Intelligence in Plants and Animals / Being a New Edition of the Author's Privately Issued "Soul and Immortality."

by Thomas G. Gentry

But radicles of seedlings, as well as those of more vigorous plants, would pass over stones, roots and other obstacles, which they must necessarily encounter in the soil. This they are abundantly able to do, for they are exceedingly sensitive just above their apices, and bend like a tendril towards the touching object. When, however, one side of the apex is pressed by any object, the growing part bends away from that object, and this seems a beautiful adaptation for avoiding obstacles in the soil, and for following the lines of least resistance.

SEEDLING OF WINTER GRAPE.
Earth Cut Away to Show Directions Taken by Tip of Radicle in Avoiding a Stone.

So feeble is the circumnutating movement of the terminal growing part, both of the primary and secondary radicles, that it can assist them but little in penetrating the ground, excepting when the superficial layer is very soft and moist. But it must aid them materially when they chance to break obliquely into cracks, or into burrows that have been made by earth-worms or larvæ. Moreover, combined as it is with the sensitiveness of the tip of the radicle to contact, it can hardly fail to be of the highest importance, for as the tip is always endeavoring to bend to all sides, it will press on all sides, and will thus be able to discriminate between the harder and softer adjoining surfaces. Consequently, it will tend to bend from the harder soil, and will thus take the directions of the least resistance. So it will act if it meet with a stone or the root of another plant in the soil, as must incessantly occur. If the tip were not sensitive, and did not excite the upper part of the radicle to bend away, whenever obstacles were encountered at right angles to its growing direction, it would undoubtedly be liable to be doubled up into a contorted mass. But with radicles growing down inclined plates of glass, as shown by experiment, it has been observed that as soon as the tip merely touched a slip of wood cemented across the plate, the entire terminal growing point curved away, so that the tip soon stood at right angles to its former direction; and thus, as far as the pressure of the surrounding soil would permit, would it be with an obstacle encountered in the ground. Thick and strong radicles, like those of the horse-chestnut, are endowed with less sensitiveness than more delicate ones, and would therefore be the better able by the force of their growth to overcome any slight impediment to their progress. Further, as radicles perceive an excess of moisture in the air on one side and bend towards this side, it is reasonable to infer that they will act in a similar manner with respect to moisture in the earth, for the sensitiveness of moisture resides in the tip, which determines the bending of the upper part. May not this capacity partly account for the extent to which drain-pipes often become choked with roots? The direction which the apex takes at each successive period of the growth of a root, ultimately determines its whole course. It is therefore very important that the apex should follow from the first the most advantageous direction. We can thus understand why sensitiveness to geotropism, contact and moisture should all reside in the tip, and why it should determine the upper growing part to bend either from or to the exciting cause. Darwin has compared a radicle with a burrowing animal, such as a mole, which wishes to penetrate vertically into the ground. By a process of circumnutation, or the movement of his head from side to side, he is enabled to feel any stone or other obstacle, as well as any difference in hardness of soil that may exist, and will therefore turn from that side; but if damper on one side than on the other, will turn thither as a more suitable hunting-ground. Nevertheless, after each interruption, he, guided by the sense of gravity, will be able to recover his downward direction and to reach to a greater depth.

Destruction of the tip of a radicle does not prevent the adjoining part from bending, if this part has already received some influence from the tip. As with a horizontally extended radicle, whose tip has been cut off or destroyed, the part which should bend most remains motionless for many days or hours, even though exposed at right angles to the full influence of gravity, we cannot do otherwise than conclude that the tip alone is sensitive to this power, and transmits some stimulus to the neighboring parts, thereby causing them to bend. Direct evidence of such transmission has been obtained. When a radicle was left extended horizontally for an hour or an hour and a half, by which time the supposed influence will have travelled some distance from the tip, and the tip was then cut off, the radicle subsequently became bent, although it was placed in a perpendicular position. Terminal portions of several radicles thus treated continued for some time to grow in the direction of their newly-acquired curvature, for being destitute of tips they were no longer acted upon by the power of gravity. New vegetative points, however, appeared, and being acted on by this influence coursed themselves perpendicularly downward as was their custom.

Investigation having shown that it is the tip of the radicle that is sensitive to geotropism in the members of such distinct families as the Leguminosæ, Malvaceæ, Cucurbitaceæ and Gramineæ, which may be represented by the Clover, Mallow, Gourd and Rye, we may justly infer that this character is common to the roots of most seedling-plants. Whilst a root is penetrating the ground, the tip must take the incipient step, as it has to determine the direction of the entire root. When, however, it is deflected by any subterranean obstacle, it is essential that a considerable length of the root should be able to bend, particularly as the tip itself grows slowly and bends but little, so that the proper downward course should be recovered. Immaterial as it would seem whether the entire growing part should be so sensitive to geotropism as to effect this movement, or that it should be brought about by an influence transmitted exclusively from the tip, we should, however, remember that it is the tip that is sensitive to the contact of hard objects, causing the radicle to bend away from them, thus directing it along certain lines in the soil where the least opposition interposes. It is again the tip that is alone sensitive, at least in some instances, to moisture, causing the radicle to bend towards its source. These last two kinds of sensitiveness conquer for a time the sensitiveness to geotropism, which, however, ultimately prevails. But the three kinds most often come into antagonism, first one prevailing, and then the other. It would, therefore, be an advantage, perhaps a necessity, for the interweighing and reconciling of these different kinds of sensitiveness, that they should all be localized in the same group of cells which have to transmit the command to the adjoining parts of the radicle, necessitating it to bend to or from the source of the irritation.

Though generally believed by authors that the modification of the upper or lower surfaces of a radicle, whereby curvature is induced in the proper direction, is the direct result of gravitation, yet there can be no question from all that has been said that it is the tip alone that is acted on and that transmits some influence to the adjoining parts, causing them to curve in a downward manner. Gravity, it would seem, does not act in a more direct way on a radicle than it does on any lowly-organized animal, which moves away when it feels some weight or pressure.

When we consider what we have written, it is impossible not to be impressed with the resemblance between the movements of plants and many of the actions performed by the lower animals. With plants an astonishingly small stimulus suffices. One plant may be highly sensitive to the slightest continued pressure, while a closely-allied form just as highly sensitive to a slight momentary touch. The habit of moving at certain periods is inherited both by plants and animals; and other points of similitude have been specified. But the most striking resemblance is the localization of their sensitiveness, and the transmission of a stimulus from the exciting point to another, which consequently moves. Yet plants do not, of course, possess nerves or a central nervous system. May we not therefore infer, and wisely so, too, that with animals such structures but serve for the more perfect transmission of impressions, and for the more complete intercommunication of their several parts?

No structure in plants seems more wonderful, as far as its functions are concerned, than the tip of the radicle. Lightly pressed or burnt or cut, it transmits an influence to the upper adjoining part, causing it to bend away from the affected side. But more surprising, however, is the fact that the tip can distinguish between a slightly harder and softer object, by which it is simultaneously pressed on opposite sides. Let the radicle be pressed by a similar object a little above the tip, and it will be noticed that the pressed part does not transmit any influence to the more distant parts, but bends abruptly towards the object. Perceiving the air to be moister on one side than the other, it likewise sends out an influence to the upper adjoining part, which deflects towards the source of the moisture. When excited by light, the neighboring part bends from the light; but when excited by gravitation, the same part bends towards the centre of gravity. In almost every instance the ultimate purpose or advantage of the several movements can be clearly perceived. Two, or perhaps more, of the exciting causes often act simultaneously on the tip, and one conquers the other, doubtless in accordance with its importance for the life of the plant. The course pursued by the radicle in penetrating the ground being determined by the tip, has acquired for it the diverse kinds of sensitiveness which it possesses; and it is hardly an exaggeration to assert that the tip of the radicle thus endowed, and having the power to direct the movements of the adjoining parts, acts like the brain of one of the lower animals, which organ, seated within the anterior end of the body, receives impressions from the sense-organs, and directs their several movements.

In animals possessed of a nervous system, contractions only follow stimuli, which are carried to the contractile elements by nervous threads, the internal energy representing the external stimulus being called nervous energy or neurism. But where a nervous system does not exist, as is the case in some low animals and in all plants, external stimuli must be justly supposed to be converted into the same form of energy, which in such organisms has a general circulation throughout the contractile protoplasm. The attainment of some position, favorable for the procurement of relief from some unpleasant sensation, or the acquisition of some agreeable one, or for both, is the important thing directly subserved by such movements in the generality of animals. While we have the best of reasons for believing this to be true in the vast majority of animals, because fundamentally their structure is similar to our own, yet the inference that the same is true of the lowest forms of life is justifiable until it is proved to be mistaken.