The Life of the Plant

Thus the principle of selection affords man a powerful means for improving organisms, for perfecting them, and the simplest application of this principle consists in the extermination of organisms that do not correspond to his aim. Let us remember this conclusion, which will later on prove to be the key to the explanation of phenomena taking place in Nature. Let us sum up what we have learnt in this lecture. There is a law underlying organic Nature according to which the cell though able to produce such giants as the Wellingtonia and the Baobab, the age of which is reckoned by thousands of years, is yet unable to reproduce itself endlessly in the same vegetative way. The main- tenance of vegetable forms requires that they should occasionally be renewed by the union of two separate cells. The significance, meaning, and necessity of this law of the existence of two sexes is quite obscure ; it is only an empirical law, based upon the conjoint testimony of all the facts known to us . 1 It may be that we are entitled to see in this law only one of the many manifesta- tions of a more general law — the law of the utility of the physiological division of labour, which expresses itself in the fact that the functions fulfilled in the simplest organisms by a single cell distribute themselves over different cells as the organism increases in complexity. The cell may be unable to reproduce itself successfully in all its parts in a long series of generations, and perhaps this labour is divided between two cells, each of which works out only a certain part of the future organism, and taken by itself may be even incapable of further develop- ment. But wherein does the difference lie between these two cells ? Which is the element of development each of them contributes ? These problems are the problems

1 It is worth mentioning that certain seaweeds present a curious instance of the sexual process taking place between three cells instead of two, a phenomenon which has no analogy in the rest of organic nature. One of these three cells, being the element fertilised by the second cell, is at the same time the fertilising element of the third. This fact, quite authentic although unique, together with the fact of the absence of any sex what- ever in the simplest organisms, prevents us from too broad generalisations, from metaphysical theories concerning the existence of a certain organic polarity, and so on. .

of the future. There is but one thing that can be inferred from the facts we know, which is, that along with the growing complexity of organisms, certain external differences arise between the sexual cells, and at the same time we observe that the degree of relationship between them increases. So far as the significance of the sexes is concerned, we are as yet completely in the dark, and it is better to abstain from any explana- tion at all than to indulge in vague hypotheses with no facts behind them.

In the previous chapters we have studied three functions of a vegetable organism, nutrition, growth, and repro- duction, which, from a certain point of view, may be re- garded as a particular case of growth. On glancing at Nature superficially, and regarding only such forms and phenomena as we meet at every step, we might easily come to the conclusion that all the vital activities of the plant are summed up in these three functions. This idea has found expression more or less from time imme- morial in some such definition of vegetable life as that plants live ( i.e . feed) but are deprived of motion, with the occasional addition of voluntary motion. This absence of motion and outward activity is looked upon as the essential point of. difference between plants and animals ; and vice versa, this is why a man, whose life is spent lazily, in little more than eating and sleeping, is said to be vegetating. But is such a general statement about the plant justifiable ? A broader outlook upon the vegetable kingdom and a closer study of the plant will soon prove how hasty such an opinion is. We find with amazement that, far from being absent, the pheno- mena of motion are even widely spread in the vegetable world.

Let us turn first of all to the microscope and study with its help a fully -developed uninjured cell under the most natural conditions possible. We choose for this purpose hairs found on the surface of stems and leaves or of young roots, and consisting either of a single cell or of a single row of cells ; or we may make a delicate section with a sharp razor of a leaf or stem of a water plant, such as Vallisneria, 1 so thin as to be transparent, being at the same time careful not to injure the uncut cells. Water plants are convenient simply because microscopic investigations are generally made in water, which means that the cell remains in its natural medium. If all the natural conditions are fulfilled, i.e. if the temperature is not too low and the cells are not injured, one of the most curious phenomena ever presented by the organic world reveals itself in a short time to our eyes. The cell-sap, or rather that part of the cell-contents which we have called protoplasm, 2 and which as a layer of thick liquid lines the inner surface of the cell-walls, or stretches in strands across the cavity of the cell, which is filled with a thinner sap, — this protoplasm moves in every cell, slowly at first, then more and more quickly. This movement is seen especially clearly in cases (such as Vallisneria ) where bright green chlorophyll grains are suspended in the protoplasm. These grains, carried by the rapid current of the protoplasm can be seen flowing along one of the longitudinal cell -walls, turning along the transverse wall to the other longitudinal wall, and from it again along the second transverse wall, to come back to the point of departure and repeat over and over again that circular voyage. This rapid rotating movement of the protoplasm can be observed in one and the same cell for hours and even days. In cells in which the protoplasm forms a general network of strands, the movement is not limited to the circular current along the walls, but is also seen to flow in thin streams across the cavity of the cell. Such a movement can be observed in any hair, such as the familiar hairs of

1 A plant to be found in any aquarium. The curious phenomena taking place in this plant during the period of pollination have been described in the previous chapter (fig. 68). Traiescantia (see fig. 15) or the stinging hairs of the nettle, and also in the cells of the flesh of fruit, as, for instance, the large cells visible to the naked eye making up the ripest parts of a water melon. We have but to take some of these cells with a needle and place them under a microscope to notice this curious streaming movement of the protoplasm in every one of them. Thus the protoplasm in these cells is in continuous movement. This movement is spontaneous : it is not provoked by any external physical agents, although such agents as heat or electricity may affect it, accelerat- ing, retarding, or even entirely arresting it. So many and such varied illustrations of this movement are known to us, that it seems most likely it is character- istic of the protoplasm of all cells, at all events at a certain period of their existence.

Sometimes this movement of the protoplasm mani- fests itself in a still more curious way, and so strongly as to be seen even with the naked eye. There is a group of organisms so peculiar that for a long time scientists wondered whether they were to be con- sidered plants or animals. Even nowadays some people reckon them as a separate third kingdom, although it might be more reasonable to range them among the simplest plants, along with the fungi. These organisms are called slime fungi, because during the greater part of their existence they present but a mass of protoplasm without any structure whatever, without any cell-walls, and therefore like slime, colourless, or of a brownish or bright yellow colour. These organisms appear on the surface of decaying wood, leaves, etc. One such organism specially well known occurs on the piles of bark accumulated in tanneries. It appears in masses without any definite shape, looking like thick cream, only yellow in colour, penetrating amongst the pieces of bark in thin filaments, or collecting on their surface in variously branched or compact masses. If

vve mark in one way or another the position of these semi-liquid masses (called plasmodia) and remember their outlines, we shall be greatly surprised in a short time to notice that they have moved considerably from the place they occupied and have also changed their shape. By observing one of the fine branches of a plasmodium, or better still by examining it under a microscope, we come to the conclusion that it actually does move. These branches form protuberances, pseudo- podia , into which the protoplasm of the neigh- bouring parts flow. The protuberance so formed is soon drawn back again and becomes absorbed into the general mass ; another one appears drawing the protoplasm in its turn. Thus stretching and contracting again the plasmodium creeps about (figs. 72 and 73) , tending for the most part in some definite direction ; it changes its position, creeps towards the light at the top of the pile, out from the inside of it, where it was concealed, crawls over every object in its path, e.g. a sheet of paper or glass — in a word it wanders, until it is arrested by the approach of the period of re- production. Then it transforms itself into an indefinite scone-like shape, the size of the palm of the hand, with a very brittle wall, and inside a very fine dust is formed, reminding us of the dust we raise when we tread on a ripe puff-ball. This dust consists mainly of minute cells, spores, serving to reproduce the organism. In germinating, the spores of our slime fungus shed their cell-wall and soon transform themselves into micro-

scopic lumps of protoplasm, which are continually changing their shape (fig. 73). Though on a smaller scale they exhibit the same creeping movement as that described in the case of plasmodia, a fact which is easily understood, since plasmodia themselves, masses of protoplasm visible to the naked eye, are formed by the coalescence of a very great number of these micro- scopic lumps derived from the spores (fig. 73) . cell, vegetable as well as animal, is endowed with a peculiar movement, inadequately accounted for as yet, and manifested indifferently whether surrounded by a wall or entirely free as in the case of the plasmodia of the slime fungi. 1

These instances do not exhaust the phenomena of movement, exhibited by the vegetable cell. So far we have studied one kind of motion, the streaming move- ment of shapeless masses ; let us now study the pro- 1 There is a satisfactory attempt to explain this movement from the physical point of view. Unfortunately we cannot dwell on it, because it would take us too far into physics; we can only say that by mixing two liquids we can obtain, under the microscope, forms and movements exactly similar to these.

gressive movements of whole cells. The spore plants give us numerous illustrations of such phenomena. Let us choose a few of them at random, taking them mainly from among plants that we see every day. If we pick up a dead fly and throw it into a glass of water, we notice in two or three days a soft white down, forming a kind of halo round the body of the fly (fig. 74) . This is a mould, i.e. a microscopic fungus. If we examine under the microscope its radiating branches, we shall observe at their ends oblong sacks, filled with colourless grains (fig. 74). If we leave some of these sacks in water under the microscope and look at them occasionally, we shall almost certainly catch one of them at the moment when its end breaks and lets out the grains it enclosed. These grains will cluster at the opening.

We shall notice that every one of them has two cilia attached to one side. But in a few moments the whole mass of them will quiver ; first one grain, then another, and then fig. 74 . stir, rush round as in a whirlpool and disperse, moving their cilia so rapidly that they are now scarcely visible. For a long time they continue to rush about, swimming across the field of the microscope, knocking against each other or anything else they meet, then bounding back and rushing in another direction.

It is impossible to distinguish this movement from that of the infusoria, and it is so contradictory to the current ideas as to the non-motility of the plant, that the first observers of similar phenomena refused to believe the evidence of their own eyes ; they would not believe that these bodies were of a vegetable nature. They attributed this movement to animals developed inside the plant. These motile cells after a time come to rest, germinate, and give rise to a new organism — they are therefore spores. In order to mark their resemblance to animals, they are called zoospores, i.e. animal-spores, or rather motile, wandering spores. Let us study one more zoospore, but from another class of plants, the water-weeds. There appears on sub- merged objects in ponds, streams, and ditches, occa- sionally also upon the surface of very damp soils, a bright green weed consisting of a single much-branched tubular cell. If we leave such a weed in summer in a glass of water, we shall notice every morning a curious phenomenon : a narrow, bright green line will appear on the surface of the water, at the side of the glass which faces the light. If we move the glass so as to turn the green edge away from the light, we shall observe that the green line will disappear, to reappear again at the side towards the light. We can repeat this experi- ment many times and always obtain the same result. This green matter is evidently capable of movement, and moves always towards the light. Let us investi- gate the constituents of this green matter and its origin. We place it in a drop of water under the microscope and notice that it swarms with green cells swimming to and fro (fig. 75, I., on the top). The cells have no walls ; they consist of a lump of protoplasm spangled with shimmering cilia all over the surface.

Let us now turn our attention to the weed itself and see what is its relation to these motile green cells. We shall notice pin-head swellings at the end of its green tubes, filled with a darker green and thicker mass (fig. 75, L). If we observe such a swelling for some time (the obser- vation must be made early in the morning, because in the day-time this phenomenon ceases), we shall notice that the green mass gathers into a round or rather an oval lump, creeps out of the sack, which is ruptured at the top, and begins to move (fig. 75, L). This is a large zoospore formed out of the protoplasm of our water- weed.

The movements exhibited by spore plants are not limited to the zoospores. We saw in the previous chapter that these plants are clearly differentiated in sex, but for the sake of simplicity we chose cases where both male and female cells are non- motile, and come into contact only by fu- sion. But in a greater number of cases the male cell is motile, and therefore seeks out the female which is enclosed in a special organ. In very rare cases both male and female cells are motile, like the zoospores just , described : their move- ments bring them to- gether, make them meet and fuse into a single p

mass, into one cell, one spore. As a matter of fact in weeds, mosses, ferns, horsetails, and clubmosses it is only the male cell which is motile ; it also assumes most fre- quently the shape of a rod twisted into a spiral and provided with cilia. These so-called antherozoids are endowed with a double movement : they move quickly forward and also rotate round their axis. Thus fer- tilisation, secured in seed plants by complicated adap- tations by which the non-motile pollen of a flower is transferred to a stigma, is here accomplished by the motility of the male cells themselves, the antherozoids. 1 The antherozoids of mosses are most easily seen. If in the spring we gather a stem of a big moss, which forms round, soft, green tufts in woods and marshes, and if we press between the fingers the unsightly brown clusters of modified leaves, seen at that season at the ends of many of the stems, small whitish drops will exude. Every such drop will contain millions of antherozoids. Fig. 75, II. shows the fertilisation of a female cell of a seaweed, found in the Baltic Sea and called Fucus . This cell is non-motile by itself, but antherozoids swarm round it, often surround it with a thick layer, and thus carry it away with them.

Thus, the vegetable world observed under the micro- scope turns out to be full of motion : in the cells of the water-melon the protoplasm moves ; in every weedy pool there swarm myriads of zoospores ; in the drops of evening dew there move the antherozoids of mosses and ferns, finding their way to female cells in order to fertilise them. But do we not notice phenomena of motion in a more obvious form, in those organs and plants that we can observe with the naked eye and which are naturally associated in our minds with the word * plant ’ ? Such phenomena can indeed be easily demonstrated, although they do not occur as often as microscopic movements. They are especially striking in plants growing in warm countries or in our hothouses ; the reason of which is easily understood. All kinds of motion in plants are accelerated with rise of tempera-

1 We have noticed in the previous chapter that antherozoids have been discovered in the pollen-tubes of some plants. In the next chapter we shall be able to appreciate the importance of this fact. ture : thus, for instance, the movements of protoplasm can be accelerated or arrested at will by subjecting the cells under investigation to heat or cold. We must differentiate two kinds of motion when we speak of movement in the organs of the highest plants. Some of them are slow and gradual ; like growth they can be observed only by their results, and they generally depend on the influence of variable external conditions. Others are rapid and abrupt like the movements of animals ; and, as in animals, they are either provoked by external irritation, or take place without any stimulus whatever, apparently quite spontaneously.

Phenomena known under the name of sleep movements belong to the first class. We mean by these the changes in position of leaves and parts of flowers at different hours of the day and night which are manifested by nearly all plants, but most obviously by some of them. If you look at a field of pink clover, you will get a different impression of it according to the time of day. In the day-time its surface will be more uniform, because the leaflets are almost horizontal, and catch the light falling right down upon them with the whole of their surface turned up to the sky. In the twilight, on the contrary, the surface of the field will look dishevelled, and if we examine more closely the separate leaflets, we shall notice that all the three blades of the leaf are raised ; they are now turning their edges instead of their surface up to the sky; the two side blades are folded together, while the third is pressed against their common edge. Other plants have the lamina of their leaves bent down in the nocturnal position, and so appear as if withered; in this case of the clover, on the con- trary, they are raised, and so it is clear that we have to do with quite a peculiar mechanical phenomenon.

Sleep movements in flower organs are even more distinctly seen. Thus, for instance, in the early morning or in the twilight we do not see any of the yellow heads of the dandelion which spoil so often the uniform green of our lawns. This is because these flowers open only in the light : on a dull day they may likewise remain closed. Other flowers, on the contrary, close in the day-time. This is the case with the goat's -beard (John-go-to-bed-at-noon), the yellow flowers of which are very much like those of the dandelion, only larger in size. They open early in the morning and close by ten or eleven. These phenomena attracted much attention among botanists of the eighteenth century. It was even suggested that floral hours might be observed ; the hours of the day being defined by the opening and closing of different flowers.

These phenomena are easily proved to depend on the action of light and heat. The crocus is especially con- venient for the purpose. Its large flower opens in the day-time and closes at night ; but the same phenomenon can be caused by placing it alternately in the light and in the shade, or removing it from a warm place to a cold one, and vice versa . A difference of temperature of ten or twenty degrees makes it close and open again in a few minutes. We can explain these phenomena by unequal growth or by the tension of tissues in the upper and lower, or outer and inner layers of the moving organ. We have already seen that light retards growth ; consequently, under its influence, the growth of the outer layers will be checked while the inner layers will outgrow them, and the organ as a whole will tend to curve outwards. As a result the flower will open. Now it is the inner (or upper) side that becomes exposed to greater illumination, while the outer (or lower) side, being shaded, will outgrow it and the flower will close. Similar reasoning can be applied to the effect of changes in temperature.

Such is the nature of these phenomena. They can ultimately be attributed to irregular growth, and, as a matter of fact, they are generally observed in organs that have not stopped growing. 1 The movements of another class are different. These take place quickly, almost instantaneously, as a result of irritation, or even without any stimulus whatever, and apparently quite spontaneously. Let us study a few cases of this kind of pheno- menon, beginning with the simplest, observed in the well- known barberry. In the centre of the yellow flowers of this plant, which are very like small roses, there is a pistil surrounded by six stamens (p, fig. 76). 2 These grow normally in the position shown at st. on the left. But as soon as we touch the base of the filament with a needle (as shown in the figure) , the stamen suddenly moves and assumes the position st'. on the right-hand side, i.e. applies itself to the stigma. It remains for a time in this position, then gradually returns to the normal position to again apply itself to the stigma as soon as irritated. Movements as the result of irritation, though of a somewhat different kind, are also characteristic of the stamens of the corn-flower, the thistle, the artichoke, and other plants.

These are all movements of very small, if not exactly microscopic, organs, and hence do not produce so start- ling an impression as does the movement of the irritated 1 Some cases of these phenomena, however, fall rather into another class; they depend on the presence of a special tissue in which the quantity of water changes, hence also the tension of the cells. It is obvious that in such cases the phenomenon may also be observed in fully developed plants. Such are, for instance, the sleep movements of leaves.

2 Fig. 76 is a longitudinal section of the barberry flower : pet. marks the position of the petals ; st, and $t', t the stamens ; p> the pistil with the stigma. leaves of the sensitive mimosa, which grows in our greenhouses. It is curious to witness the amazement of a person who has never before heard of this plant, and sees for the first time the way it folds its leaves on being slightly disturbed. It is only then that we realise how deeply rooted is the conviction, based as it is upon daily experience, that motion is not a characteristic of plants. Normally, the leaf of the mimosa has the appearance shown in fig. 77 on the

right-hand side. This is a so-called compound leaf. Its main leaf-stalk bears four stalks spread out like a fan, and each of these in its turn bears a con- siderable number of leaflets distributed in pairs. We have only to touch such a leaf or irritate it in some way and it will move. The leaflets will raise them- selves in pairs, and fold like the wings of a butterfly at rest. The four spreading stalks will lay them- selves together, and the main leaf-stalk will eventually droop, and sink downwards. The whole plant will look like the leaf on the left-hand side of fig. 77. The higher the surrounding temperature the quicker the movement. When the irritation ceases the leaf will gradually reassume its former position. Fresh irritation produces again a similar phenomenon.

Apparently we have here an impetuous movement, caused by some external stimulus, reminding us very strongly of the movement of an animal when it tries to avoid some irritating contact. Can we give any explanation of this phenomenon ? Yes and no. We can indicate the proximate mechanism of the movement, but as yet cannot explain the nature of the stimulation, caused by irritation and in its turn producing the move- ment. This movement takes place at the points where the leaflets are attached to the stalks, where these are attached to the main petiole, and, lastly, where the latter is attached to the stem. At all these points, the joints or articulations, special swellings or cushions are found. These leaf cushions or pulvini are formed of a tissue, the cells of which are overfilled with sap, and as a result these parts are in a constant state of tension. The moment the leaves are irritated the tension is suddenly released ; it is even reversed. Thus, for instance, the tension of the tissues of the lower half of the pulvinus which forms the base of the leaf-stalk supports it horizontally, and even in a rather uplifted position (as on the right-hand side of the figure). But whenever irritated this part of the cushion loses its tension, becomes floppy, loses its turgidity ; it is then unable to support the petiole, which sinks or rather is bent down by the upper half of the pulvinus, which has preserved its turgidity. Thus the two parts of the cushion — the upper and the lower — are in constant antagonism. In a normal condition the tension of the lower part predominates and the petiole is held up ; at the moment of irritation, when the tension of the lower part ceases, the preponderance is in favour of the upper part which bends the leaf down.

At places where individual leaflets are attached to the stalks, the reverse phenomenon happens : the upper part of the pulvinus (appearing here in the form of a white knob the size of a millet seed) is always more strained than the lower ; the leaves are consequently widely spread horizontally, or even slightly bent down, but at the moment of irritation the tension of this upper part breaks down, and the leaves, being left under the influence of the tension of the lower part only, rise and draw together in pairs. So the cause of this movement lies in the sudden, almost instantaneous loss of turgidity in the tissue of one of the two halves of the pulvinus : from being turgid it suddenly becomes flaccid, the balance between the two antagonistic halves of the organ is upset, and the leaf or leaflet moves in the corresponding direction. But how can we account for this sudden collapse, this loss of tension ? The microscope reveals the fact that the tissue which has this curious property of losing its tension consists of cells with thinner walls than the cells of the opposite antagonistic side of the pulvinus ; and, moreover, that the cells of this irritable tissue alternate with spaces filled with air. At the moment of irritation these spaces become filled with a liquid, as is easily proved. We have only to fix our eyes on the thickened pulvinus at the base of the petiole to see a sort of shade pass across that place at the moment of irritation ; the spot suddenly becomes darker. The same thing happens even more distinctly if several pairs of leaves are suddenly but gently clutched with both hands in such a way that they are irritated, but at the same time not allowed to fold. We shall notice, then, that the thickened pulvini, compared above to the millet seeds and lying at the base of every leaflet, will change in colour; from dull white they become transparent green. The moment we let the leaves go, they will fold.

What causes this sudden change in colour ? It is the same cause which produces a dark spot on the white surface of the snow, a filter paper, or ground glass when we sprinkle them with water. The whiteness in all these cases depends upon the reflection of light by countless minute surfaces in contact with the atmosphere ; but whenever water is substituted for the air, there is no longer the same reflection ; the bodies become more transparent and hence less bright. A direct experiment, however, shows this explanation to be the true one. We have only to make a slight incision on the lower side of the pulvinus of the leaf-stalk to see that a drop exudes from the in- cision at the moment of motion. If a similar incision is made on a leaf, which has already drooped owing to irritation, the drop of water will not exude. This water, exuded from the cells and occupying the intercellular spaces in the tissue, is absorbed or evaporated in the course of time; the cells become refilled with water, and the tissue regains its tension until a subsequent irritation.

Ultimately, therefore, the cause of the phenomenon which has attracted our attention resolves itself into the fact that water is rapidly exuded from the thin walled cells of the irritable tissue overfilled with it, and consequently this tissue as quickly loses its turgidity. But why is irritation followed by the exudation of water, and what is the energy that forces the cells to be over- filled with water ? We are unable as yet to answer these questions, but very probably we are dealing here with electrical phenomena, as we shall see further on.

Let us pass on to another example. At the end of the eighteenth century a plant was discovered in the marshes of North America, the movements of which are more striking still. I mean the so-called catchfly (fig. 78). The upper part of the leaf has the form and function of a trap. Whenever we touch the hairs upon ts surface, or whenever an insect imprudently creeps on to it, the two sides of the trap immediately fold together and do not let their victim out again. The more agitated the entrapped insect becomes, the more tightly do the walls of its prison close. This struggle between the plant and the animal ends always in the death of the animal.