[ANALOGIES OF ANIMAL AND PLANT LIFE.]
The boundary-line between the lowest forms of animal and vegetable life is of a most indefinite character. Nature would seem to have been guilty of many inconsistencies in her arrangement of these organisms; for a being which at one period of its existence exhibits the common characteristics of a plant, may at another period possess the attributes of an animal. Such an organism is found in the form of a fungus which grows on the surface of tan-pits. Under slightly altered conditions it becomes a locomotive creature capable of feeding upon solid matter. Naturalists have therefore always felt a difficulty in deciding which of these doubtful organisms should be classed with the one kingdom and which with the other. Indeed it has been seriously proposed to form a separate class for their reception, a kind of ‘no-man’s land’ to which they might by general consent be relegated.
It would at first appear that a sufficient distinction would be made if such organisms as possess the power of spontaneous movement were at once called animals. But this classification would prove to be most erroneous, for many plants possess the power of movement in a very high degree. The swarm-spores of such algæ as seaweeds, for instance, swim actively about by means of minute filaments or cilia. They were on this account long supposed to be animalcules, and it was not until they were found to ultimately develop into the plants from which they sprung, that their real place in nature was determined. These swarm-spores, common enough in the sea and in pools and ditches all the world over, are particles of matter which detach themselves from their parent cells, and after a longer or shorter time of activity, come to rest and form new algæ. They are provided with two or more vibratile cilia—minute processes which we more fully alluded to in a recent paper on ‘Bell Animalcules.’
The suggestion that animals should be distinguished by their motor powers is also fallacious, for the reason that many animals do not possess this power. Sponges, for instance, are organised bodies which remain stationary attached to rocks. But their system of pores and vents, through which a constant circulation is maintained, and by means of which they are supplied with particles of solid matter as food, most certainly entitle them to be ranked as animals.
The similarity between the lowest organisms of the two kingdoms does not seem so extraordinary after all, when by the help of the microscope we examine their structural details. In both we find a similar semi-fluid matter called protoplasm, which has been defined as ‘the physical basis of life.’ In the cellular tissues of many plants this fluid may, with a sufficiently high magnifying power, be seen in a state of ceaseless activity. It is composed of four elements, namely carbon, hydrogen, oxygen, and nitrogen. An analogous substance is found in white of egg, and protoplasm itself is one of the constituents of blood. Many of our readers will know that the colour of blood is due to innumerable red bodies called corpuscles, so minute, that myriads will be contained in one drop of the vital fluid. But there are also other corpuscles quite devoid of colour. These are minute particles of protoplasm, and like the same matter in plants, they exhibit peculiar phenomena of motion, allied to those seen in the Amœba or ‘Proteus-animalcule.’ We may therefore conclude that the vital principle in both animals and plants is the same, and that the tissues of both are built up of this protoplasm; the point of difference being that, whereas animals obtain it ready-made from plants, the latter are the manufacturers of it from mineral or inorganic sources.
There are of course, besides the mere chemical constituents of protoplasm, other conditions necessary to vitality. A certain range of temperature would seem to be the most important, if we except perhaps the presence of water, without which life can hardly exist. But even here a curious exception is presented to us in the Rotifera or wheel-animalcules—formerly alluded to in this Journal in an article on ‘Suspended Animation’—which may be kept in a state of dried dust for many years, and which, on the addition of a drop of water, will resume their original vigour and rapid movement. The so-called mummy-wheat which is said to germinate after a burial of some thousands of years, is an instance of this retention of the life-principle in plants. Light as well as heat also plays an important part in the mystery of vitality, although it is a curious but well-authenticated fact that the mere growth of plants is most rapid in darkness. We may see an instance of this in the stems of a growing plant which is placed near a window. They will all be bent towards the glass. Hence it is a common saying that they are attracted by the light. But the real reason for this bent form is, that their darker side grows more rapidly than the rest of the plant, forcing it to assume a curved form.
It is in the nature of their food that plants and animals shew the most marked points of difference. We may state as a broad rule that all living things have the power of taking in foreign matter, wherewith to supply and replenish their various parts. This process, in which the many units which make up the structure are constantly dying away and being reproduced, constitutes what we call growth. In carrying out this function, animals convert organic into inorganic matter, whilst plants do precisely the reverse. They may both be described as digesting their food—if we accept as a definition of the term digestion, that process by which insoluble food is reduced to a soluble form fitted for absorption. In the animal this process is performed by means of glands or their analogues in lower animals, which open upon the internal surface of the stomach, and which secrete an acid fluid called the gastric juice. This fluid contains pepsine—a dried preparation of which, obtained from the stomach of the pig, forms a valuable remedy in the treatment of indigestion. Its power of dissolving organic matter is so subtle, that even after death it may act upon the stomach itself, as well as upon any of the other organs with which it may come in contact. The problem as to why the stomach is during life preserved from destruction by its own secretion, was long a puzzle to physiologists; but it has been decided according to one opinion, that the alkalinity of the blood, which constantly circulates through the tissues, protects them from injury by its neutralising influence.
In plants the function of digestion is the same in principle, although the absence of a mouth and special digestive organs renders it different in detail. Plants require inorganic matters for support. Potatoes and turnips will, for instance, withdraw immense quantities of alkaline matter from the soil. Beans and peas will rob the ground in like manner of its lime, while the various kinds of grasses will choose silica for their nourishment. It is this selective property of plants which renders necessary the rotation of crops. A succession of alkaline plants would in time render the ground quite unproductive of vegetation of that kind; but if a proper rotation of crops be observed—the soil, whilst giving up one of its constituents, is gradually regaining those which it has previously lost. A consideration of these conditions of agriculture forms the very groundwork of scientific farming.
Exceptions to the rule that plants consume inorganic matter are furnished by certain fungi and also by the insectivorous plants. One of these latter, the Dionæa muscipula, or Venus’s flytrap, we fully described some months ago; but the subject is so replete with interest that we shall not hesitate to recur to it and to refer to some of the other members of the same family.