Exhalation is a function common to vegetables and to animals. By the stomata of leaves, as by the pores of the skin of animals, watery vapour and various gases, according to the vital phenomena which take place in the interior of the tissues, are constantly being disengaged.
Absorption takes place in both kingdoms. If you pour water on the lower surface of a leaf, you will see that it will be absorbed with great rapidity. Sprinkle a bouquet of flowers with water, and the freshness of the withered blossoms will revive. Absorption is even more active in vegetable than in animal tissues.
The circulation of liquids in the interior of plants is accomplished by a complicated system of channels and vessels of every order and of every calibre, absorbent vessels, exhalant vessels.
Nothing is more varied than the disposition of these channels in the interior of plants, and their multiplicity indicates a circulatory function as complicated as that of animals.
It is then evident that vegetables have the same physiological functions as animals, but as yet we know those functions very imperfectly. It is very strange that while animal physiology is so far advanced in our day, vegetable physiology is almost in its infancy. We know very well how the digestion of food takes place in man and animals, we know how our blood circulates in a double system of vessels, called arteries and veins, and we know the central organ, the heart, through which the two liquids are carried by this double system. We see and we touch the organs of sensation and motion, that is to say, the nerves. More than this, we distinguish the nerves which produce sensation from those which rule motion. We know that the centre of nervous action in man and animals is double; that its seat is equally in the brain and in the spinal marrow.
Briefly, science has shed its brightest light on all the functions belonging to animal organization, while vegetable physiology remains in obscurity. Notwithstanding the labours of naturalists within the last two centuries, we cannot explain the life of plants with certainty. We cannot positively state how the sap, which is vegetable blood, circulates in their channels. We do not even know with precision whether a tree grows from the outside to the inside, or from the inside to the outside. All the physiological functions in the vegetable kingdom are hidden from us by a thick veil, and it is only by lifting a corner of it with great difficulty that we can catch a few gleams of light through the obscurity. Nevertheless, all unexplained though they be as yet, physiological functions do exist in plants. Considering these numerous functions, it appears entirely impossible that plants should not have received the gift of sensibility. It is difficult to believe, as Linnæus would have us believe, that they possess life, and nothing more.
We shall be told that vegetables have no nerves, and that in the absence of every organ of sensation, we cannot accord them the faculty of sensibility. But, we reply, that the imperfect state of vegetable anatomy and physiology forbids us to come to any conclusion touching the existence or the absence of nerves in plants. We are convinced that these organs exist, but that botanists do not know how to discern them, or have no means of distinguishing between them and other organs.
4. The manner of multiplication and reproduction among plants and animals is so analogous, that it seems impossible, when we consider this extraordinary resemblance in the most important functions, to refuse sensibility to plants, and accord it to animals.
Let us consider the various modes of reproduction proper to vegetables. Reproduction, or rather the fecundation which precedes it, is executed in certain vegetables, by means of an apparatus of the same typical form as that of the animal kingdom. It is composed of a male organ, the stamen, which contains the impregnating dust, pollen, and of a female organ, the ovary, supported by a stalk, the pistil. The pollen impregnates the ovula contained in the grains of pollen in the ovary, as the seed of the male impregnates the ovula contained in the egg of the animal. In both cases the fruit of the impregnation develops itself afterwards with the aid of warmth and time. The vegetable egg grows and ripens, just as the animal egg grows and ripens.
We may add that the analogy between the modes of reproduction, in the two kingdoms, animal and vegetable, does not limit itself to these conditions of likeness; we may observe resemblances in the specialities of the function. Particular vitality, a turgid state of the tissues, accompanied by elevation of the local temperature, occur in the case of certain plants at the moment of impregnation, especially in the species of the family of Aroïdes. On placing a thermometer, at that time, in the great floral covering of the Arums, an excess of from 1° to 2° on the temperature of the surrounding air will be denoted, an extraordinary fact in vegetable life, for vegetables are always colder than the external air. How can we believe that the plant in which this excitement takes place has no feeling of its own condition? The plant, like the animal, has its seasons of love, can it be that it has no consciousness of them? Are we to believe that the plant which becomes warm, in which life rises at the moment of impregnation, has no more sensation than a stone? Such is not our opinion. We cannot understand life without sensibility—the one appears to us to be the indication of the other.