The Life of the Plant

The mimosa and the catchfly are instances of plants capable of manifesting movements in response to the slightest external stimulus ; but here is also a plant in which movements take place without any external stimulus whatever — Desmodium or Hedysarum — coming from the West Indies and belonging to the so-called Papilionaceae and therefore akin to our bean, clover, and other plants. Imagine that, of the three leaflets which form the compound leaf of the clover, the uppermost were to become greatly elongated and the two side ones were but slightly devel- oped so that they were much smaller than the third. Such is the leaf of

Desmodium, shown in fig. 79. If now we take our stand on a bright hot day before this plant, which is often grown in our hot-houses, we shall notice in less than a minute a slight shudder passing here and there over its leaves. Let us concentrate our attention on some par- ticular leaf which arrests us by such unexpected move- ment, and we shall soon perceive one of the most striking phenomena in the vegetable world. At first the two leaflets are in a horizontal position. Suddenly one of them changes this position by an abrupt movement, by a bound, so to speak, and it is found at a considerable angle with the horizon. Another and yet another such abrupt movement, and it is raised vertically. Mean- while the opposite leaflet is lowered by a similar series of abrupt movements, by internal shocks. Then the leaflets change their parts. The raised one falls and the lowered one rises. This movement seems to be regular, produced by inner pulsation, provided the plant has sufficient light and heat. As the temperature falls, the intervals between every two movements will become longer, and eventually the movement will be no more jerky, but slow and continual, and only to be noticed when attention is drawn to the relative position of the leaflets. Finally, if the temperature fall, say, to 68° F., the movements will cease altogether, the plant will become chilled. But warmed again it will recommence to wave its small leaflets.

The study of all these phenomena of motion leads of necessity to the question as to the purpose they serve for the plant. Apparently their significance varies according to the case. The movements of spores and antherozoids, of the stamens of barberry, etc., are obvious : they are useful, if not necessary, for purposes of fertilisation and the reproduction of plants. The sleep movements of flowers, the folding of their outward wrappings during the night, probably saves them from cooling, which is so dangerous to them. The sleep movements of leaves have probably the same effect : by folding or turning their edges upwards in the condition of sleep the leaves present a comparatively small surface from which heat can radiate ; consequently they 'escape

too great cooling, and are less accessible to morning frosts, during which plants are frequently frozen (actually because of irradiation) although the thermometer may not have fallen below 32 0 F. The purpose of the movements of the catchfly is obvious : the very name of the plant expresses it ; for, as we shall see later on, together w r ith certain other plants, the catchfly in reality feeds upon captured insects. The use of motion in the irritable leaves of mimosa is less obvious. Nobody has apparently even tried to explain its purpose. Only more or less probable suggestions can be brought forward in this connexion. Any one who has observed the effects of heavy showers and hailstorms will certainly have noticed how it chips the foliage of our trees. Such delicate organs as the leaves of mimosa would suffer still more from tropical storms, if the first drops of rain did not cause them to gather their outstretched leaflets together and fold them against the stem. These leaves, therefore, behave like the rush in the fable : they weather storms which shatter oak-trees. I repeat, this is only a conjecture, the accuracy of which can be verified only by observation on the spot where these curious plants grow. It is much more difficult to explain the object of the continuous movements of the leaflets of Desmodium, unless we admit that these movements serve the plant to scare away pernicious insects, attracted by its sweet juicy foliage. 1 If this be true, plants would seem to use their capacity for movement for two different ends : for getting rid of enemies on the one hand and for catching and feeding upon them on the other.

1 The same explanation may hold with regard to mimosa; at all events I have observed cases when mimosas in our hot-houses perished from certain white lice, which found their abode at the very articulations of the leaf. This is possible only in cases where the leaf has lost its irritability. The tissue of the articulations must specially attract insects, on account of the abundant sugary substances it contains. Let us leave these conjectures for the present 1 and study another question : Are we entitled to consider the movements of plants described above as similar to those in animals, or can some essential difference be established between these two categories of phenomena ? So far as the movement of protoplasm is concerned, it does not present any difference whatever in the two kingdoms. The same may be said of the movements of the zoospores and antherozoids : here also no difference can be established between the movements of vegetable and animal organisms, which is proved by the fact that the earliest investigators refused to believe their own eyes and mistook for animals the motile organs of plants.

The case is rather different when we compare the movements of the higher plants with those of animals. At all events we do not find in the plant any special tissue adapted for movement, any muscular fibre capable of contraction. Yet we can hardly base upon this difference in structure a fundamental difference be- tween the phenomena. A comparison of the conditions which determine and accompany the movements of the higher plants and animals points rather to similarity than difference between them. We know, for instance, that motion in animals is closely bound up with respira- tion : the contracting muscle absorbs more oxygen, gives off more carbonic acid than a resting muscle, and it is this oxidising process that serves as the main source of the energy used in muscular activity. Now, does the plant present phenomena similar to the respiration of animals ? We have already met with cases which prove this in the affirmative. During the germination of seeds, the development of buds, especially during the flowering period, these vegetable organs greedily absorb oxygen and give off carbonic acid,

1 All these explanations brought forward as conj ectures (in 1876) have been proved by subsequent investigators and adopted by almost all botanists. while their temperature noticeably rises. This pheno- menon is manifested by all parts of the plant during its whole lifetime, but with the difference that the green parts also decompose carbonic acid and give off oxygen much more energetically under the action of light, so that this process conceals the respiration taking place simultaneously with it. 1 But is respira- tion connected with motion ? Experiments answer in the affirmative, although the nature of this connexion is not as yet clear to us. If we arrest the supply of oxygen to the plant, all the phenomena of movement will cease accordingly ; the protoplasm will stop flow- ing, the stamens of the barberry, the leaves of the mimosa, will lose their irritability, and only after being placed for a considerable time in an atmosphere con- taining oxygen will these phenomena recur. Conse- quently the movements of plants as well as the move- ments of animals are closely connected with respiration.

Let us proceed with our comparison. Muscles become heated by contraction ; their temperature rises to a measurable though insignificant degree : the same is to be observed in plants. By applying to the pulvinus of the leaf-stalk of mimosa a very sensitive thermometer called a thermopile, it has been found possible to show that the temperature rises at the moment of motion. In muscles at'rest, as also in a state of tension, the presence of an electric current is noticed. If a circuit connected with a sensitive galvanometer is closed (with all due precautions) by means of a muscle of a frog, the needle of the galvanometer will swing and manifest the current in the circuit. A similar though weaker current will become apparent, if instead of a muscle, a leaf of the catchfly is introduced into the circuit. The similarity does not stop there. If the muscle is made to contract, a slight decrease in the strength of the current is ob- served at the moment of contraction, the needle of the

galvanometer goes back: this is the negative variation of the current. The same phenomenon takes place in the case of the catchfly. At the moment when the two parts of the leaf fold together a negative variation in the current becomes apparent. Again a very small but always measurable interval of time passes between the moment of irritation and the contraction of a muscle : this is known as the latent period of excitation. A similar though more considerable in- terval is noticed in the case of the leaf between the moments of irritation and movement.

Thus the similarity between the movements of the catchfly and of animals does not lie solely in the out- ward manifestations of these movements but also in the internal processes which accompany them. As we have seen, it is true that the movements of vegetable organs resolve themselves in the most obvious cases into the exudation of water from the cells of the irritable tissue overfilled with it, while the movements of animals result from the contraction of muscles, a change in their actual form ; but the contraction of muscles is not an elementary phenomenon. Will it not likewise reduce itself ultimately into the mutual rearrangement of the elementary solid particles and liquids which enter into the composition of the muscle ? 1

Thus the main wall of partition breaks down which used to differentiate the vegetable kingdom from the animal. Movement is not exclusively confined to animals ; it also takes place among plants. But if this distinction does not hold good, can we not find something else in its place ? Let us consecutively review the characteristics of the life of organisms, 1 It is possible that the overfilling of cells with water, its sudden exudation, the presence of a current and its variation — all these details of the process of motion in vegetable organs will find an explanation in the phenomena of electro-diffusion. It would be out of place to enter Into greater details about them at present,

which were supposed to be peculiar to the represen- tatives of this or that kingdom. Let us commence with the mode of nutrition. Plants are generally said to feed upon simple inorganic matter : carbonic acid, water, salts ; while animals feed upon complicated organic compounds. Generally speaking this is true, but the rule has nevertheless many excep- tions. For instance, the large class of fungi comprises plants which feed exclusively upon complicated organic compounds, hence these organisms can exist only on soils rich in humus — decomposing organic matter — or else as parasites, feeding upon other organisms. But fungi are not alone in feeding upon ready formed organic matter ; there are parasites also among the highest representatives of the vegetable kingdom. Some of these are lacking in green colouring matter, such as the Dodder ( Cuscuta ), for instance, which clings to our field plants, like the hop, and lives entirely at the expense of its victim. Others, such as the mistletoe, which attacks oaks, fruit trees and other trees, although capable of independently pro- viding themselves with food, probably also use in great measure substances formed by the plant on which it lives.

Moreover, modern researches, especially those of Darwin, have acquainted us with a whole series of plants which, though provided with green organs, feed upon animal food and, what is more curious, digest this food in the same way as animals. These are known under the name of insectivorous plants. Here are some examples. The above mentioned catchfly is the most curious of them all. If a leaf of the catch- fly once gets hold of an insect, it does not reopen until it has sucked out of it everything it can, leaving behind a bare insoluble skeleton. A similar experiment can be made by taking instead of the insect a piece of raw or cooked meat, or the hard-boiled white of an egg.

The leaf will immediately fold up and only reopen when all trace of the food supplied has gone. The catch- fly, as we have already said, lives in the marshes of North America, but some of our own marshes provide us also with a plant, which, though related to the catch- fly, yet attains the same end, i.e. feeds upon insects, by means of a somewhat different adaptation. This plant is the Sundew ( Drosera ). Its small leaves are covered with a certain kind of hairs, the ends of which secrete drops of a viscid liquid. This liquid used to be taken for dew, hence the name of the plant. The insect which imprudently settles upon the leaf sticks to it. The leaf then quickly manifests an extremely curious kind of movement. The hairs from all sides tend towards one and the same point, where the imprisoned animal lies ; the glands at the tips of the hairs secrete their sap profusely, and it dissolves the solid particles of the nutrient substances, transforming them into a condition in which they become easily absorbed by the cells of the hairs. When the food is completely absorbed the hairs expand again and are ready to meet another visitor in the same way. The large peculiar leaves of Nepenthes, Sarracenia, and Cephalotus found at lower latitudes are not less curious, as well as the minute leaflets of the bladderwort ( Ultricularia ) in our streams and ponds. One part of the leaf of the three former plants develops into a pitcher, which in the case of Cephalotus is, moreover, covered with a lid, while in the bladderwort the finely sub- divided submerged leaves are provided with similar little organs. The pitchers were long ago observed to contain a liquid. Formerly this liquid was thought to be water, and only recently it has been proved to possess the property of dissolving solid organic food substances.

Closer investigation of these pitchers has revealed in them very complicated adaptations for catching insects. They contain parts which secrete sweet sap and therefore serve to attract insects, and also very smooth surfaces over which the insects cannot help sliding down into the trap, and lastly, hard hairs with tips turned inwards to prevent the victim from coming out of its trap. But what is this liquid secreted by the plant, what is this process by which the solid food is dissolved, and has it anything in common with what we generally call the digestion of food in an animal organism ? As we have already seen , 1 careful investi- gations have proved that there is a remarkable similarity between these two processes.

As in the gastric juice of animals, in which the dis- solution of albuminoids takes place under the influence of a special ferment, pepsin, so also in the liquids secreted by all insectivorous plants the presence of a ferment has been discovered. Just as there pepsin acts only in presence of a small quantity of free acid, so also here, at the moment when the plant is irritated, an acid reaction of the secretion can be easily demonstrated . 2 Thus the process of nutrition cannot afford us any sure criteria for differentiating plants from animals : fungi, parasites, and especially insectivorous plants, present a complete analogy with the nutrition of animals. In fact, if, taking into consideration all that has been said about these plants, we had described an imaginary organism, which by means of antennae caught insects and conveyed them into a cavity of its body covered with glands ; if, moreover, we had said that these glands secreted a juice that dissolved

2 Recently doubts have arisen as to the utility for the plant of this absorption of organic matter. While paying a visit to Darwin I had an opportunity of seeing some unpublished experiments of the great scientist, which prove the significance of the process above described from the point of view of nutrition. Several sundew plants were grown by him under a glass bell, so as to save them from insects. Half of them received meat, half did not. At the time I saw them (in July) the plants which received meat were far bigger and more healthy in appearance than the others (Note to the first edition.)

albuminoids and then absorbed them as food, every one would certainly have believed that we meant an animal of some kind ; yet all these features would have been those of very distinct and typical plants. If the nutrition of plants can thus so closely resemble the nutrition of animals, perhaps the nutrition of animals, on the other hand, will never present us with a parallel to the pheno- menon, which is characteristic of plants, of nutrition at the expense of inorganic substances. But neither can this be maintained, the property of decomposing carbonic acid being, as we have seen, peculiar to a special organ, namely the chloroplast, and we can name several animal organisms containing chlorophyll . 1

Now let us proceed to another supposed distinction based upon the process of respiration. When the interchange of gases taking place in plants, and resulting in the decomposition of carbonic acid and the accumula- tion of carbon, was erroneously compared to respiration, the following antithesis used to be brought forward ; the respiration of animals consists in the absorption of oxygen and the giving off of carbonic acid ; the respira- tion of plants — in the absorption of carbonic acid and the giving off of oxygen. We now know, however, that the decomposition of carbonic acid cannot be compared to respiration, that this is a case of nutrition — a peculiar kind of nutrition by means of air ; we also know that another process takes place simultaneously — real respira- tion, but that this latter process can be observed only when we investigate either colourless organs, or green organs in the absence of light when the opposite process of decomposition does not take place. This process of respiration will certainly appear sluggish if compared with the respiration of a mammal, or a bird. Carbonic add is given off in great quantities by the latter, and the resulting rise of temperature above the temperature of

1 This chlorophyll belongs, however, probably to algae which have found their way into the animal organisms in question. the surrounding medium is very apparent ; whereas in the majority of cases plants passively acquire the temperature of the medium. But if we compare the respiration of plants with that of the so-called cold- blooded animals, e.g. with frogs, or even with the respiration of mammals in a state of torpor (for in- stance during their winter sleep), we shall see that in both cases the respiration does not differ so sharply from the respiration of plants, either in the quantity of gaseous interchange, or in the difference in temperature of the organism from the surrounding temperature.

A question arises here ; Is respiration one of the processes absolutely necessary to the organism of the plant ? We have seen that motion of any kind ceases in the absence of oxygen. For a long time the growth of cells was also believed to be impossible without oxygen, but it now appears that another chemical process can be substituted for respiration, a process similar to it in its results. This process is called fermentation, and consists in the decomposition of sugar— glucose — into alcohol and carbonic acid. This process underlies the manufacture of alcohols, i.e. the manufacture of wines, the distilling of spirits, the brewing of beer, etc. In all these cases fermentation takes place owing to the development in the fermenting liquids of a peculiar microscopic organism, a fermenting fungus known as yeast. The yeast cells grow and multiply without oxygen. The process of fermentation differs essentially from respiration in the fact that it is not accompanied by the absorption of oxygen, but is similar to it in that in both cases carbonic acid and heat are given off . 1

1 During respiration carbonic acid is formed at the expense of the oxygen of the air, while during fermentation it is formed at the expense of oxygen contained in sugar itself. Something like the combustion of tinder or gun-powder takes place here. We know that both these substances can burn without air at the expense of the oxygen in the saltpetre which enters into their composition. It is probably this very heat that furnishes the plant with the energy necessary for its development. Fer- mentation appears to be a sort of succedaneum of respiration. But this process is not as useful to the plant as respiration, because for the same expenditure of sugar much less energy in the form of heat is generated. Formerly the process of fermentation was considered essentially peculiar to the yeast fungus, but now it appears that any plant or vegetable organ placed in an atmosphere devoid of oxygen begins to give off car- bonic acid, without absorbing oxygen, and forms alcohol, i.e. begins to decompose its stores of sugars and to ferment. The yeast fungus, which ferments the sugars in the liquid where it develops, evidently cannot suffer from this process in the same way as the higher plants, which destroy their own substance unproductively during fermentation. This circum- stance, together with the accumulation of alcohol in their cells — given off by yeast into the surrounding liquid — probably explains why it is that higher plants cannot maintain their life by means of the process of fermentation ; in the absence of oxygen all their movements as well as growth come to an end, and if they are kept a long time in such an atmosphere they finally die.

Therefore fermentation can only maintain the exist- ence of lower organisms, and even these but for a short time, because they also appear to need respiration from time to time ; whereas higher organisms cannot endure fermentation even for a vexy short time. Happily natural conditions in Nature do not expose them to this danger. They begin to ferment only when exposed to an artificially confined atmosphere, e.g. when they are enclosed under a glass bell, from which oxygen has been removed — in a word, when they suffocate. We cannot simply say to an organism : Cease to live. It is true it either lives or dies ; but while it lives it clings to

life. Failing to find in the surrounding medium the necessary conditions for life, it reacts on itself, and during this internal process breaks down, becomes exhausted, and dies. But the moment we remove it from the suffocating atmosphere, and by taking off the glass bell give the organism the chance of breathing freely, fermentation stops at once ; the pathological process of fermentation is replaced by the physiological process of respiration, the work of destruction is replaced by that of construction ; healthy and normal life asserts its rights, and motion and development follow as fellow travellers. Hence respiration conditions the very exist- ence of organisms, be they animals or plants.

Much has already been said about the impossibility of establishing any difference between the two kingdoms, on the ground of the presence or absence of movement. One more question remains to be discussed. Is the plant capable of voluntary movement ? Before answering this question we must agree as to what we mean by voluntary movement, or speaking generally by a voluntary pheno- menon. If the term implies a phenomenon produced without cause science will not admit such even in the sphere of animal life ; if the term implies a phenomenon produced by internal, hidden causes, then voluntary may also in the meantime be understood to connote the movements of protoplasm, antherozoids, and the leaves of Desmodium, because all these movements take place without any apparent stimulus, under the influence of internal forces peculiar to the organism. But if capable of movement why may not a plant also feel ? If we allow the response to stimulus, i.e. irritability, stimulation, to be a sign of feeling we are bound to re- cognise this faculty in the plant. In fact if a man is pinched, tickled, or pricked without responding to these stimuli, we decide that he has become insensible (be- come unconscious) ; but as soon as he begins to respond to them, by some movement, we say that he has regained

his senses (regained consciousness). 1 If this is the indication to be followed, then mimosa, the catchfly, etc., are apparently endowed with sensitiveness, because they do respond to different stimuli, be it a prick, or a light touch, a burn, an electric shock, or some chemical action. There are some especially striking cases in which plants do not respond to every stimulus in the same way, but seem to discriminate between them. For instance, contact with nitrogenous organic substances causes quicker movements in the hairs of the sundew, and a more energetic secretion of its digestive juices, than contact with particles of inorganic matter, which cannot serve it as food. Were it an animal we should say that its mouth waters, that it greedily throws itself upon a dainty morsel.

Granted that the plant is endowed with sensitiveness, can we not deprive it of this property, make it in- sensible to all kinds of stimulus ? Experiment proves that we are actually able to do so ; moreover, that we attain this end by using the same means as when we wish to bring a man into an insensible condition. We can anaesthetise a plant in the same way as we anaesthetise a man before a serious surgical operation. We oblige it to inhale the vapour of ether or chloroform. For this purpose we have only to cover a pot of mimosa with a glass bell, and place a sponge under it filled with ether or chloroform. After remaining a certain length of time under the glass bell the mimosa will lose its capacity for movement. However much stimulated, it will no longer fold its leaflets, but after being exposed again to fresh air, free from noxious vapours, it will regain its sensitiveness, its irritability. In order that this experiment may succeed we must not expose the

1 We know, however, that the reverse conclusion is not correct. By the action of some poisons we can deprive an animal of the capacity for responding by a movement to stimulation, without at the same time depriving it of the capacity for feeling. plant too long to the influence of anaesthetics, or it may never revive, but will irrevocably perish. The same thing happens with a man’s organism. Unfortunately sad cases of death happen fairly frequently as a result of the imprudent use of chloroform.

A nervous system has often been taken as the attribute of an animal ; but, as a matter of fact, a nervous system is not found in all animals. On the other hand, if the existence is established of special tracks in plants (as is suggested by some scientists) , by means of which irrita- tion is communicated more quickly than by others, we shall have to acknowledge that these are physiologically somewhat analogous to nerves. Thus irritation in mimosa, for instance, is transmitted by a special system of tubes, by means of hydrostatic pressure. Such an apparatus may best be compared with a pneumatic bell. This case obviously does not present any real analogy to the nervous system.

Now a last question : Is the plant endowed with consciousness ? We shall answer this question by raising another : are all animals endowed with it ? If we do not refuse it to the lower animals, why should we deny it in the case of the plant ? And if we do refuse it to the simplest animal, tell me, where, at what degree of organic development, does this threshold of consciousness lie? Where is the limit beyond which an object becomes a subject ? How can we escape this dilemma ? Shall we not rather admit that consciousness is wide- spread in Nature, that it smoulders in lower beings and glows in a bright spark only in the mind of man ? Or, had we not better stop at the point where the thread of positive knowledge breaks off, at the border line beyond which stretches the limitless province of speculation ever captivating us by its elusive vastnesses, ever escaping the limits of experimental inquiry ? 1