The Life of Plants

a) Plants as Living Bodies — b) Plants as Non-Sentient Living Bodies

a) Plants as Living Bodies

A living body must be an organic body which is capable of self-perfective immanent activity. And such self-perfective action means action which tends to the upbuilding or development of the body, to its preservation by means of constant repair and replacement of worn-out and discarded elements, and to the discharge of connatural function in propagating its species or kind. Now, a plant has the capacity for such self-perfective immanent activity, and, given fair opportunity, it will infallibly exercise all of its functions. It will develop and grow to maturity; it will actively tend to maintain itself in being and perfection; it will tend to be fruitful and thus to generate other plants of its own nature, kind, and species. Therefore a plant is truly a living body. It is not a mere automaton or a machine-like arrangement of parts which operate under the action of physical, chemical, and mechanical energies or forces. Such forces are manifestly at work in the living plant, but they are under a direction not their own. There is a unifying and constituent principle in the plant which uses these lifeless energies or forces as the instruments of the plant’s activity and various operations. This principle is the substantial form of the plant; it is the vegetal life-principle; it is the plant soul.

There have been in times past, and indeed there are to-day, physicists (from the atomists of ancient Greece to the Cartesians of the past three centuries and the materialists and mechanists of the present) who maintain that plants are not alive at all. But this contention stands fully confuted by the fact that plants have life-activity, and hence a life-principle, and therefore life itself. Plants are alive. The plant has its own fixed and determinate mode of action, and its action is really its own: it is immanent action, performed by, in, and for the plant itself; it is action originated by the plant, directed by the plant, and finished by the plant. Thus, for example, a plant takes food or nourishment, and shows a nice discrimination in selecting and assimilating what suits its nature. It transforms the food into its own substance, building up and maintaining the various parts of a highly complex and delicately interbalanced whole. Now, no operation of lifeless bodies or of lifeless forces (physical, chemical, mechanical) is thus self-originating and self-directive and self-perfective. Chemical affinities, physical union, gravitation, cohesion, inertia, electrical vibration or impulse, local movements,—all these and all other lifeless forces or energies are, in non-living bodies, exercised by the wholly extrinsic influence of one bodily thing upon another, even when this influence ends in the substantial union or fusion of the bodies in question. There is nothing self-directive in lifeless activities considered in themselves. There is in them no inner drive or tendency to keep on functioning for the benefit of the bodies in which they are found; there is rather the tendency, excited externally or extrinsically, to exercise their mutual function and have done with it; there is a tendency to equilibrium, and rest, and inertia. Thus lifeless forces are always transient and extrinsic in their manifested activity; they show no tendency towards development, preservation, and propagation in themselves or in the bodies which they affect. Living bodies, on the contrary, tend, not to equilibrium and rest, but to continuous, unremitting, self-perfective action; and the plant is, on this score, a truly living body. The plant, as we have seen, manifests immanent and intrinsic activity. And even when the plant employs lifeless forces (physical, chemical, mechanical) as it constantly does, it controls these, and applies them, and directs them, by a power not resident in these forces but in itself, towards its own well-being. Plants are, therefore, not to be classed with lifeless bodies. Plants are truly living bodies.

b) Plants as Non-Sentient Living Bodies

A sentient living body has the powers of sensing, appetizing, and moving locally. It has the power of sensing, that is, the power of being aware, of knowing, by means of body-structure or organ, certain bodily objects. A sentient living body has one or more senses, that is, organic powers of knowing bodily objects, and among these powers the basic and fundamental and essential one is the sense of touch or feeling. The most imperfect sentient body has at least the sense of touch. Indeed, it is by the manifestation of the sense of touch that certain very imperfect animal bodies show that they are truly sentient. It is by discerning the presence of this sense that the scientist is enabled to classify the lower animals (such as the one-celled animals) as sentient or animal bodies, and to distinguish them from plants.

A sentient body has the power of appetizing, that is, of tending towards what the sense apprehends as good or pleasing or desirable, and away from what is grasped as bad or displeasing or harmful. This appetition or appetite is the natural outcome of knowledge; appetition necessarily follows on sensation.

A sentient body has the power of moving, that is, of locomotion. Appetition would be a great hardship if the appetizing body could not move to carry out the tendency consequent upon sense-knowledge.

Now, manifestly, plants are not sentient. If they were, they would necessarily give some outward signs of it. Life is an inner capacity and force, but, in living bodies, it inevitably manifests itself in organic (and hence outer) action. In an earlier chapter we learned that immanent activity may be accompanied or evidenced by outer and transient effects (Cf. Chap. I, Art. 1, b). This is always the case with sentient activity which necessarily involves some modification or change in the organ affected by such activity. To put the matter more simply: all bodily life must be manifested in a bodily way: function follows essence. Now life-functions are ceaseless while life endures, and, in the living body, these functions are continuously exercised through and by means of the bodily organs or parts. Therefore we must say that, if plants were sentient, they would infallibly give signs of sentiency. But, as a matter of fact, plants do not give signs of possessing sentiency. In the first place, they are not equipped with the special organic system necessary for sentient functions. And, in the second place, such parts or organs as plants have, manifest no sign, or beginning of a sign, of a tendency towards sentiency. We conclude that plants are not sentient. They are living bodies; but they are not sentient living bodies.

There is a saying, axiomatic among philosophers, that “Nature does nothing in vain.” In other words, no natural thing, no natural body, will have powers that it cannot use. But if plants were sentient, their sentiency could not serve them; they could not make use of it; and, in consequence, it would be an utterly “vain” piece of natural equipment. For consider: sentiency involves appetition and the power of local movement. But plants have obviously no power of local movement; they are regularly rooted and fixed in one spot. The tree slowly sends its roots far abroad to obtain nourishment, but the tree itself does not stir abroad in quest of desirable food, nor does it move, or tend to move, to avoid the advancing axman. Conscious appetition would be a vain thing, and a great hardship, in a living body which lacks the power of local movement.

It may be objected that there are certain plants with such unusual functions that they have been called sensitive or sentient plants. There is the plant which shrinks away from a touch; and there is the plant which closes the petals of its flower upon insects. These and other “sensitive” plants are not really sentient at all. Their reaction to outer stimulus is marked, and much more evident to the casual observer than that of more common plants, but it is a vegetal and not a sentient reaction. Every plant closes upon its food in one way or another, by the action of vessels in root or leaf or flower. Every plant reacts in some measure to certain outer agencies, and the shrinking of a flower from a harsh touch is no more wonderful than the closing of the morning-glory when darkness comes, or the drooping of certain plants because of excessive dryness or excessive moisture. These activities, far from indicating sentiency, appear, at first sight, to be merely mechanical and chemical in their nature; but, as we have seen, closer investigation shows them to be radically vital.

Summary of the Article

In this short Article we have studied the manifestations of vegetal life or plant life. We have seen that, while physico-chemical and mechanical energies are used as the instruments of the plant’s functions, the plant itself applies, directs, and controls these forces to its own development, maintenance, and propagation. We have seen that the plant, in its life-activities, is self-directive and self-perfective; that, in a word, the plant gives unmistakable evidence of perfective self-movement, that is, of life. But we have found that the plant, although truly a living body, has only the lowest grade of life, and does not possess sentiency.