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.

Without reproducing our description of the Dionæa, we may assist our readers’ memory by shortly stating that the leaf of the plant is formed of two lobes joined by a midrib, and that each half of the leaf is furnished with three sensitive hairs. On a fly or other insect settling on the leaf and so irritating these hairs, the two lobes gradually close and imprison the intruder. The most remarkable property of the plant is that it not only kills insects in this way, but that it actually digests them in a manner exceedingly similar to that by which animals are nourished; for after the prey is secured, a liquid secreted in the upper part of the leaf is exuded, and this liquid is analogous with that furnished in the case of animals by the glands of the digestive mucous membrane. The closeness of the analogy will be better understood by referring to an experiment which was made with a view to testing the solvent powers of this secretion. A slice chipped from a dog’s tooth was placed between the lobes of a Dionæa leaf. After some days the lobes were separated, and the piece of tooth was found to be in such a soft fibrous condition that it was torn to shreds by the slight force employed in removing it. This energetic power of the secretion will remind the reader of what we have already said regarding the action of the gastric juice upon the animal tissues after death. Another curious point of similarity between the two fluids is observed in the fact that in both cases the secretion is stimulated by the presence of food.

It seems almost incredible to think how such a peculiarity in a plant should have, until very recent years, remained in obscurity. It is true that more than a century ago an English naturalist described it, and submitted his observations to Linnæus. But since that time the matter had aroused very little interest, until some few years ago when Darwin published his wonderful book on Insectivorous Plants. This want of attention is evidently due to the fact that Linnæus himself merely looked upon the plant as one, like the sensitive plant, having an excitable structure. He regarded the imprisoned insects as merely an accidental occurrence, stating it as his opinion that they were probably released when the leaf re-opened. The matter was thus quietly set at rest by a great authority, and no more was heard of the Dionæa until an able naturalist of North Carolina, where the plant is indigenous, again called attention to it.

Another plant belonging to this group has several peculiarities which are worthy of notice. We allude to the Sarracenia, which is found in the eastern states of North America. This plant grows in bogs and similar moist neighbourhoods. The leaf consists of a trumpet-shaped tube half covered with an arched lid. This tube exhibits a smooth and slippery surface for some distance down its interior; but lower still it is studded with bristles, its lowest depths being filled with a fluid of intoxicating properties. Round the mouth of the pitcher thus formed exude drops of a sweet viscid fluid. The Nepenthes form another branch of the family of Pitcher-plants, including many different species. Indigenous to the Asiatic Archipelago, their appearance is that of a half-shrubby climbing plant, the leaf of which terminates in a long stem, to which is attached a hanging pitcher. These pitchers vary in length from an inch to a foot, or even more; indeed some are large enough to entrap a bird or small quadruped. Their structure is not so complicated as those of the Sarracenia, although in other respects they greatly resemble them; while in both cases the digestive functions are closely allied with those of the Dionæa. But the most seductive of all these traps for unwary insects is certainly the Darlingtonia. Its victim is first of all attracted by the bright colour of its petals, and after it has settled upon the plant, and helped to fertilise it by the movement of its body against the pollen, it slips into a treacherous pitcher, to be first intoxicated, and then totally annihilated. Surely there will be no difficulty in finding an analogy here to certain social institutions belonging to the higher order of animals!

The electrical phenomena common to both plants and animals must next claim our attention. The celebrated Galvani was the first to direct attention to the existence of an electrical current in the muscle of a frog’s leg. Volta disputed this, and insisted that the current produced by Galvani was due to certain metallic connections which he employed, and not to any inherent electricity in the muscle itself. Since Galvani’s time, however, numerous investigators have followed up his researches; and it is now an accepted fact that every exertion of muscular force is accompanied by a current analogous to electricity, the strength of which is in exact proportion to the mechanical power called into play. It is a curious fact that this peculiar force remains in the muscle for a certain time after death, but it is totally lost so soon as rigidity sets in, and no earthly power can recall it. It may therefore be considered as essentially a vital phenomenon. It is moreover greater in mammals than in birds, and is least noticeable in reptiles and fishes. But we must not omit to mention that among the latter are found several which have a powerful electric battery as their chief defensive power. The Mediterranean torpedo—one of the Ray or Skate family of fishes—after which our most modern engines of war are named, is the chief of these.

Although it has long been known that currents of electricity existed in plants, such currents were attributed to chemical reaction between the external moisture and the internal juices of the plants themselves, and also to atmospheric disturbance. They have therefore hitherto borne very little analogy to the muscular electricity of animals. But very recently the subject has received great attention; in fact the electrical disturbance consequent on the excitation of the leaf of our old acquaintance the Dionæa, formed part of the subject of a paper lately read before the Royal Society. The authors of this contribution to our knowledge of a very obscure subject, proved by numerous delicate experiments that the current which accompanied the closure of the leaf in question was in every respect similar to that obtained from the muscles of animals.