DEVELOPMENTS IN SCIENCE AND INVENTION

Is Plant Intelligence a Possibility

S. LEONARD BASTIN IN MONTHLY REVIEW December 1 1906
DEVELOPMENTS IN SCIENCE AND INVENTION

Is Plant Intelligence a Possibility

S. LEONARD BASTIN IN MONTHLY REVIEW December 1 1906

Is Plant Intelligence a Possibility

S. LEONARD BASTIN IN MONTHLY REVIEW

Within the last few year3 revoluti' nary changes have taken place in the sphere of human knowledge. Startling as have been the discoveries of the past, s-. ill more so will be those of the future. At this intermediate stage in the elucidation of nature’s problems, it were well to cultivate the habit of the open mind.

THERE are few more fascinating propositions than those which have been advanced in connection with the possibility of an intelligence in the plant. To most people the suggestion may seem to be scarcely worthy of consideration: the point having been settled long ago, to their way of thinking—so fondly do we cling to the traditions of our forefathers. Yet when one comes to approach the matter unhampered by any prejudices, it must be admitted that, far from being settled, the question of r iant intelligence, until very recently, has never been the object of any serious inquiry at all. It is now an established fact that plants can feel, in so far as the phenomenon of sensation is understood to be a response to external influence ; this being so, there is nothing unreasonable should we go still farther and seek for evidence of something approximating to a discerning power in the vegetable world.

It is always wise to keep before

one the near relations of the great living kingdoms. As is well known, the exact line of demarcation between the two worlds has not been, and probably never will be, definitely fixed ; in a sphere of life of which we should be quite unconscious were it not for our microscopes, plants and animals appear to blend imperceptibly together. Higher up the scale it is sufficiently obvious that the organisms have developed on very different lines, although one can never forget the extremely close connections at the start. To animals we freely grant a limited amount of intelligence, and it does not appear that there should be any vital objection to making a similar concession to plants, if due allowance is made for the difference of structure. It is the purpose in the present paper to gather together a few instances which seem to point to the presence of a limited intelligence in the vegetable kingdom ; each one of these is either the outcome of personal observation, or else gathered from the record of

an indisputable authority. In all cases they are selected as being examples which it is not easy to explain as direct response to any special stimuli, and cannot therefore be referred to as plant sensation.

The interesting group of plants, almost world-wide in distribution, which have developed carnivorous habits, has always attracted a good deal of attention. Each one of the many species offers an infinity of fascinating problems, but for the present purpose it will be sufficient to confine our observations to the Sun Dew group—Droseraceae. Our indigenous Sun Dews are attractive little plants, found commonly in bog districts. The leaves of all the members of the family are densely covered with clubbed hairs, and a fly settling amongst the tentacles is immediately enclosed by these organs ; meantime, a peptic fluid is exuded from the glands of the leaf. An interesting experiment may be conducted with the Sun Dew, proving that the little plant has a certain discriminating power. Place a tiny pebble amongst the tentacles ; these at once close in, it is true, but not the least attempt is made to put out the digestive liquid. How does the Sun Dew know the difference between the fly and the pebble ? Still more remarkable were some investigations conducted a few years ago by an American lady, a Mrs. Treat. She proved conclusively that the leaves of the American Sun Dew were actually conscious of the proximity of flies even when there was no direct contact. Pinning a living insect at a distance of half an inch from a healthy leaf, we are told that in about a couple of hours the organ had moved sufficiently near to enable it to secure the prey by means •f its tentacles. A member of the

same natural order as the Sun Dews —the Venus Fly Trap (Dionaea musciplua)—is quite one of the strangest plants in the world. The species, a native of Southern Carolina, is sometimes grown in glass-houses in this country, and the general form of its leaves must be fairly familiar. Designed in two bristle-fringed lobes, both hinged together, the leaf, when fully expanded bears a striking resemblance to a set spring trap. On the upper surface of each side of the leaf are arranged three sensitive hairs, and should any object touch one of these, no matter how lightly, the lobes snap up together, the bristles interlock, and the catch, should there be any, is a prisoner beyond any hope of escape. It is not surprising to find that such a highly specialised plant will give us an incontrovertible instance in support of the theory of plant intelligence. The leaf of the Dionnaea will enclose anything which irritates its sensitive hairs, and to induce the plant to accept a small piece of cinder, for instance, is a simple matter. But it does not take very long for the plant to find out—how, it is not easy to suggest—that its capture is inedible, and, acting upon this impression, it slowly opens its leaf and allows the substance to roll away. Now try the same leaf with a fly, or even a morsel of raw beef; so tightly clenched are the two lobes that nothing short of actual force will separate them until after the interval of several days, when the plant has drained the fragment of the desired nitrogenous elements. Unless one admits the presence of some kind of discerning power on the part of the Dionaea, it is not easy to explain its behaviour.

The whole subject of the relation between plants and insects is one which is full of mysteries : it is not

always easy to see just how these relations have been established, even though one admits that they must have been developed side by side. In hundreds of cases plants have specially adapted their floral organs for the reception of one kind of insect, often so arranging the processes that others are excluded.

Even more remarkable are those instances in which a definite compact seems to have been arrived at between the plant and the insect ; the former tolerating, and at times even making some provision for the latter. The case of a species of fern is a typical one. This plant provides little holes down the sides of its rhizomes for the accomodation of small colonies of ants ; the exact services which these insects render to their host is not very clear. The following instance of a Central American acacia is quite romantic in its way, but it is vouched for by good authorities. This tree (A., spherocephala) grows in districts where leafcutter ants abound, and where the ravages of these insects are so dreadful that whole areas of country are at times denuded of foliage in a fewr hours. The acacia has, however, hit upon a unique way of protecting itself against the assaults of these enemies. At the end of some of its leaves it produces “small yellowish sausage-shaped masses, known as 'food bodies.’ ” Nov/ these seem to be prepared especially for the benefit of certain black ants which eat the material greedily, and on this account it is no matter for surprise that these insects (which are very warlike in habit) should make their homes in the acacia, boring out holes in the thorns of the tree to live in. It is not very difficult to see how this arrangement works out. At the approach of an army of leaf-cutting

ants, the hordes of black ants emerge, fired with the enthusiam which the defence of a home is bound to inspire, with the result that the attacking enemy is repulsed, and the tree escapes unscathed. Explain it how one will, it is impossible to deny that it is very clever of the acaia to hire soldiers to fight its battles in the manner decribed above.

When plants find themselves in extraordinary positions they often do things which seem to be something more than just cases of cause and effect. There really appears to be such a thing as vegetable foresight, and by way of illustration reference may be made to the manner in which plants in dry situations strive to come to maturity as soon as possible. Specimens growing on walls are most instructive in this connection. It is almost always noticeable that plants in such positions run into flower and produce seed much in advance of their fellows living under more normal conditions ; by so doing they have made certain the reproduction of their kind long before the hot summer has arrived, at which time any active growth on a wrall becomes an impossibility. It is willingly conceded that shortage of water discourages a luxuriance of growth, and tends to induce an early maturity, but to any one who has watched the habits of plants under these circumstances there seems to be something more than this. Some thing which enables the plants to grasp the fact that their life can only be a very short one, and that it is their duty at the earliest possible time to flower and produce seed ere they perish.

Generally speaking plants are most desirous to obtain as perfect an illumination as is possible of their foliage. Of course, light is so neces-

sary to bring about the formation of perfect green tissue, that it is not surprising to find that it is a sufficient stimulus to cause vegetables to move their organs to the direction from which the illumination is coming. But there are parts of the world in which plants find that the direct rays of the sun, where this orb is nearly vertical as in Australia, are more than they can stand. The Blue Gum trees, for instance, find that the solar heat is too great for their leaves, and accordingly adopt an ingenious way out of the difficulty. As young plants growing under shelter, the eucalypti develop their leaves in lateral fashion, fully exposing their upper surface skywards. Later on, however,, as the plants grow into trees and rise above any screening shade, the Blue Gums turn their leaves edge-way fashion, so that no broad expanse is exposed to the scorching sun. Some plants direct certain organs away from the light, as in seen in the case of the vine where the tendrils always seek dark corners. The value of this tendency is very apparent, for it must be seen at once these organs, whose sole object it to obtain a hold somewhere, would be much more likely to do so

in some cranny, than if they took their chance by growing out into the open. This habit is exceedingly interesting when we remember that the tendrils are modified shoots, parts of the plant which certainly do not shun the light. Indeed, these tendrils seem to be working against their inherent tendency.

The instances which have been detailed above might be multiplied indefinately. They have only been selected out of an immense mass of evidence which is at the disposal of any student who will take the trouble to watch the members, of the great vegetable kingdom. To say that plants think, as has been suggested by an enthusiast, is probably carrying the matter too far ; the word used in its accepted sense scarcely conveys a right impression of the mysterious power. Rather one would refer to the phenomenon as a kind of consciousness of being, which gives to each plant an individuality of its own. It is likely, and indeed highly probable, that it is impossible for the human mind to grasp just how much a plant does not know, but in the face of proved fact the existence of some kind of discriminating power in the vegetable kingdom will scarcely be denied.

He who respects his work so highly and does it so reverently, that he cares little what the world thinks of it, is the man about whom the world comes at last to think a great deal.