Evolution and Adaptation

by Thomas Hunt Morgan

As has been pointed out in the case of the seedling plant, the main stem responds positively and the roots negatively to gravity. In addition to this, the lateral position taken by the lateral roots and branches and by underground stems are also, in part, due to a geotropic response. In this case also the effect is produced by the increased growth on the upper side when the response is positive, and on the lower when it is negative. Leaves also assume a transverse position in response to the action of gravity, or at least they make a definite angle with the direction of its action.

The most striking case of geotropic response is seen in plants that climb up the stems of other plants. The twining around the support is the result of a geotropic response of the sides of the stem. The young seedling plant stands at first erect. As its end grows it begins to curve to one side in an oblique position, and this is due to an increase in growth on one side of the apex of the shoot. As a result the stem bends toward the other side. Not only does the end “sweep round in a circle like the hands of a watch,” but it rotates on its long axis as it revolves. As a result of this rotation “the part of the stem subjected to the action of the lateral geotropism is constantly changing; and the revolving movement once begun, must continue, as no position of equilibrium can be attained.” This movement will carry the end around any support, not too thick, that the stem touches.

Most climbers turn to the left, i.e. against the hands of a watch, others are dextral, and a few climb either way.[^[32]] Strasburger states that whenever any external force, or substance, is important to the vital activity of the plant or any of its organs, there will also be found to be developed a corresponding irritability to their influence. Roots in dry soil are diverted to more favorable positions by the presence of greater quantities of moisture. This may, I venture to suggest, be putting the cart before the horse. The plant may be only able to exist whose responses are suited to certain external conditions, and these determine the limits of distribution of the plant or the places in which it is found.

[32]. These cases recall the spiral growth of the shell of the snail, but the spiral in the latter is due to some other factor.

A number of plants climb in a different way, and show another sort of tropism. Those that climb by means of tendrils twist their tendrils about any support that they happen to come in contact with, and thus the plant is able to lift its weak stem, step by step, into the air. The twining of the tendrils is due to contact, which causes a cessation of growth at the points of contact. The growth of the opposite side continues, and thus the tendril bends about its support. In the grape and in ampelopsis the tendril is a modified branch. The stalk of the leaves in a few plants, as in Lophospermum, act as tendrils. Other climbers are able to ascend vertical walls owing to the presence of disks, whose secretions hold the tendril firmly against the support, as in ampelopsis.

It is interesting to find in practically all these cases that, whatever the stimulus may be, the results are reached in the same way, namely, by one part growing faster than another. The fact of importance in this connection is that the plant is so constructed that the response is often beneficial to the organism.

Before leaving this subject there is one set of responses to be referred to that is not the result of growth. Certain movements are brought about by the change in the turgidity of certain organs. The small lateral leaflets of Desmodium gyrans make circling movements in one to three minutes. No apparent benefit results from their action. The terminal leaflets of Trifolium pratense oscillate in periods of two to four hours, but do so only in the dark; in the light the leaflets assume a rigid position. There is nothing in the process to suggest that the movement is useful to the plant, and yet it appears to be as definite as are those cases in which the response is of vital importance. Had these movements been of use, their origin would, no doubt, have been explained because of their usefulness, and the conclusion would have been wrong.

The leaves of the Mimosa respond, when touched, and it cannot be supposed that this is of any great advantage to the plant. The sleep movements of many plants are also due to the effect of light. In some cases the leaflets are brought together with their upper surfaces in contact with one another; in other cases the lower surfaces are brought together. Darwin supposed that these sleep movements served to protect the leaves from a too rapid loss of heat through radiation, but it has been pointed out that tropical plants exhibit the same responses. We have here another admirable instance of the danger of concluding that because we can imagine an advantage of a certain change, that the change has, therefore, been acquired because of the advantage. In the Mimosa not only do the leaflets close together, but the whole leaf drops down if the stimulus is strong. Other plants also show in a less degree the same movements, Robinia and Oxalis for instance, and certainly in these latter the result does not appear to be of any advantage to the plants.

The preceding account of some of the tropisms in animals and plants will serve to give an idea of how certain movements are direct responses to the environment. Some of the reactions appear to be necessary for the life of the individual, others seem to be of less importance, and a few of no use at all. Yet the latter appear to be as definite and well-marked as are the useful responses. I think the conviction will impress itself on any one who examines critically the facts, that we are not warranted in applying one explanation to those responses that are of use, and another to those that are of little or of no value. Inasmuch as the Darwinian theory fails to account for the origin of organs of little or of no value, it is doubtful if it is needed to explain the origin of the useful responses. If, on the other hand, we assume that the origin of the responses has nothing to do with their value to the organism, we meet with no difficulty in those cases in which the response is of little or of no use to the organism. That great numbers of responses are of benefit to the organism that exhibits them can be accounted for on the grounds that those new species, that have appeared, that have useful responses, are more likely, in the long run, to survive, than are those that do not respond adaptively.

We may now examine some of the more complicated responses and instincts, more especially those of the higher animals. Some of these are pure tropisms, i.e. definite responses or reactions to an external exciting agent; others may be, in part, the result of individual experience, involving memory; others, combinations of the two; and still others may depend on a more complex reaction in the central nervous system of the animal. These cases can be best understood by means of a few illustrations.