The Life of the Plant

To begin with, we distinguish in such a cell between its thin and perfectly transparent wall and the actual contents of the cell. At first the cavity of the cell is filled by a uniform, semi-fluid mass called protoplasm, with a round body called a nucleus embedded in it, which we shall study later on. Subsequently little spots appear in the semi-fluid protoplasm, like cheese eyes, so to speak, filled with liquid. Thus the contents of the cell become separated into two parts, the proto- plasm and the liquid cell-sap, becoming more and more frothy. Later still the proportion of sap to protoplasm increases ; the volume of the protoplasm diminishes relatively as that of the cell augments. In the end almost the entire cavity of the cell becomes filled with the watery sap, and the protoplasm remains only as a thin layer, lining the inner wall of the cell, or stretching from one wall to the other in little strands. In Trades- cantia such a differentiation of the contents of the cell is particularly well marked, because the cell-sap is violet in colour while the protoplasm is colourless. Besides these two substances, protoplasm and cell-sap, we also frequently notice in the cavity of the cell something of a different kind — small, shining drops with an oily appearance, or round, colourless little grains, the char- acteristics of which will be studied later. At a later stage the contents of the cell sometimes disappear, and the cavity fills with air. Such a skeleton of a cell must be considered dead. The dry, sapless part of a tree, for instance, may be considered as formed of such dead cells. Thus in a living, active cell the microscope

reveals the following substances : the wall, the proto- plasm, the sap, and occasionally other bodies such as drops or grains. So much for the microscope. Now let us return to chemistry with its balance and reagents ; but this time let us stop a little earlier in our analysis without reducing the plant right down to its elements. We shall try to separate out the different substances which enter into the composition of the plant without destroying them, dealing with them as they actually exist in the plant. In a word let us study the proximate constituents of a plant — I say proximate in contradistinction to the ultimate constituents, which are the elements.

Evidently it is impossible to study here all the various substances which the vegetable world produces — everything we find at our grocers’ and chemists’ shops, at the carpenters’ and the confectioners’, in spinning factories and at dyeworks. We shall limit ourselves to the commonest bodies, or rather groups of bodies, without a study of which it is impossible to understand vegetable life. Let us choose for an illustration some vegetable organ, say grains of corn. Let us take them in a powdered form, as flour. As we shall see in a moment, flour, represents a heterogeneous mixture of substances. To separate them let us prepare a small lump of dough, and wash it a long while with water, working and knead- ing it with our hands. At first the water runs off milky- white in colour, but gradually it becomes quite clear. We have now instead of dough a lump of something, greyish-white in colour, sticky, and flexible like india- rubber or leather. This is called gluten, and is that constituent part of flour which makes dough sticky. If, on the other hand, we let the water stand which ran off during the washing we observe that it becomes quite clear, while a very thin white sediment, quite soft to the touch, forms at the bottom of the glass. This is

starch, the well-known substance which is used for dressing linen and also in the kitchen. Thus we have separated the flour, simply by washing it, into two of its components : gluten and starch. If we had mixed the flour with ether and let it stand, then poured off the ether and let it evaporate in an open dish, we should have obtained an oily residue. Thus flour or grains of corn consist chiefly of three substances : gluten, starch, and oil. The methods of separating these substances which have just been described may serve as a rough but obvious example of a so-called proximate analysis. In such an analysis we try if possible to extract sub- stances, without altering them, by taking advantage of their properties of dissolving or not dissolving, of volatilising, crystallising, and so on.

These three bodies, starch, gluten and fat, may be taken as representatives of the three principal and most widely diffused groups of vegetable substances. These groups are known as carbohydrates, albumin- oids, and fats. Other substances are generally met with either in comparatively small quantities or else in exceptional organs or plants, and consequently do not affect the general phenomena of vegetable life. Here is a table giving the proportions in which these proximate constituents are present in various widely differing vegetable products. These analyses fully endorse what has just been said about the large mass of the plant consisting of the three classes of compounds which have been enumerated.

The carbohydrates are so called because hydrogen and oxygen are combined in them in the same ratio as they are found in water ; since they also contain carbon, they seem to be composed of carbon and water. The following substances belong to this group of carbo- hydrates: common cane sugar, beetroot sugar, and grape sugar, or glucose , which is found in old raisins ; gums, such as the gum which oozes out of the stems of cherry trees ; starch ; and, lastly, cellulose, the sub- stance which forms the solid skeleton of the plant, its cell-walls, and which is used in our cotton and linen cloths, and in paper. The carbohydrate group is some- times spoken of also as the sugars, because some of the members of the group, as we have just seen, are actual sugars, while others can be easily changed into sugar. For instance, by treating starch with dilute sulphuric acid starch sugar is obtained. Cellulose can also be changed into sugar if treated with the same acid. The same method will transform old rags into sugar. The carbohydrates we have mentioned seem to fall into a series : cane-sugar and glucose are easily soluble in water and capable of crystallisation ; gums, like cherry gum for instance, are soluble in water, forming a thick viscous liquid, but are incapable of crystallisation ; starch does not dissolve in cold water, but swells in hot water, forming a sort of paste ; lastly, cellulose neither dissolves nor swells in cold or hot water.

Now let us see how we can detect the presence at least of the chief of these substances. They are all colourless, but we possess means of px'oducing in them certain characteristic colour changes. The colourless liquid in this glass is a solution of grape sugar, the other glass contains a bright blue liquid. I pour the colour- less liquid from the first glass into the blue liquid in the second, and slightly heat the mixture. It becomes turbid, then turns a dirty green colour, and finally forms a yellow precipitate which turns brown,

then bright red, and sinks to the bottom of the glass, leaving the liquid colourless. Therefore grape sugar produces a red precipitate in our blue liquid ; or, in other words, this blue liquid, otherwise called Fehling’s solution, by changing colour reveals the presence of grape sugar. This reaction is so delicate that it will betray in a liquid the presence of the most minute quantity of this sugar. Thus we have in Fehling’s solution a valuable reagent for detecting the presence of very small quantities of grape sugar. In iodine we have a similar reagent for detecting the presence of starch. I take a large beaker of water, add to it a few drops of starch solution and stir. I have thus in the liquid minute traces of starch. I add to it a few drops of iodine solution, yellow in colour, and the liquid at orice turns blue. In the same way if I drop iodine solution on a lump of dough or a piece of bread, I get a dark blue, almost black spot, because starch is contained in both substances ; but if I drop some iodine solution on a piece of gluten, I do not get any black spot, because the starch has been previously washed out with water. So iodine stains the colourless starch blue, and therefore serves as a reagent for detecting starch. We have now to find means for a similar detection of cellulose. Iodine by itself does not stain it blue, but iodine and zinc chloride will. We have only to drop this solution on a sheet of white paper, which, as we know, is cellulose, to produce on it a blue spot. Such are our reagents, our means for detecting the most widely diffused carbohydrates, grape sugar, starch and cellulose.

Now let us pass to another group, that of albuminoids. These are found either in solution, as in the juice of the cabbage, or in a solid form, e.g. the gluten we have just obtained from our wheat grain. As soon, however, as we heat cabbage juice, we see it turn into flakes : the albumen has ‘ set ’ or coagulated in the same way as an egg ‘ sets ’ when it is boiled. Chemistry presents a whole series of reagents by which we can detect the presence of albumen. Let us experiment with one of these reagents, the most obvious if not the most certain. I have in a glass a certain quantity of the white of an egg in water. I add to it some ordinary syrup of sugar, together with concentrated sulphuric acid. A pre- cipitate forms which dissolves again, and all the liquid gradually turns a splendid red colour. In this way albuminoids can be detected by means of sulphuric acid and sugar.

There remains a third group, that of oils and fats. We have no clear and simple reagents to produce in them such characteristic changes of colour; but instead, as we have already noticed, we have only to treat a sub- stance in which the presence of oil or fat is suspected with ether, and the ether will dissolve them. Then if we expose this solution to the air, and let the ether volatilise, we get oil or fat with its characteristic properties. Now we can reproduce in cells under a microscope all the reactions we have mentioned. Suppose w r e add sugar and sulphuric acid to water in which a cell is being observed. We shall notice the protoplasm turning pink, which proves that it consists chiefly of albuminous substances. Let us use Fehling’s solution, and if the cell-sap contains any trace of grape sugar we shall get a red precipitate. We add a drop of iodine solution, and notice that the small colourless grains in the cavity of the cell turn blue : this indicates the presence of starch. We take next iodine dissolved in zinc chloride solution, and the whole cell-wall turns blue, which means that it consists of cellulose. Finally we add ether,, and notice that the drops which had attracted our attention by their oily appearance have disappeared, have dissolved, which proves that they were drops of oil. Such is the way that chemical analysis and microscopic investiga- tion work hand in hand, mutually supplementing each

other. Analysis shows (see table on p. 43) that the substances most abundant in the plant are carbo- hydrates, and the microscope confirms this fact, showing that carbohydrates form the cell -wall, appear in the shape of grains of starch, or are dissolved in the cell-sap in the form of sugar. Analysis shows that in relative abundance albuminoids take the second place, and also that the younger parts of a plant are comparatively richer in nitrogenous substances than older parts ; the microscope demonstrates that protoplasm consists chiefly of albuminous substances, containing nitrogen, and that this protoplasm is the predominating constituent in young cells. Lastly, both microscope and analysis point to the presence of fatty substances in the plant and in the cell.

We have now made acquaintance with the principal substances contained in a vegetable cell. Already we had come to the conclusion that the cell builds up all these substances from gases, salts, etc., which surround it. In other words it must feed from the outside. Every cell must draw its food from the soil, from the air, or from some neighbouring cell. A question naturally arises here : in what way can this cell, this little bladder without any opening, or any mouth or jaw, attract and absorb surrounding substances ?

To explain this first phase in the nutrition of the vegetable cell we must turn aside from it for a while, we must turn aside even from botany itself, and study some purely physical phenomena ; we must study certain general properties of matter manifested in dead as well as in living nature. We shall often use this method in the future. It is the only sure method whenever we wash to find the explanation of vital phenomena ; for, in the language of physiologists, to explain means to reduce complicated vital processes to more simple physico-chemical phenomena.

Physics teaches us that particles of matter are endowed with motion, that we do not know any matter without motion. This motion is most clearly manifested in fluids, and more especially in the gaseous state of matter. Particles of gaseous matter are endowed with rapid motion : they tend to disperse until they fill up all spaces unoccupied by them ; this goes on until they are equally distributed everywhere throughout the region accessible to them.

This capacity, this tendency of matter to spread in space, is called diffusion. It is a simple matter to prove the existence of the phenomena of diffusion, especially in respect of gaseous and volatile substances. We have only to sprinkle a few drops of ether to smell it in an instant not only in the immediate neighbourhood but also in the remotest corners of the room. The ether has changed into vapour, and that vapour has distributed itself throughout the whole room. The diffusion of liquids is also easily demonstrated. I only need to remind you of the probably well-known experi- ment with water and wine. We gently pour some claret on to the surface of water, and notice that the liquids form two distinct layers ; a b but little by little the sharp

boundary between them disappears, the wine permeates the water and the water the wine, so that both liquids mingle together. We can perform here a similar but still more striking experiment (fig. 16). Here are two almost colourless liquids which, when poured into each other, produce a blood-red liquid. We pour the denser of the two liquids into this long, narrow beaker, then with care the lighter one on the top of it. A narrow layer of red solution appears between them; but in time

this narrow, hardly visible red line will broaden, and at the end of this lecture will be several inches in breadth ; while in several hours, or it may be days, the whole liquid will be a uniform red colour. Apparently both liquids interpenetrate each other. This depends on motion — peculiar to their particles, and therefore invisible — on their tendency to spread in space ; otherwise we cannot explain how, in spite of the force of gravity, the lighter particles sink to the bottom while the heavier rise to the surface.

Different substances are endowed in a different degree with this property of diffusion — in other words, particles of different substances move with different velocities. This is best demon- strated with gases. This vessel (fig. 17) made of very porous clay water coloured red. Both vessel and tube contain air. The object of this apparatus is to demonstrate the slightest change in the volume of air, contained both in the vessel and the tube. If by any chance the volume increases, air will begin to escape in bubbles through the coloured liquid. On the other hand if the volume of air in the apparatus decreases, the coloured liquid will rise in the tube. In the meantime neither happens because the air in- side the vessel is just like that outside. But if we surround the pic. 17.

with another gas, it is clear that a mutual interchange of gases will take place through the porous wall, which can be penetrated by them ; each of them will strive to permeate the other. Obviously if the two gases tend to spread, and their particles move at different velocities, a temporary change of volume will result in the apparatus ; the volume will increase or decrease according as the gas without enters more quickly or more slowly than the gas within escapes. A similar phenomenon will take place in a few minutes on the threshold of this hall. Let us suppose that there are three hundred persons at present in the hall ; and, further, that one hundred of them are bored with this lecture (all too prolonged), and are impatient to hear the end of it and to leave the hall ; while another hundred persons are standing outside waiting to enter for the next lecture. If the former leave the hall in the same hurry as the latter enter it the number of people in the hall will remain the same. But if those outside, not feeling so weary after an hour’s mental effort, should prove more energetic, the number of people in the hall in the first instance will increase, the hall will fill up, and only later, when those wishing to leave actually do so, will the number remaining decrease to the original three hundred. The same thing happens here. If I surround this porous vessel with gas, the particles of which enter more quickly than the particles of air contained in it pass out, the vessel will for a short time contain more particles of gas than it can actually hold, and the superfluity of gas will escape in bubbles from the end of the tube. I take a glass bell full of hydrogen. Since this gas is lighter than air, it can be kept for a certain time in a vessel with its opening turned down. I lower the bell (c) over the porous vessel (a). The inside of the vessel contains ordinary air ; outside, under the bell, is hydrogen.

If particles of hydrogen are endowed with more rapid motion than particles of air, the inner volume of gas must increase, and you hear and see the bubbles of gas bubbling through the coloured liquid in the beaker. I lift up the bell now ; the conditions are altogether reversed ; hydrogen is now inside the vessel, air outside it ; hydrogen moves towards the outside, air passes in ; but particles of hydrogen move more quickly than those of air, so the volume inside the apparatus decreases, and you notice the red liquid rising quickly in the glass tube (b).

Therefore gases, even more than liquids, are capable of diffusion, i.e. are capable of permeating all spaces as yet unoccupied by them. The hydrogen rushed into the vessel only because the latter contained no hydrogen, and later rushed out of the vessel only because none was present in the air of this hall. Likewise all gaseous matter and also matter dissolved in liquids tends to occupy the whole space accessible to it, and to spread uniformly through it.

_ Now let us see in what relation the phenomena of the diffusion of gases and liquids stand to our question concerning the nutrition of the cell. Here is an appar- atus reminding us pretty closely of a cell (fig. 18). It is a thin bladder moistened with water and transparent as glass, made out of a substance like cellulose or rather actually made of cellulose itself, only a little modified chemically. This is nothing but collodion, such as is used by photographers. The bladder (A) is joined to a horizontal glass tube (B) which contains a drop of a coloured liquid (a). We can judge whether the volume of air in the bladder increases or decreases according to the movement of the drop towards or from the bladder. I let the bladder down into the broad and empty vessel (C) and pour into it some carbonic acid. You cannot see it because carbonic acid is a gas as colourless as air. But I am correct in saying that I pour in the carbonic acid because it is heavier than air, and so can be poured from one vessel into another while remaining totally invisible. Carbonic acid can be kept for a certain length of time in a vessel open at the top in the same way as hydrogen can be kept for a short time in a bell open at the bottom. After having introduced carbonic acid into the vessel surrounding the bladder, the drop of coloured liquid trembles and then runs along in the direction of the arrow, thus demonstrat- ing that carbonic acid has begun to penetrate through the moist wall of the bladder, which is our artificial cell, and moreover that it is doing so more quickly than the air is escaping from the bladder. In the same way a vegetable cell has no need to attract or imbibe gases, such as carbonic acid, which, owing to its property of diffusion, will penetrate independently any cell devoid of it.

Now let us observe the behaviour of vegetable cells towards substances dissolved in the water of the soil. Let us take some oblong bladders made of the same collodion and fix them to the extremities of lamp glasses (fig. 19). Suppose these collodion bladders represent root-cells, by means of which the plant comes into contact with the nutrient substances contained in the soil. A plant, as we know from its chemical com- position, needs among other things iron salts. We choose these for our illustration, because they give

very obvious reactions by which it is easy to detect slight traces of them in a solution. I have, for instance, some water in this glass. I add to it a few drops of an iron salt and then some of another liquid (a solution of tannin), and the solution previously as colourless as water turns as black as ink ; in fact it is not quite correct to say as ink, because it actually is ink. We put into the vessel which contains water a bladder of collodion also filled with water (x). Then we pour some

iron salt into the vessel and some tannin into the bladder. Almost immediately a greyish tint appears near the inner wall of the bladder, and in a few minutes all the liquid in the bladder is turned into ink (2) . We notice therefore that the iron salt spontaneously penetrates into our cell ; and we know that this process will continue as long as the solution of the salt in the cell is weaker than that in the vessel outside it, for only then will as many particles enter the cell as pass out of it — in a word, until equilibrium is established. But here a question arises : can an equilibrium of that kind

be reached in our illustration ? Apparently not ; as soon as our iron salt penetrates into our cell, it enters into combination with the tannin, forming what for the sake of brevity we shall call ink. Therefore the cell will contain ink, but no more iron salt, and if it does not contain any iron salt a fresh supply of the salt will enter from the vessel outside ; this in turn will become ink, and so on. If the bladder contains a sufficient quantity of tannin, the equilibrium will never be reached, and the iron salt will diffuse into our cell in a continual stream. Thus we have only to take the collodion bladder containing the solution of tannin, and put it down into the vessel containing the solution of iron salt, to withdraw from this solution the whole of its salt and transfer it into the bladder. Let us put aside this apparatus for a few hours or days, and we shall find that there is no more iron salt in the outer vessel : our artificial cell will consume it, will absorb it completely.

Apparently we are approaching the simple physical explanation of the way nutrient substances enter into the vegetable cell. We have seen that a gaseous or soluble substance penetrates into a cell spontaneously, and goes on penetrating into it until it finds itself present equally on both sides of the cell-wall. We have noticed further that this equilibrium will never be established if the substance which penetrates into the cell is transformed there and enters into a new combina- tion. In that case it will rush into the cell in a per- petual and continuous stream, and become precipitated therein. We perceive here one of the reasons why the mass of a plant increases, i.e. why matter becomes accumulated in it ; but to complete our explanation we require one more link in the chain. The process of the accumulation of matter in a cell will become quite clear only in so far as we admit that substances enter freely from outside into the cell, and that those into which

these are transformed inside the cell, i.e. substances within the cell, do not again escape from it. The experiment we have just performed fully confirms this. In fact the liquid blackens only inside the collodion bladder ; outside, it is as colourless as water. This would not have happened could tannin, or its compound with the iron salt, ink, pass through the cell-wall. To verify this let us perform the converse experiment. Let us pour some iron salt into the cell and some tannin into the outer vessel. In a few moments black streams will be noticed in the outer vessel, and in the end the whole liquid in it will become so black that the bladder will be invisible (3, fig. 19). Let us take it out of the vessel — the solution inside of it is as colourless as it was at first. Without doubt it is only the iron salt which passes freely through the membrane, with equal ease in either direction ; but neither tannin, nor its compound with iron, can pass through it. It follows that two kinds of substances exist : some of them are capable of passing through the membrane of the cell, others are not ; the iron salt serves as an illustration of the former kind and tannin of the latter.

Indeed these two substances may serve as types of two great classes of chemical bodies. Those of the one class pass easily through vegetable or animal membranes; those of the other pass with difficulty. We have noticed in speaking of the diffusion of liquids that some diffuse more quickly, others more slowly ; some are more mobile, others less. We may now add that those substances which diffuse slowly are precisely those that pass still more slowly through membranes. Chemists call substances of the former class crystalloids , since they are all capable of crystallisation ; substances of the latter class they call colloids, i.e. gum-like substances ; these are all incapable of crystallisation.

We have here at once an explanation of our experi- ment, and a general key to the phenomena which take place in the nutrition of a vegetable cell. It is the iron salt which moves toward the tannin, not the tannin towards the iron, because the iron salt is a crystalloid, whereas tannin is a colloid. Going back to the nutrition of the cell we meet, roughly speaking, the same pheno- menon. What, in fact, are the substances a cell finds in its environment ? Gases, water, and salts dissolved in water ; that is, crystalloid substances — which means, generally speaking, extremely mobile substances which easily pass through the cell membrane. What sub- stances does such a cell contain, into what does it transform the substances absorbed from the outside ? It transforms them chiefly into albuminoids, oils, gums, starch, or cellulose — in other words, into colloids, scarcely mobile substances which will not pass through membranes or into other substances totally insoluble. This may be easily grasped with the aid of the following table