THE INFLUENCE OF HABIT IN PLANT-LIFE.
The old maxim regarding the power of habit is usually and rightly regarded as exhibiting a thorough application to the regulation of animal life. Not merely in human affairs is habit allowed to be ‘a second nature;’ but in lower life as well, the influence of use and wont is plainly perceptible. A dog or cat equally with a human being is under the sway of the accustomed. That which may be at first unusual, soon becomes the normal way of life. Even, as the physiologist can prove, in a very large part of ordinary human existence, we are the creatures of habit quite as much as we are the children of impulse. It is easily provable, for example, that such common acts as are involved in reading, writing, and speaking, are merely perpetuated habits. At first, these acquirements present difficulties to the youthful mind. A slow educative process is demanded, and then, by repetition and training, the lower centres of the brain acquire the power of doing the work of higher parts and centres. We fall into the habit, in other words, of writing and speaking, just as our muscles fall into the way of guiding our movements. No doubt, a large part of the difficulty is smoothed away for us by the fact that we inherit the aptitude for the performance of these common actions. But they fall, nevertheless, into the category of repeated and inherited habits; and equally with the newer or fresh ideas and tasks we set ourselves, the actions of common life may be regarded as merely illustrating the curious and useful effect of repeated and fixed habit on our organisation.
Recent researches in the field of plant-life, however, it is interesting to note, show that habit does not reign paramount in the animal world alone. The plant-world, it has been well remarked, too often presents to the ordinary observer the aspect of a sphere of dull pulseless life, wherein activity is unrepresented, and wherein the familiar actions of animal existence are unknown. Nothing is farther from the truth than such an idea. The merest tyro in botany is nowadays led to study actions in plants which are often indistinguishable from those of animal life. Instead of the plant-world being a huge living domain which never evinces a sign of sensation or activity, the botanist can point to numerous cases in which not only are the signs of sensibility as fully developed in the plant as in the animal, but in which also many other phases of animal life are exactly imitated. We thus know of plants which droop their leaves on the slightest touch, and exhibit as delicate a sensitiveness as many high animals, and a much finer degree of sensibility than most low animals. Then, again, when, with the microscope, we inspect that inner plant-life which is altogether hidden from the outer world, we see that the tissues of plants exist in a state of high activity. Currents of protoplasm are seen to run hither and thither through the plant-cells, and active movements to pervade the whole organisation of the living organism. Vital activity is the rule, and inertness the exception, in plant-life; and the discovery of this fact simply serves to impress anew upon us the danger and error of that form of argument which would assume the non-existence of higher traits of life in plants, simply because they are invisible to the unassisted sight.
The effects of habit on plant-life are nowhere better seen than in the curious differences which exist between the food and feeding of certain plants and the practices of their more familiar plant-neighbours. The food of an ordinary green plant, as is well known, consists of inorganic matters. Water, minerals in solution, ammonia, and carbonic acid gas, constitute the materials from which an ordinary plant derives its sustenance. It is curious to reflect that all the beauty of flower and foliage merely represents so much carbonic acid gas, water, and minerals, fashioned by the wondrous vital powers of the plant into living tissues. Yet such is undoubtedly the case. Between the food of animals and green plants, we perceive this great difference—namely, that whilst the animal demands water, oxygen gas, and minerals—all three being inorganic materials—it also requires ready-made living matter to supply the wants of its frame. This ready-made living matter the animal can only obtain from other animals or from plants; and as a matter of fact, animals demand and require such materials to feed upon. In one sense, the plant, then, exhibits higher powers than the animal, for it is more constructive. It can build up its frame from non-living matter entirely; whilst the animal, less constructive, requires a proportion of already living matter in its food. What has just been said of the food of plants applies to those which possess green colouring-matter associated with the plant-tissues. This green colour, so universally diffused throughout the plant kingdom, is called chlorophyll by the botanist. It exists in the cells of plants in the form of granules, and is intimately associated with the living matter or ‘protoplasm’ of the cells. The presence or absence of green colour in a plant makes all the difference in the world to its habits. The want of this chlorophyll, in fact, converts the habits of the plant into that of the animal.
If we select a plant which possesses no green colour, we may be prepared for some startling revelations respecting the mode of life of such a plant. Examples of a total want of chlorophyll are seen in the fungi, that large group of plants which harbours our mushrooms, toad-stools, and like organisms as its familiar representatives. If we inquire how the non-green fungus lives, we shall discover, firstly, that it is like an animal in respect, firstly, of the gas on which it feeds. The green plant, we saw to feed on carbonic acid gas; but the fungus, like the animal, inhales oxygen. Furthermore, a still more remarkable fact must be detailed respecting the difference between the habit of the green plants and their non-green neighbours. When an ordinary green plant takes in the carbonic acid gas which it has obtained from the atmosphere—whither it has come from the lungs of animals and elsewhere—it performs a remarkable chemical operation. The green colour enables it, in the presence of light, to decompose the carbonic acid gas (which consists of carbon and oxygen) into its elements. The carbon is retained by the plant, and goes to form the starch and other compounds manufactured by the organism. But the oxygen, which is not required, at least in any quantity, in the living operations of the green plant, is allowed to escape back to the atmosphere, where it becomes useful for animal respiration. Thus, what the animal exhales (carbonic acid), the green plant inhales; and what the green plant exhales (oxygen), the animal inhales. We have here a remarkable cycle of natural operations, which suggests how beautifully the equilibrium of nature is maintained. It may be added that the want of light converts even the green plant to somewhat animal habits. In the dark, the decomposition of carbonic acid is suspended, chlorophyll alone being insufficient for the analysis. Then, the green plant seems to inhale oxygen and to emit carbonic acid, like the animal and its non-green relative; to return, however, to its normal habit with the returning light. At the same time, the plain difference of habit in respect of the want of green colour in the fungi and other plants, is in itself a remarkable fact of plant-life.
Other differences in habit may also be noted between the plants which possess green colour and those that want it. We have already alluded to the fact that green plants feed on inorganic or lifeless matters, and that they build up these matters into their living tissues. On the other hand, the habits of the fungi and non-green plants lead them to resemble animals in that they feed upon organic materials; that is, on matter which is derived from other plants or animals. As a matter of fact, most fungi are found growing in places where decaying organic matters exist. The gardener, in growing edible fungi, supplies them with such materials in the form of manure. Again, those fungi which cause skin-diseases in man (for example, ringworm) feed on the tissues in which they are parasitic, and in so doing absorb organic matter. The plants which are not green, in this way appear to prefer organic matters, like animals. In habits, therefore, they present a striking contrast to their green neighbours.
The habit of parasitism, however, which has just been alluded to is a powerful means of inaugurating and maintaining change of life and living in plants. A parasitic being is one which lives in or upon some other living organism. There are degrees of parasitism, however: some parasites are mere ‘lodgers,’ so to speak; others both board and lodge at the expense of their host, and these latter are of course the more typical parasites of the two. But there are even degrees and differences to be seen in the behaviour of plant-lodgers and boarders. For example, mistletoe is a plant of peculiar habits, in respect that whilst its roots enter the substance of the tree-host to which it is attached, and drink up so much of the sap that host is elaborating for its own use, it also can make food-products for itself. For the green leaves of mistletoe, like the leaves of other plants, take in carbonic acid gas, and decompose it, as already described, retaining the carbon, and setting the oxygen free. On the other hand, a parasitic fungus will not elaborate any food-products for itself; and hence it is, if anything, a more complete and typical ‘boarder’ even than mistletoe. The effects of habit in plant-life are here seen in a double sense and aspect. Not only is it through the exercise of ‘habit’ that a plant becomes a parasite; but it is a variation in the parasitic and acquired habit for a parasitic plant to develop its own special ways of feeding. Habit within habit is thus seen to operate powerfully in bringing about the existent phases of the life of plants.
Plants without green colour are, however, not the only members of the vegetable world in which the habit of feeding like animals has been inaugurated. Some of the most remarkable chapters in botany have been recently written on the habits of so-called carnivorous or insectivorous plants—that is, plants which subsist on insects in other forms of animal life, and which lay traps designed to capture their unwary prey. The Common Sundew (Drosera) of our bogs and marshes catches flies and other insects by means of an ingenious arrangement of sensitive tentacles which beset its leaf, aided by the gummy secretion of the leaf itself. The Venus’ Flytrap (Dionæa) captures insects by converting its leaf into a closing trap; the alarm to close being conveyed to the sensitive parts of the plant by the insect touching one or more of the six sensitive hairs which are seen on the surface of the leaf. The Side-saddle plants (Sarracenia) of the New World and the Pitcher plants (Nepenthes) of the Old World likewise capture insects. Their leaves form receptacles, in which, as is well known, flies and other insects are literally drowned. Within the Sarracenia’s hollow leaf, a honey-secretion is found, together with a limpid fluid found at the bottom of the pitcher. There seems little doubt that flies and other insects, attracted by the honey-secretion, pass into the pitcher, and are then suffocated by the fluid found below. This much has been proved—namely, that the fluid has an intoxicating effect on insects, and that, once entrapped, the insects ultimately perish in the pitchers. It is equally notable that their retreat is cut off by the presence of pointed hairs, which, on the facilis descensus principle, and by pointing downwards, allow the insect easy admittance, but present an array of bayonet-points on its attempt to escape. In the Nepenthes or Pitcher plants of the Old World, insects are similarly captured, and are prevented from escaping by various contrivances, such as a series of incurved hairs or hooks, or allied apparatus.
At first sight, there seems a plain reason for classifying together all these insect-capturing plants, especially when it is discovered that they utilise the insects they capture for food. Botanists did not realise till recently that the capture of insects by plants was a strictly utilitarian and purposive act—namely, that its intent was to feed and nourish the plant. Once awaking to this truth, much that was formerly mysterious in the life and ways of these plants became clear. They captured the insects and fed upon them; in these words were found the clue to and explanation of a seeming anomaly in plant-life. These plants might thus be supposed simply to differ from other green plants, and to resemble the fungi in their preference for an animal dietary, in part at least. For, with their roots in the soil, and possessing green leaves, they appear to subsist partly upon the matters on which ordinary green plants live, and partly upon organic matters, like mistletoe. But a further study of these curious plants shows that the whole facts of the case are hardly to be comprised within this somewhat narrow compass. Habit within habit again appears as the principle which has wrought out important differences between the various kinds of insect-eating plants. Taking the case of the Sundew first, we discover that this plant actually digests its insect-food. From glands with which the leaf is provided, fluids are poured out which resemble the gastric juice of our own stomachs in their digestive properties. The matter of the insect-body is thus absorbed into the substance and tissues of the plant, just as the substance of our own food passes, through digestion, to become part and parcel of our own tissues. Of the Venus’ Flytrap, the same remarks hold good. This plant will digest fragments of raw beef as readily as its own insect-prey. The closed leaf is converted into a kind of temporary stomach, within which the imprisoned insect is killed, digested, and its tissues absorbed, to nourish the plant. In the Pitcher plants, a similar result happens to the insect-prey. Digestion and absorption of the nutrient parts of the prey are the duties performed by the modified leaves.
The foregoing facts would therefore seem to present a remarkable uniformity in the life of the plants just mentioned. Similarity of habits would seem to reign supreme, under variations in the method of capturing the insect-prey. Turning now to the case of the Side-saddle plants and their allies, we discover how remarkably the habits of these plants have come to differ. Investigation has shown that the flies, which are apparently drowned in the pitchers of Sarracenia in a manner exactly similar to that in which they fall victims to the artifice of the Pitcher plants, in reality are subjected to a widely different action. The Pitcher plant digests its flies, as we have seen; but in the Side-saddle plants no digestion takes place. What happens in the latter appears to consist of a simple process of decay. The insects are allowed to putrefy and decompose amid the watery fluid which drowns them; and in due time, the pitcher becomes filled with a fluid which has been compared to ‘liquid manure.’ It is this decomposing solution, then, which is duly absorbed by the Sarracenia. Rejecting this idea, there can be no other explanation given of the use of the elaborate fly-catching ‘pitchers.’ And, moreover, analogy would force us to conclude that the explanation just given is correct. If fungi feed on decomposing organic matters, why should not a Sarracenia exhibit like habits? No reasonable reply can be given save that which sees in the Sarracenia a curious difference of habit from the apparently similar Pitcher plants. The latter, in other words, eat their meal fresh; the Sarracenias, like humanity with its game, eat their meat in a ‘high’ state.