The Psychology of Plants

I

THE subject of this paper is chosen with the full realization that it savors of heresy at the outset. The answer to the question, Are plants conscious? seems so self-evident, that this question would scarcely be chosen for use in an intelligence test for college freshmen. It perhaps might find a better place among the conundrums which the psychiatrist holds in store for his patients.

It was none other than Aristotle who stated clearly and conclusively: Animals have souls, plants do not. And then, as if to make it doubly sure, the renowned Linneus began his great work by writing: —

(Stones grow; plants grow and live; animals grow, live, and are sensitive.) Indeed, the very word ‘animal’ comes from the Latin anima, meaning soul, spirit, mind. And therefore, by derivation itself, plants are found to be devoid of consciousness.

Charles Bonnet, nearly two hundred years ago, arranged natural beings in a single series, varying all the way from complete soullessness to complete soulfulness. The series is as follows: rocks — corals — truffles — plants — mimosa — sea-nettles — polyps — worms — crustaceans — snails — snakes — eels — fish — flying fish — birds — bats — quadrupeds — man — angels — archangels — seraphim.

It will be observed that, in this list of twenty-one forms, the fourth from the soulless end is the plant, and the fourth from the other extreme is man. Charles Bonnet would say, then, in modern language, that the question as to whether or not the plant has a soul is a matter of relativity. As compared with rocks, plants do have souls; as compared with man, they do not. A similar situation would obtain with respect to man. As compared with plants, he has a soul; but in relation to archangels and seraphim, he has none.

Professor Titchener writes: ‘The plant mind, if there is such a thing, must be so extraordinarily rudimentary and so totally different from our own, that it is hopeless to try to form an idea of it.’ And so I hereupon apologize to my readers for inviting their attention to a hopeless proposition.

However, I have the conviction that, had Dr. Titchener devoted as much time to the study of plant psychology as he did to human psychology, using the same methods, he would have arrived at the conclusion that the mind of the plant, far from being ‘extraordinarily rudimentary,’is really extraordinarily well-developed.

In the first place, we must agree to expect nothing of plant-consciousness which we do not expect of human consciousness. The domain of human consciousness is limited; and no small part of our hesitancy about plant-consciousness, is, I believe, due to the fact that we look for feats of psychic behavior in plants which really transcend the limits of human consciousness, and, not finding such, we turn away disappointed, or, shall I say, happy in the feeling of ‘I told you so.’

We must realize that human consciousness has nothing at all to do with the fundamental processes of life. Consciousness may step in and control for a short period of time the rate at which we breathe; but it can have no effect in the long run upon the amount of oxygen which passes into our bodies, or the amount of carbon dioxide exhaled. Much less can it have any effect upon the intimate intracellular processes of respiration and assimilation. Neither does consciousness enter into the process of digestion (though perhaps the same cannot be said of indigestion). Not only are growth and development entirely free from conscious control, but, I presume, growing pains are the only instance of their ever pushing up into the conscious realm at all. The human being is not ordinarily conscious of the clotting of blood or of the healing of wounds, to say nothing of exercising any psychic control over them. The most we can do is to make conditions favorable for such processes.

It is not reasonable, therefore, for us to expect that the fundamental lifeprocesses of plants should prove to be conscious acts. If we find that synthesis, assimilation, respiration, conduction, digestion, transpiration, growth, and regeneration are not conscious processes in plants, we shall not be at all surprised. In fact, if the reverse should prove to be true, we should indeed be very much amazed.

II

There appear to be three chief aspects of the domain of conscious human behavior. First among these is protection in an emergency; that is, enabling the organism to avoid danger. This includes mechanical injury, danger from extremes of temperature, from unfavorable gas-relations, from animal enemies, and from injurious substances in our foods. And so the sense-organs of taste, smell, contact, temperature, sound, and sight set up processes in our minds, enabling us to adjust ourselves with respect to our environment.

The second phase of human consciousness consists in enabling the organism to get food. As evolution has gone on, the animal has found it necessary to devote less and less conscious effort to protection, and more and more conscious effort to getting food. Even the physician, who is devoting all of his superior intelligence to protecting the community, and the policeman, who is devoting all of his superior physique to the same end, must confess to themselves that they are only earning their daily bread.

And the third phase of human conscious effort is in procreation. While the developmental processes which that act initiates are entirely beyond intelligent control, yet the reproduction of the species is in its inception a conscious act. And with it are bound up many of the feelings and emotions which play such a large part in art and music and the movies, and which make for so much of weal and woe.

It would be the logical thing, then, in looking for the existence of consciousness in plants, to investigate the possibility of its expression in protection, food-getting, and reproduction.

But, the skeptic will say, plants cannot move anyway, and therefore they cannot protect themselves; plants make their own food, and therefore they have no need to go out and look for it, or dig for it, or fight for it; furthermore, insects carry the pollen from one flower to another, and so plants have no need of love or hatred in relation to their sexual life. But to this logic let us not yield too readily.

The most valuable method of the psychologist is introspection. In fact, it is the only method which gives conclusive results. I know that I have a mind; but I do not know positively that anyone else in the world is conscious. Someone has written: ‘Mental processes are those which form part of the experience of one person only.’ How consoling this thought must be to the psychopathic individual, to know that, however hard the psychiatrist may try, he cannot find what is in his patient’s mind. And who knows but that plants laugh in like manner at our hopeless efforts to understand them?

All study of minds other than my own, then, rests upon analogy entirely. You behave in a similar way to me, and therefore I conclude that you must have a mind. You have the same structure of sense-organs, brain, and motormechanism that I have, and therefore I conclude that your psychic life is like mine. My contention, therefore, in this paper, is that I can demonstrate just as well that the plant has a mind as that you have a mind. Both are matters of analogy of structure and behavior.

The monkey and the parrot have sense-organs and brains; their behavior is quite similar to mine; and therefore I conclude that they too have minds and are conscious. And so on down the animal kingdom: I find no place where I can draw a line and say the animals on this side are conscious and those on the other side are not. Jennings, Mast, and others have shown such wonderful feats of behavior in the lowest group of animals, the Protozoa, that I find it impossible to separate them psychologically from the higher forms.

But the lowest forms of animal life are indistinguishable from the lowest forms of plant life. Let me challenge anyone to tell whether the flagellate, Euglena, is an animal or a plant. If the Protozoa have minds, then Euglena must have a mind. And if Euglena has consciousness, then must also Chlamydomonas, which behaves almost exactly like it. And before we know it, we have climbed down out of the animal tree and are starting up the plant tree, without having touched terra firma at all. In fact, we find that the two trees have a common root-system, and that the latex of human kindness permeates the veins of all their branches alike.

As for structure, plants are composed of cells as are animals. The living part of the plant cell is like that of the animal cell, with the exception that the former in some instances contains plastids. These are the centres of food formation. It is very difficult to imagine how the possession of such structures in relatively few cells would result in a lack of consciousness.

Unlike the animal cell, however, every plant cell is enveloped by a more or less rigid cell-wall. This wall prevents the plant cell from readily changing its shape or size, and hence the development of motor tissue in plants is all but entirely prevented. The plant is thus structurally almost totally deprived of the ability to express internal experiences, either physical or psychical, by movements of its body. We must take this into account when we attempt to determine whether or not plants display mental traits. They may be just as sensitive as are animals; they may feel just as keenly, and think and associate just as well; but they rarely have the opportunity of expressing their sensations or feelings by motor-activity. Do we think the mourner who bears his grief in silence less sensitive than the one who weeps? If we should place an animal in a plasterof-paris cast and, after stimulating it, find that it is unable to react, should we conclude that it does not experience sensations or feelings?

But, despite the general inability of plants to express their psychic life by motor-responses, the plant nevertheless has sense-organs, tracts for the transmission of impulses, and some motor-organs for response.

The sense-organ of equilibrium of the plant is constructed upon the same general plan as are the semi-circular canals within the human ear; but it is much more delicate and efficient in its response. In the very tip of the root there are very small grains inside of cells. If these grains are brought into contact with any side of the cell-membrane except the normally lower side, as they would be in case the root were placed horizontal, a stimulus is initiated in these cells, which is carried up the root several millimetres to a motor-organ, and the root gradually bends down until the tip becomes exactly vertical again.

The human mechanism for equilibrium is not very delicate when compared with these plant organs. The human body must be moved several degrees from the vertical before any impression is made upon consciousness; but the plant responds to the slightest departure from the vertical. Aviators tell us that they do not know whether they are right-side-up or upside-down after they have been in the air for a time. Thus, when put to a severe test, the human mechanism for equilibrium becomes entirely inadequate for the situation. Not so with the plant. I can rotate a plant on a clinostat for hours, or even days, and it will respond to the stimulus of gravity at the close of this interval just as readily as a plant that has been standing upright.

There are sense-organs for contact in plants, especially prominent in the case of tendrils, of leaves of certain insectivorous plants, and of the stamens of the barberry. If the hairs on a leaf of Dionea are touched, a stimulus will be transmitted to a motororgan in the mid-vein. This will result in the leaf closing, as does a book.

There are sense-organs for the reception of light in leaves, and motororgans which respond. In most leaves these motor-organs are microscopic in size, and hence the ordinary observer does not see the movement of a part of the plant in response to light. However, in the sensitive plant, Mimosa, the movement is of such magnitude that it is plainly visible to everyone. This plant has its leaflets folded when in the dark. If light is thrown upon them, they open. The light stimulus is received by the sense-organs, transmitted by conductive tracts to motororgans, and we see the response, in just the same way you rise to draw the shade when the sun beams in upon the book you are reading.

The special tracts along which the impulses are transmitted are known as sieve tubes. These very much elongated cells are arranged end to end, and form a continuous line of communication from the tip of the leaf to the tip of the root. Furthermore, they extend into every vein of every leaf, forming here a wonderful network of paths of transmission of stimuli. Miss Lewis has recently discovered that in practically all ordinary leaves every cell is within one tenth of a millimetre, which is one two-hundred-and-fiftieth of an inch, of the nearest vein. In the human hand there are some cells as much as one half a millimetre from the nearest nerve. This close proximity of the sieve tubes to every part of the plant indicates the readiness with which stimuli may be caught up and carried to other centres.

That these sieve tubes do transmit impulses is shown by the fact that, if they are severed in the leaf-stalk of a sensitive plant, the leaf will not move when one strikes it; whereas any other tissue may be cut without affecting the ability of the leaf to react. Evidence still more conclusive has been published in the Proceedings of the Royal Society of London for March of this year. A glance at the data which Dr. Bose here presents shows that the tissue marked phloem, in which the sieve tubes are located, transmits impulses with from six to sixty times the speed with which they travel in other tissues of the leaf-stalk.

In the higher animals there are senseorgans and tracts for the transmission of impulses, association centres, and motor-organs. We have shown that all these are to be found in plants as well, with the exception of the association centres, which must now be considered. Are there any structures in plants which resemble the ganglia and neurones of the nerve-tract of animals? In the sclerenchyma tissue of plants we have cells with many elongated projections, which make contact with other cells at their tips. Of the exact nature of these contacts we know as little as of the nature of synapses in nerves. But it seems that there are just as good possibilities of make and break in one case as in the other.

III

We will now turn to behavior. The simplest psychic process is sensation. I can cause the leaflets of a sensitive plant to close by using several different stimuli: by placing it in the dark, by contact, by shock (jarring), by ammonia gas, by altering the temperature or the moisture relations. But, in each case, one would notice the ability of the plant to distinguish different intensities of the stimulus. If I stroke it gently, the leaflets close; if I strike it sharply, the whole leaf bends downward.

The tendril of the wild cucumber is far more sensitive to pressure than is a human being. It will respond to as small a pressure as 0.00025 of one milligram. The roots of all plants are far more sensitive to electrical stimuli than are we. The weakest current that can be felt by a human being is about 10 milliamperes. A root exposed to 0.025 of one milliampere will curve in the direction of the cathode. At 0.2 of a milliampere it will turn toward the cathode first, and then toward the anode. And at 0.5 of a milliampere it will curve at once toward the anode.

The tendrils of the passion flower will curve downward if the under surface of the tendril is stroked gently. It is, therefore, evident that the lower side is sensitive to contact, and a stimulus there received is transmitted to a motor-organ which accomplishes the curvature. Now, if we stroke the upper side, the tendril will not curve at all, and we might therefore conclude that the upper side is not sensitive. But we should be wrong in such a conclusion. The upper side is just as sensitive as the lower side, but there is no positive motor-organ which will enable the tendril to curve up. I can demonstrate that this is the case, if simultaneously I stroke both sides with equal intensity. In this case the tendril will not curve in either direction. Sensations have been received on both surfaces; they have been transmitted to motor-organs respectively, and the responses of the two motor-organs have counteracted each other, so that there is no resulting movement of the tendril. Me have then shown the existence of a negative motor-organ on the upper side of the tendril — an organ which is unable to act independently, but which is able to counteract the operation of other motor-organs. I know of no better evidence that plants do receive sensations, and that these impulses are transmitted to motor-organs; but that, because of the existence of cellulose cell-walls, the plant is unable to make a visible response, although it is able to counteract other reactions.

Now, the responses to sensations in plants may become fixed into habits, just as they may be in our kind. If a plant of Oxalis Bowei is left in the greenhouse, under normal conditions of illumination during the daytime and of darkness at night, and then is placed in a bright light at night, the leaflets will remain folded. Leaflets of these plants have been opening in the daytime and closing at night for generations; and so firmly has the habit become fixed, that this plant folds its leaflets during this night as usual, though it is in strong light. If this plant remain in a dark room, illuminated at night, and be left in the dark during the day for a period of one week, it will change its habit, so that it nowwill open at night and close during the day. It does not acquire this new habit during the first twenty-four hours, nor during the second; but gradually, within a week, it completely changes its daily periodicity. In this change of habit, the plant has shown the same reluctance and obstinacy as the man working at the factory in changing from the day to the night shift. It is to be noted further, that this plant will resume its old habit upon return to normal conditions more rapidly than it acquired the new one, displaying what our grandmothers would have called backsliding

The flowers of the common marigold are very interesting in this connection. The day and night periodicity in the opening and closing of these flowers can be reversed in the same way as for the leaves of Oxalis. But in this case the lengths of the intervals can also be varied. If I illuminate for eight hours in place of twelve, and then leave them in the dark for eight hours, the flowers will, in time, learn to open and close alternately for eight-hour periods. Or I can establish a six-six periodicity in them. But if I try it for four hours of light and four of darkness, the plant apparently becomes disgusted and reverts to the normal twelve-twelve periodicity. We thus have a display of temper, which is, I take it, a very highly developed psychological trait.

Now, many responses of plants — and this is true also of human beings — may be regarded as reflex acts, but not so these of Oxalis and Calendula. The psychologist holds that Consciousness emerges when reflex acts will not meet the needs of the situation. When we reverse the illumination, therefore, we have an instance of a reflex act which is not meeting the situation. During the first night of illumination the leaflets remain closed. The plant has encountered a new situation which it has never met before; neither have its ancestors, in all probability, ever experienced strong light at night. Whatever benefit the plant derives by opening in the light is vitiated under these new conditions. The reflex act is not equal to the task. Does it not seem reasonable then to assume that consciousness here steps in and aids the plant in meeting a problem by the formation of new habits?

Not only do plants experience sensations, and form habits and break them; they also exhibit memory. If I expose a pot of oat seedlings, the sheaths of which have not yet broken, for twelve seconds to one-sided illumination, and then cover them from the light, they will curve over in the course of about forty minutes, so that the tips will come to point in the direction from which the light had been received.

One evening last summer a rabbit wandered into my yard, and found an apple lying under one of the trees. The next evening the rabbit returned and nosed about under the same tree. He had remembered the place where he had found the apple, and had returned to the exact spot. And just so the oats remember the direction from which came the light needed in their food-making processes, and turn in that direction, even though the light has disappeared, and is no longer present to their senses.

IV

Passing now from the simple psychological processes of sensation, habits, and memory, to the more complex ones, let us first inquire whether or not plants display reason. Mathematics constitutes the most extensive and ideal system of logic. Let us see if the plant mind is mathematically inclined.

The basis of mathematics is addition and subtraction, and it can readily be shown that plants perform both these operations. If, in place of exposing these oats for twelve seconds continuously, I had exposed them for six seconds, then waited a short time and exposed them six seconds more, they would have reacted in exactly the same way as in the case noted above. Or, if I expose them for four three-second periods, or twelve one-second periods, they will add up the individual periods and, if the sum is twelve, they will react. Or they will add periods of unequal lengths, such as a three-second period, a four-second period, a twosecond period, a one-second period and another two-second period, and if that makes twelve, they will react. If it is less than twelve, they will refuse.

Furthermore, if I expose them for fifteen seconds on one side and four seconds on the other, they will subtract and, finding the answer to be eleven seconds, will not respond. But if I expose them for seventeen and onehalf seconds on one side and five and one-half on the other, they will respond, because the answer to that problem is twelve. Finally, I can change the minimum presentation time from twelve to almost any other interval by varying the intensity of the light. So that I can cause these plants to add and subtract almost any intervals of time, and they will give me the answer with a precision which seems almost uncanny.

Do plants exhibit feeling? Feeling is defined by one psychologist as a mental process which accompanies an approach to, or a departure from, a condition of equilibrium. The approach toward equilibrium is pleasurable and the departure from equilibrium is unpleasant or painful. Now the condition in which one finds a Mimosa plant in the dark may be taken as a condition of equilibrium for that plant. In this condition the leafstalk is raised and the leaflets folded. The appearance of light causes the leaflets to unfold; this must therefore be an unpleasant experience, because it involves a departure from equilibrium. If, however, I now stroke the leaflets gently, they will fold again. They must be pleased over this, because it is returning them to a condition of equilibrium again. But, if I strike the leaf sharply, the leaf-stalk drops. This is a departure from equilibrium and hence must be painful. So that, when I stroke the leaf gently, it is pleased, and when I strike it sharply, it experiences pain; just as my dog is pleased when I pat his head, and pained when I kick him.

Most people, however, will not be satisfied with habits or memory, or even with reason and feeling, as evidence of a psychic life in plants. They must be shown an instance of volition on the part of the plant before they will be convinced. Now, it must be remembered that there is still some question as to whether or not any living thing really has a will-power which it can assert; or whether, on the other hand, our actions are not simply determined in a mechanical way, while our consciousness sits by, like the purchaser of a Liberty bond, watching the war going on, with a sort of egotistical feeling that he has a good deal to do with the outcome.

I presume that the average person exercises will-power most commonly in connection with the things he wears and the things he cats. Inasmuch as plants wear no garments, they are deprived of one of these opportunities to show their volition. In regard to selecting materials which are taken into their bodies, the plant displays the same ability of choice that the animal does. The human being exhibits two successive selective processes. The first is a conscious act of selection, and we make a great deal of our ability to decide whether we wish malted milk or hot fudge sundae. And then, later, after the concoction has passed into our alimentary canal, the cells which line that tract further select materials, rejecting the indigestible and, to an extent, the undesirable and harmful.

But both these processes are far from efficient. We choose for eating many things which are not good for us; and after we eat them, the cells lining the alimentary tract do not successfully protect us by refusing to absorb injurious materials. The many internal poisons are such because they are not rejected when they get into our alimentary tract. The body of the diabetic continues to take up sugar, though that substance may cause serious results.

The plant, on the other hand, finds itself in a soil of which the constituents are highly variable in kind and in quantity. From this soil it selects, by a single operation, the substances which it uses in metabolism. The clover plant absorbs just five and two thirds times as much calcium as does the barley plant growing beside it; and the barley takes up just eighteen times as much silicon as does the clover. The result is highly more efficient than the selection of materials by the human being. We know of very few substances winch are poisonous to plants. The plant prefers to reject the injurious compounds rather than to absorb them and thus avoids suffering the consequences as does the animal.

In the reaction of roots to temperature and light, we have further evidence of volition in plants. The optimum temperature for the growth of corn roots is 34 degrees Centigrade. If they are grown at a lower temperature, say at 28 degrees, and a hot plate is brought near them, they will curve in the direction of the plate, that is, they will grow in the direction of the temperature at which they grow fastest. If they are being grown, however, in a temperature above 34 degrees, say at 40 degrees, and a hot plate is brought near them, they will curve away from it, that is, they will again grow in the direction of the temperature at which they will grow fastest. Does it not seem proper, then, to conclude that these roots show intelligence with respect to temperature and to assume that they are happiest in the temperature at which they will grow best, inasmuch as they display a choice of that temperature, as opposed to any other temperature above or below the optimum? Human beings do not always choose the food which is best, for their bodies; but the plant invariably chooses from the soil those salts which are best for it, and also chooses the temperature at which it will grow best. Does not the plant, then, display greater will-power; does it not yield less to temptation than does mankind ?

Now, a similar phenomenon can be shown with respect to light. The function of the root of the plant is to absorb water and salts, and to anchor the plant. It is, therefore, to its interest to grow away from the light; and this characteristic of roots is one of the things which causes them to grow downward into the ground. The stem of the plant, on the other hand, bears leaves, which must have light in order to manufacture the food of the plant. It is, therefore, necessary that the stem shall grow in the direction of light, in order to carry the leaves to a position of maximum illumination. If I place a tumbler of white-mustard seedlings in one-sided illumination, the stems will bend toward the light and the roots away from the light. The plant has thus exhibited will-power in controlling the direction of these organs so that they will operate to the best interests of the entire organism.

But while we marvel at the intelligence of this plant, let us not lose sight of the mechanism by which the movement is accomplished. In fact, we can in this case follow the whole chain of events on purely physical grounds. Light retards the rate of growth of all parts of all plants. The reason, therefore, that the stem turns toward the light is not that the will-power of the plant has directed it so to do, but that the side of the stem nearest the light is retarded in its growth by the rays of light themselves, and therefore the opposite side, growing faster, causes the stem to bend toward the light.

But then, why does not the root also bend toward the light? The reason is not difficult to find. The root is a slender translucent organ. The curved surface of the root on the side toward the light focuses the rays of light through the root and upon the opposite side, so that, as a matter of fact, as Blauuw has shown, the side of the root away from the light is really the most highly illuminated. This side will then grow more slowly, and the root will curve away from the light.

The question is, then, Is the plant conscious of its reactions to light? Before we had arrived at a physical explanation of this behavior, we were perhaps ready to ascribe to it an intelligence. Once we have found a physical explanation we rapidly seek to abandon the idea of a psychical experience in the plant. But is that necessary? When we find the physical basis for a certain behavior of an animal, shall we then abandon the idea of a concurrent consciousness in that animal? Perhaps we may some day find the physical basis for the fact that I prefer to go to the Pagoda for luncheon rather than to Whetstone’s. Will that prove that I have no will-power, no intelligence?

Driesch challenges the mechanist to conceive a machine which will accomplish results such as those performed by the animal. Indeed, may not the plant be such a machine? May we not hope to explain some day all its reactions on purely physical grounds; and yet is there not some evidence of a psychic life as well? My plea is this: let him who would explain nature from the vitalistic standpoint, study the plant as evidence of the vitalistic nature of physical phenomena; and let him who would explain nature on purely mechanistic grounds, study the plant as evidence of the mechanistic basis of psychic phenomena. In this way shall we all come nearer to grasping the true conception of the unity of the universe.