds itself entitled to take rank above the vegetable kingdom in virtue of the higher development of consciousness, owes this power entirely to the pre-eminence of the latter in organic reference, since the latter forms for it the materials, to the leisurely consumption of which it owes its own higher consciousness. If, now, the consumption of material, which is formed in foreign organisms, suffices to define the action of parasitism (for the dwelling of the parasite is indifferent, as, for example, the chamber-bug), the animal kingdom as a whole may be called a parasite of the vegetable kingdom. In this respect the animal kingdom resembles the great class of fungi, which, although, according to morphological analogies, hitherto reckoned to plants, yet can only be termed vegetable parasites. There are wanting to them, namely, the vegetable “philosopher’s stone,” the arcanum, by whose help the vegetable converts inorganic matter into organic chlorophyll, and they are therefore just as much as the animal kingdom consigned to the consumption of ready-formed organic matter.
This contrast of formation and consumption is now, however, not to be taken so strictly, as if the plant merely produced, the animal merely consumed, rather do we see in every animal also processes partly of the higher elaboration of received material (e.g., the formation of cerebral fats), partly of the conversion of the same without relapse, partly of decomposition and recomposition in the course of the process of digestion and assimilation. On the other hand, we see in every plant a consumption here and there of the products, which it has itself formed at other places (we have only to think of the processes of retrogression in flowers, their inspiration of oxygen and excretion of carbonic acid). In yeast, fungi, and some other unicellular plants, we even find a remarkable ambiguity of such a kind, that they are able to take up, indeed, the nitrogen requisite for their organic production from ammonia, the carbon, however, only from higher ternary combinations. There can, accordingly, on both sides be a dispute only of a more or less; every animal is in part of vegetable, every plant, in part, of animal nature; where one side clearly dominates the other, we rightly term the whole according to that side; but where both are in tolerable equipoise, a onesided designation is difficult—nay, even inadmissible. We can now also find it no longer wonderful, if one and the same being exhibits for one part of its life an eminently vegetable, during another part an eminently animal nature. There is no greater metamorphosis at those stages near the point of indifference, than that of insects, frogs, or fishes. Certainly, whoever regards animals as animated organisms, but plants as merely empty soulless husks, such an one must be brought to despair by that fluctuation of the boundary of the two kingdoms, and the harmless passing over from the one to the other. We shall, however, see in these facts, in conjunction with the previous considerations of this chapter, only one more proof that plant and animal have much more in common than our age is accustomed to suppose.
As concerns the external general form, plants lose at lower stages their foliate type, and assume simply articulated, or rotundate, more or less enclosed forms (e.g., Confervæ, Fungi). On the other hand one finds striking similarities with higher vegetable forms among the lower animals. “Some (coral animals) grow as leaves rolled one over another, like a cabbage-head, others consist of delicate, crisp, irregularly arranged leaflets. The surface of every leaf is covered by polyp-blossoms, by whose growth and secretion it has arisen. Not less may we detect resemblances with an oak or acanthus twig, with fungi, mosses, and lichens” (Dana in Schleiden’s and Froriep’s Not., 1847, June No. 48)
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The chemical substances can certainly not establish a difference. Linnaeus thought we must regard several calcareous marine plants, such as corallines, as animals, just because he regarded the formation of chalk as a monopoly of the animal kingdom. Silicious coats of mail are found both in vegetable (Diatomaceæ) as in animal (Infusoria) organisms. The similarity of the vegetable and animal proteids is well known; the Fungi especially are rich in animal-like compounds; in the mantle of the Ascidians and other salpae-like Tunicata is found ligneous fibre; chlorophyll (leaf-green) has been proved to exist in Turbellariæ (rotifers) and in Infusoria.
Often different species of a genus were reckoned partly to the vegetable kingdom, partly to the animal kingdom, e.g., the species of Alcyonium are all in the main of so similar a nature, that Linnaeus certainly was not wrong in including them in one genus. Nevertheless, some of them are the not inaptly named Animalia ambigua (according to Pallas), which accordingly are very well ranked among the Amorphozoaria, e.g., Alcyonium cidaris (Donati), cydonium (Leba), and ficiforme (Sotander, Ellis, and Marsigli). Others were generally reckoned to the vegetable world, thus, especially, e.g., several specimens of the synonymous genus Peziza, so rich in species. In yet others not only the animal, but even the polyp nature is so decidedly evinced, that they have been separated from the Sponges and ranked among the Polypifera at the same time with the addition of a second generic name, so that Lobularia digitata, palmata, and arborea, from the Alcyonaria of the animal corals, are synonymous with Alcyonium lobatum, palmatum, and arboreum. The primæval species Manon peziza is compounded of an animal and a plant name. Here we find only phenomena from other departments of the animal kingdom again, where, e.g., some rotifers are reckoned to the worms, others to the infusoria, a species Cercaria to the worms, other species of the same genus to the spermatozoids (?)
The small vesicles of which the red-colouring matter of snow consists (Protococcus nivalis), were regarded by Agardh, Decandolle, Hooker, Unger, Martin, Harvey, Ehrenberg, as Algae; the latter sowed them even on fresh snow and observed their propagation; the tiny young plants bore a finely-grained, lobed receptacle and rootlets, but no trace of animal character. Voigt and Meyen subsequently found, that the red-colouring matter presented rather the form and movement of infusoria, and Shuttleworth finally distinguished therein partly Algae, partly Infusoria. These contradictions are cleared up by Flotow’s careful observations on a quite allied little plant or animal (Hœmatococcus pluvialis) living in rain-water. This showed at first merely a vegetable nature, was converted however in infusions under suitable circumstances by different intermediate steps clearly traceable, into an infusory animalcule (Astasia pluvialis) with, proboscidiform, sometimes even furcate feelers and all the signs of independent movement. Shuttleworth’s Astasia nivalis showed itself related in red snow. Kützing (“On the Metamorphosis of Infusoria into the lower forms of Algae, Nordhausen 1844”) observed, that the infusorium Chlamidomonas pulvisculus changed many times, e.g., into a decided species of Algae, Stygeolconium stellare, and into other formations of an algous character, which certainly in form partially resembled sedentary forms of infusoria (Tetraspora lucbrica or gelatinosa, Palmella botryoides, species of Protococcus and Gyges). The same asserts the conversion of the infusorium Enchelys pulvisculus into a Protococcus and finally into an Oscillatoria. In a whole series of algae (Zoospermae) and yet other lower plants (Fungi, Star-jelly), the germinal granules, spores or sporidia have an infusorian-like form, and move by means of cilia or whipshaped organs, and there are even certain forms among them, which Ehrenberg recognised as infusoria. An altogether similar state of things is observed in the embryos of many polyps and Medusæ, they too go through a period, in which by means of cilia they produce a simultaneous rotatory and progressive movement, before they settle themselves for further evolution, they have also the form of infusoria and no mouth. Unger (“The Plant in the Moment of becoming Animal”) observed of the sporidia of a small Alga (Vaucheria clavata, or Ectosperma clavata), that, when liberated from the mother cell, they at first raise themselves in the water and revolve several times in-quick movement similar to an infusorium, that then moments of repose arbitrarily alternate with movement, and that they carefully avoid all obstacles in the most surprising manner, wind themselves extremely cleverly through the threads of the Vaucheria, and always avoid each other in such a way that two never collide.
The emission of mucilaginous filaments indistinctly formed and again coalescing, which is characteristic of many kinds of inferior animals, is also found in certain plants (Myxomycetes).—A small thread-like species of Algæ exhibits, as long as it actively vegetates, a threefold movement—an alternating slight curvature of the anterior filament, a half-pendulous, self-elastic, to-and-fro bending of the anterior half, and a gradual advance. “These movements have something strange, I might say uncanny, about them” (Schleiden, Grundzüge, ii. 549). The Oscillatoriæ and the Zoospores of several kinds of algæ (e.g., Vaucheria sessilis) move just like polyps to the illuminated part of the vessel; other Zoospores (e.g., of Ulothrix speciosa) flee from the same, yet others (those of the families of Stephanosaura) avoid both intense illumination and darkness, and collect at semi-dark places.—Pandorine, an Alga living in fresh-water pools, affords an example of the family of the Volvocineæ; it consists of sixteen pyramidal cells, which; with their bases turned outwards, form by their close attachment to one another an ovate collective body. Every cell has at its base a colourless spot on which rest several cilia, by means of which the organism swims about. From this mobility its animal nature was for a long time inferred, and Ehrenberg called the red pigment granule found beside each ciliated part, an eye.
We see that all the marks which have been set up on different sides as decisive do not hold, such as partial or total locomotion, spontaneous movement, morphological and chemical differences, mouth and stomach. As concerns the oral aperture, it is in the sea-blubber (Rhizostoma Cuvieri), a Medusa of the Mediterranean one to two feet in diameter, replaced by numerous openings and canals in its eight arms; further, a mouth is entirely wanting in many intestinal worms, Cercarisæ, infusoria, and embryos; the Gregarinæ, which are found in abundance as parasites in the digestive canals of insects and other animals, have not only no oral aperture, but also no cilia—no visible organs at all; they are simply cells with visible nuclei. To speak of a stomach where the mouth is wanting is without meaning, for then we may call the interior of every cell its stomach.
These statements may suffice to justify the previous general remarks.—What is now contributed by this examination to the solution of the question regarding the consciousness of plants is as follows:—We have seen that plant and animal have something distinct, somewhat in common, and that we may fairly well collect the total of the common characters, if in both kingdoms we descend so far down the scale of organisation, until we come to those structures where the differences disappear, and essentially only the common element remains. If we now find that in this common element sensation and consciousness is still included, that thus the lowest vegetable organisms possess sensation and consciousness, we shall look round for the material conditions to which here sensation and consciousness appear to be linked; and supposing these material conditions are fulfilled in higher plants in the same or still higher degree, we shall consider ourselves warranted in ascribing also to the higher plants a similar, but higher, measure of sensation and consciousness to that which we may suppose to exist in those lower ones. Since we do not directly know how the plant feels, but only how we ourselves feel, we descend the animal scale guided by analogy, turn round again at the indifference-point of animal and plant, which forms the connecting link between the two kingdoms, and likewise under the guidance of analogy ascend the scale of plants.
Further, we call to mind the result of the conclusion of the first introductory chapter and of C. Chap. iii., according to which each sensation excited by material movement, as soon as it at all arises, also arises conscious
ly, whilst if the material movement lies below the threshold of stimulation, not only no conscious, but altogether no sensation at all comes to pass. So far then as we can trace signs of a sensation excited by material stimuli, so far shall we also be obliged to regard sensation as conscious, and thus must allow the existence of a consciousness, no matter how barren its content may be.
We must here recur once more to the already frequently repelled prejudice (comp. A. Chap. vii. 1. a., vol. i. pp. 173, 174), according to which nerves are the conditio sine qua non of sensation. That on this globe, and up to this time, they are the form of matter most suited to the production of sensation is certainly not to be doubted. It by no means, however, follows from that that they are the sole form; on the contrary, a multitude of facts prove that other forms may be substituted for them. The tactile papillæ in the epidermis are found at several parts of the body at tolerably wide intervals (as the sizes of the ellipses prove, within which two contacts are felt as one). Nevertheless every spot of the skin is alike sensitive, even to thermal and chemical stimuli, where the mere propagation of mechanical pressure or conduction of heat is excluded. Burdach states that even nerveless parts of the human body may become sensitive as soon as, with increased blood-pressure and loosening of the tissue, their vitality is increased; so that, e.g., the new flesh formed in healing wounds may be highly sensitive without any nerves, and an inflammation of the nerveless cartilage and kidneys may be even much more painful than an inflammation of the nerves themselves. Wundt shows (Beiträge, pp. 392–395) that these pains are always accompanied by specific organic sensations. Here certainly the pain of which the man is conscious is localised only in the brain, but the nerve-like function of those parts is thereby proved, i.e., their capacity to propagate currents of molecular vibrations which are similar to those in nerves. Where, however, forms of vibration exist similar to those of the nerves, they will also excite sensations which are similar to those excited by nerves, supposing that they do not lie beneath the threshold of stimulation. The latter is in no case to be assumed, since the part reaching the brain after so great a resistance causes still so many violent pains. Further, we have often seen the mind act on the body without nerves; e.g., in the embryonic states before elaboration of the nerves, in the action of the nerves beyond their own limits in muscles, secreting glands, everywhere where the mass of the organs concerned must themselves undertake the last portion of the conduction, in the sudden turning grey of the hair after emotion, &c. But now, if the mind can act upon the body even without or beyond the nerves, yet in the thoroughgoing reciprocal action of the relation of body and mind, the body also, without or beyond the nerves, can act upon the mind, i.e., evoke sensation.
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