The Life of the Plant

These are the bundles, and they are either long and straight, running parallel to one another, or they inter- lace in a complicated fashion, forming a network of which only a faint idea is obtained on simply glancing at the leaf. In order to realise the thinness and delicacy of this network, we have only to let the leaf rot in water for some time ; then we shall be able without difficulty to remove with a soft brush both the epidermis and the fundamental tissue of the leaf, and separate this network of nerves, to which no lace can be compared for delicacy. 1 The name ' nerves ’ is not very happily chosen, since these organs have almost nothing in common with the nerves of animals. If any analogy is necessary it would be more appropriate to compare them to a skeleton and a vascular system combined, since they form the hard skeleton of the leaf, and also a system of canals for the translocation of nutrient substances. I have expressed myself rather carefully in saying that they have almost nothing in common with the nerves of animals; because, as we shall see, there is a theory that they are the paths by which irritation is transmitted in the plant. If this theory be confirmed we shall evidently have to recognise in them a certain, though remote, resemblance to the nerves of animals.

The same nerves or veins so apparent in the leaf extend also into the stem, where they are less sharply defined, and do not strike the eye in the same way. Different plants, however, present a very different structure in this respect. Let us examine two very common examples. In the monocotyledonous 2 class of plants, to which for instance our cereals, asparagus, and palms belong, these bundles are scattered in the fundamental tissue as is shown in the transverse section of fig. 45, I. A longitudinal and transverse section of

1 Other parts of the plant can be treated in the same way, and then semi-transparent ‘ phantom ' bouquets, so to speak, can be made of them. 2 So called on account of their possessing only one cotyledon. one of these bundles with its surrounding fundamental tissue, from the stem of maize, is shown very much magnified in fig* 44. 1 Here it is clearly seen that the bundle consists of different vessels, with spiral, net- like, or ring-shaped thickenings, etc., sieve-tubes, and also fibres ; whereas the surrounding tissue consists of ordinary cells.

more clearly seen if a section of it is placed for a time in a suitable stain, say a solution of fuchsin. Then, as in fig. 45, I., the bundles in the transverse section will appear as red spots on the colourless ground of the fundamental tissue. 1 A transverse and longitudinal section has been taken of the stem of maize, showing a single vascular bundle together with the surrounding fundamental tissue. The large aperture, surrounded by small ones, is the opening of a very large vessel. A number of smaller vessels are cut longitudinally.

Quite a different structure is presented by the stems of conifers and dicotyledons, 1 to which all our forest trees belong, such as the oak, lime, maple, etc. In order to understand the structure of these stems, it is necessary to enter into some anatomical details, without which further ex- position of the subject would be useless. Botanists, as well as non- botanists, differentiate three parts in the transverse sections of the trunk of a tree : the bark or rind, the wood — showing a series of concentric rings — and the pith (fig. 45, III.) - But botanists go further and distinguish also between the fundamental tissues and the fibrous and vascular bundles, the same distinction as has been already clearly seen in the stems of the mono- cotyledons. Let us try and make this point clear. We I . see the predominance of the fundamental tissue in the stem of a monocotyledon ; bundles are scattered in it indiscriminately, and are also surrounded by it. But suppose these bundles were distributed regularly in a circle, and, moreover, were so much developed that only comparatively narrow layers of fundamental

tissue were between them, we should then have such a stem as is shown in fig. 45, II. 1 Such in reality is the structure of a very young stem of any of our forest trees in its first year. The fundamental tissue in it is found in the middle of the ring of vascular bundles ; this is the pith or medulla. We also see it in the form of narrow rays diverging from the pith in between the bundles ; these are the so-called * medullary rays * : lastly, we see on the outside of this ring the so-called primary cortex , generally green and sappy, and com- posed of small cells. Thus every vascular bundle en- closed between two medullary rays has in transverse section the form of a triangle with its apex turned towards the centre.

This triangular form of the bundles is also preserved in an old stem. The dark diverging rays seen in fig. 45, III. represent medullary rays, and the lighter triangles between them are vascular bundles. Thus the bundles form the predominant part in the perennial trunk of a tree ; the fundamental tissue is found in between the bundles in the form of narrow medullary rays, some- times almost invisible : hence it is clear also that the difference between the bundles and the fundamental tissue is not so well defined as in the case of mono- cotyledons, and is only seen under the microscope. 2

The principal part of the stem of our forest trees consists, therefore, of vascular bundles ; this is not, however, their most striking peculiarity. They differ from monocotyledons, such as palms, in that they in- crease in bulk during the whole of their existence — which is quite impossible to the monocotyledons — owing to the following anatomical arrangement. We all know that the rind of our forest trees is sharply 1 Fig. 45 I. The stem of a palm or of asparagus. II. Structure of the stem of an annual dicotyledon. III. Stem of a dicotyledonous tree. All three are shown in transverse section.

2 See fig. 62, showing a small section of wood cut across, with a medullary ray. differentiated from the wood ; in spring when the plant is full of sap it even peels off easily. Non-botanists presume, and formerly even botanists also presumed, that there is a space in between the rind and the wood, which is filled, especially in spring, with a thick liquid, out of which the new parts of the plant can be formed. Exact microscopic investigation has proved that there is no such space, but at that part of the stem there is a ring of exceedingly delicate juicy tissue capable of continually forming new cells — hence its name of formative tissue, otherwise cambium. In fig. 45, II, the cambium is shown as a dark ring which cuts across the vascular bundles as well as the medullary rays, the whole stem being divided by it into two parts — the wood lying inside the ring, and the rind lying outside it. Owing to the presence of this continuous circular formative layer, which is absent from the monocotyledons because their vascular bundles are scattered instead of being distributed in a regular circle, the stems of dicotyledons and conifers are capable of long continued growth in thickness. Every year this formative tissue deposits new rows of cells towards both the wood and the bark ; only the wood increases more rapidly (fig. 45, III.), and the rows of its cells are deposited more evenly ; this is why it presents the regular alternation of annual rings that we notice on every transverse section of a tree. Let us now see what is the anatomical structure of these two kinds of vascular tissue on either side of the formative tissue, i.e. in the wood and in the rind. On the wood side we find almost exclusively fibres called wood fibres (fig. 42, 6) and various vessels, pitted, reticulate, spiral, and so on, but no sieve-tubes. On the bark side we find very elongated fibres with very thick walls (fig.

42, 5) , similar to the fibres of the plants used for spinning that we mentioned before, and the only tubes we meet are the sieve-tubes mentioned above (fig. 43, II.). The fibres form the part of the rind generally called bast ; it is highly developed in lime trees for instance, and is used for splints, bast-strings, and so on. It is from this bast that all fibres having the structure we have described, wherever they are found, have received the name of bast. The transverse section of an old tree will, there- fore, present the following parts : on the outside the part which we have called the primary cortex ; here, as we shall soon see, a special tissue serving as a. protection to the tree is formed at a later period ; then under the primary cortex there is a layer of vascular tissue consisting mostly of bast and sieve-tubes ; we shall call this part the secondary cortex to differentiate it from the primary ; then comes a ring of formative tissue ; nearer still to the centre is the wood, and, lastly, in the very centre the pith. We shall content ourselves with these particulars ; they may have seemed rather tedious, especially when presented in such a necessarily terse form, yet they are indispensable for the understanding of the physiological activity of the stem. Now we enter upon the investigation of the question : what are the paths by which the sap of the plant moves, i.e . by means of which the natural interchange is effected between the substances absorbed by the root and elaborated by the leaves ?

Let us start with the former, as being the simpler case, that is with the passage of substances from the root towards the aerial parts of the plant, the so-called ascending current. This current furnishes all parts of the plant with the water they require, and in addition with the salts dissolved in it. It is quite easy to dis- cover the path of the moving water, because the want of the necessary quantity of it is generally betrayed by the plant fading. Therefore by making trans- verse incisions at different places on the stem of a living plant, and observing where and when it begins to fade,

we can easily see whether we have cut across the path of the ascending current of water or not. Experiment has proved that we can cut through the whole of the rind, and even peel it right off, without making the plant wither, which means that its aerial parts go on receiving water from the soil. We can also cut through the pith, which by the way often dies naturally in old trees, leaving hollows at the heart of them ; in course of time decay spreads also to the inner layers of old wood, and yet the tree does not seem to suffer for a long time. Apparently the ascending current of water must flow through the wood, and moreover through the young wood. This inference is also proved by another experiment which has already been mentioned, by the experiment in which the vascular bundles are stained by coloured solutions. This experiment gives specially clear results with leaves which are variegated, or wholly white. In a short time the whole network of veins stands out in colour on the white ground. Microscopic investigations show that the wood vessels are the first to become stained, which means that wood is to be considered as the course for the ascending current of water in the stem.

And how can we explain the reason of this ascent of water, sometimes to a height of three hundred feet ? The reason of this movement must lie in the stem, as well as in the root ; in the stem, because stems and branches when cut off a tree continue to absorb water, carrying it to the leaves ; in the root, because if the stem be cut off close %o the ground, and even more so if the upper part of the root be cut, water will exude from the cut surface of the part remaining in the soil. Let us first study this phenomenon of the exudation of water from the upper cut end of the root, which is apparently the primary cause of the penetration of water into the stem. It was long ago noticed that sap flowed in abun- dance from some wounded or cut stems ; to this pheno-

menon the name of ‘ bleeding ’ has been given. It was thought to be the exclusive property of certain woody stems, and to take place only at certain seasons of the year ; this bleeding is specially profuse in the vine in spring. Comparatively recent investigations have shown, however, that this phenomenon is common to all plants, herbs as well as trees ; and that it takes place all the year round, although certainly with very varying intensity. In order to see this bleeding, and to measure its force, the following method is used : a stem is cut across not far from the ground, and we attach to the stump, with the help of an india-rubber tube, a small

bent glass tube, if we only want to gather and measure the quantity of liquid exuded; whereas, if we want to find out the pressure under which the sap is driven out through the section we use another tube of the form shown in fig. 46 on the left. This doubly bent tube, filled partly with water and partly with mercury, is simply a manometer, serving to measure the pressure under which the sap of the plant is exuded. The sap drives the mercury before it as it flows into the tube ; it is by the rise of the column of mercury in the open bent part of the tube that we estimate the pressure. Experiment has shown that this pressure can be equal to that of a column of water thirty-six feet high, i.e. water is driven out of the section so vigorously that it might still come out even if a column of water thirty-six feet high had been made to press upon the cut surface. How can we explain this property of the root to raise water to such a height ? The following experiment will give us the answer. Let us take a small glass bell (fig. 46 on the right, and b ) , close its lower opening with a bladder and introduce a cork with a glass tube into its neck, and sink the whole as shown in the figure into a basin of water. If the glass bell also contained water, there would be no interchange between the water of the outer and inner vessels, so long as the level is the same in both vessels, at n, because otherwise water would soak through the bladder under its own pressure from the vessel where its level is higher into the one where it is lower. But let us suppose that a solution is introduced into the inner vessel instead of water, a solution of some substance found in the cells of plants, say sugar, which, as we know, is found in abund- ance in the roots of the beetroot, for instance.

Then a phenomenon rather perplexing at first sight will be observed, even rather contradictoiy to what has just been said about the tendency of water to reach a common level in two vessels communicating with one another through a membrane. The solution of sugar — which can be coloured to make it more conspicuous — will quickly rise in the glass tube and soon reach a considerable height (/) . The explanation of this fact is as follows : accord- ing to the laws of diffusion the water and the solution of sugar tend to mix, the one moving towards the inner vessel, the other towards the outside of it. But the particles of water move more quickly than those of sugar ; therefore the water will pass into the sugar with greater rapidity than the sugar into the water ; moreover, water passes through a bladder far more easily than sugar; therefore, by the joint working of these two causes, the current of water into the inner vessel will be far more rapid than the opposite movement of sugar, and hence the rise of the solution in the tube which at first seemed incomprehensible because it contradicts the laws of hydrostatics. We should get the same effect, though less clearly, if instead of sugar we took albumen, gum, or some other substance gener- ally found in vegetable cells. Therefore here also the phenomenon resolves itself into diffusion, complicated by the presence of the membrane. Phenomena of this kind have been called osmotic. The rate of this diffusion, all other conditions being equal, will also depend on the area of contact between the two liquids, in our case on the size of the opening closed by the bladder. Suppose we grant that our apparatus presents a certain resem- blance to a root-cell, a root-hair, and remember how large is the area of contact between such root-hairs and the water of the soil : we shall then soon realise what the result must be if such an apparatus is multiplied a million times, however microscopic it may be. Every cell greedily absorbs water and squeezes it through its presumably thinner inner wall into vessels which send it up the root into the stem.

Such is the explanation we can give of the water-raising capacity of the root or root pressure . Alone it is probably insufficient to explain the raising of water to the summit of the highest trees ; and we also know that cut stems sunk into water are themselves able to absorb it. But in order to explain the reason of this absorption of water by the stem, we must first of all begin by studying the part played in this process by the leaves. The best way to do this, is to perform the following experiment. We cut off a small branch, say of a birch-tree, covered with leaves and dip its cut end in water. On taking the branch out of water we notice a drop suspended at the cut part, but in half a minute, if not less, the drop will be absorbed ; we dip the branch in again and notice once more the rapid disappearance of the drop, showing the greediness with which our branch absorbs the water provided for it.

The root drives water into the stem ; the stem greedily absorbs it and drives it further on. What will become of this water when all the parts of the plant are saturated with it ? Apparently, if it continually enters at one end it must pass out at the other. This passing out of water is very clearly observed under certain special circumstances. If on a warm damp evening in May or June we bend to the ground after the sun has set and glance at the surface of a field of oats, we shall observe round drops on the very tips of the upright blades. If we patiently observe one leaf for some time, we shall see the little drop growing larger and larger until it rolls down ; in its place, at the very edge of the leaf, another drop will appear and so on. The same pheno- menon will be observed if oats are sown on a plate covered with a glass bell. Small drops of water will continually appear at the tips of the leaves and dis- appear whenever the glass bell is removed. This phenomenon is seen in some plants much more clearly, and they give off water in considerable quantities. Anatomical investigation has even revealed special apertures in these leaves at the points of exudation.

But such exudation of water in the form of drops is a comparatively rare case. It generally happens under the conditions just described, i.e. when the surrounding- air is saturated with water vapour ; but, as a rule, plants give off water in great quantities in the form of invisible vapour. We can realise the amount of water evaporated by plants from the following approximate calculations : one acre of oats evaporates during the summer from 150 to 250 tons of water ; an acre of mixed meadow grass something like 750 tons. We can determine this amount of evaporated water in many ways. Here is one of the simplest and most exact methods. The pot in which a plant is growing is placed in a glass or tin vessel and covered with a glass or tin plate with a hole through it for the stem (fig. 47). In this way evaporation from the surface of the soil and the pot is prevented, and if we weigh the whole apparatus from time to time we know

that the loss in weight is due to evaporation from the plant. We can also take two glass bells of equal size, a little smaller than the leaf the evap- oration of which we are going to investigate, so that the leaf can be held tightly between them (carefully, of course, so as not to crush it, but at the same time in such a way that the greased edges of the bells are tightly fixed to it) . Under each bell we place some substance which greedily absorbs water vapour, such as sulphuric acid for instance, so that it may absorb the water which evaporates from the leaf. By weigh- ing the vessels containing the sul- phuric acid we shall find out the amount of water it has absorbed. In this way we can solve many interesting problems. We learn, for instance, that it is the lower side of the leaf, i.e. the one which, as we have seen, bears the stomata, 1 that gives off water vapour most actively. It appears that these stomata are to be looked upon as the regulators of evaporation. When the plant is saturated with water the slit-like aperture of the stoma opens wide (fig. 48 b) and evaporation increases ; but as soon as the leaves begin to fade, whether on account of too much evapora- tion, or too little water, the stomata contract and almost close (fig. 48 a ) ; evaporation decreases and the plant revives. We also learn from similar experiments that leaves with a bright shiny surface evaporate less than leaves such as grass ; this is the reason why plants with shiny leaves apparently stand dry, torrid climates more easily. Lastly, such experiments teach us that

young leaves evaporate more quickly than old ones of the same plant, and this fact affords a partial explanation of why the nutrient sap flows precisely to these young growing organs. Having learnt what a considerable quantity of water is evaporated by the leaves, we can return to the in- vestigation of the actual mechanism of this movement in the stem. This question has attracted special notice of late, and yet it cannot be said to have been settled quite satisfactorily. It is true that there are many explana- tions of the phenomenon, but their very abundance proves that none of them is completely satisfactory. Let us then dwell only on facts easily verified by experiment. To begin with, it was necessary to decide what course the stream of water takes up the stem : does it pass by way of the cavities, or within the walls of the vessels ? Contrary to the first most natural supposition that the sap circulates in the cavities of the vessels, since this would appear to be their simplest course and since these cavities form continuous channels through the plant, it has been pointed out that vessels are not generally filled with liquid, but contain bubbles of air alternating with columns of liquid. Yet this very presence of air, which at first was an objection to the theory that water circulated through the vessels, is now taken as a key to the explanation of the phenomenon. It happens that this air generally exists in a very rarefied state, and that owing to this circumstance every vessel acts as a pump. We can demonstrate this fact by the follow- ing simple experiment : a stem of any kind is bent down into a vessel containing mercury until one part of it is sunk under the mercury, and it is then cut across under the mercury . If we make longitudinal sections of the same stem later on, we shall see that mercury has penetrated into the cavities of the vessels

in the form of the finest threads. This phenomenon is seen best if the microscopic preparation is lighted from above instead of in the usual way from below ; then we clearly see in the vessels bright threads of mercury like that seen in the capillary tube of a thermometer (fig. 49, I). 1 Let us remember that mercury does not rise by itself as, for instance, water does in a capillary tube, but that on the contrary it can only be forced into such tubes by pressure — the narrower the tubes the greater must the pressure be. But the diameter of plant vessels is much nar- rower than the diameter of the capillary tubes with which experiments in physics are generally made. Hence we can measure approxi- mately the degree of its rarefaction that brings about this aspiration of mercury. Two questions naturally arise: why is no equilibrium estab- lished between the rarefied gases in the vessels and the external atmosphere ? and, what is the reason of that rarefaction ? The first question is answered very simply : the air inside the vessels is separated from the external parts of the plant, containing air at the general atmospheric pres- sure, by a layer of impenetrable tissue, which entirely isolates it from the external atmosphere. But when- ever the internal parts of an organism come into con-

1 Microscopes for handing round to the audience at lectures are specially convenient and have been much used of late years. They are provided with concave metallic mirrors which concentrate the light upon the upper surface of the object. tact with the atmosphere by means of a transverse section, the equilibrium between the internal and external atmosphere is suddenly established. This Is why it is necessary to cut through the stem under mercury. However quickly we dip a cut stem into mercury, we shall always remain without any result whatever. But if we leave the end of a cut stem sunk in mercury for some time, we shall find that mercury will begin to rise in its vessels. This experiment will give us an answer to the second of the questions raised above — how is the origin of this rarefied atmosphere to be explained ? Its explanation is as follows. Leaves evaporate water, and hence more concentrated solu- tions of substances contained in them are formed in their cells. These solutions, as we have already seen (fig. 46 on the right), draw fresh quantities of water from the neighbouring cells, and thus from cell to cell absorb the water stored in the vessels. But if water be drawn out of the vessels, the air of the bubbles alternating with, it takes its place, and increases in volume, i.e. becomes rarefied. As a result of this rare- faction, a fresh quantity of water is absorbed by the vessels from the cells of the root. The truth of this inference can be proved by direct experiments. If we make a section of the tip of a stem (bearing leaves) transparent enough to be placed in a drop of water under the microscope, we shall be able to learn the following facts. If small particles of a powdered sub- stance are suspended in the drop of water, we shall notice them tending towards the apertures of the vessels and flowing right through them. The bubbles that we see in the vessels will either decrease in volume with the decrease of evaporation from the leaves, or else increase, i.e . the air will become rarefied, with the increase of evaporation from the leaves (fig. 49, II, a and b ). 1

1 In Fig. 49 II, a and b , two consecutive stages of one and the same vessel Despite, therefore, the many doubts cast upon the subject, the part played by the vessels, as the aqueducts of the plant, can no longer be disputed. Together with the explanation of the part played by the vessels, the significance of one more peculiarity of their structure which long puzzled the most cele- brated anatomists has also been explained. This is the so-called bordered pit found in vessels and conduct- ing fibres (tracheides) . They are easily observed in the wood of our fir-trees, e.g. in a very thin section of almost any match-stick. If we make a longitudinal section in the plane of the axis and diameter of the trunk

(a radial section), we shall notice on the walls of the vascular fibres numerous rings with double outlines (fig. 50, A, a and b). On closer observation we notice a third less definite circle ( c ) lying between a and b. If the longitudinal section does not lie in the plane of the diameter of the trunk, but intersects it at a more or less acute angle, the figure will change. Instead of the sur- face view of the pit we shall see it at an angle, say of 45 0 (B), and ascertain that there generally are two inner (small) rings. In order to understand the structure of the pit more clearly, we must make a third section

are shown under the microscope during vigorous evaporation of water from the leaves. By comparing them we notice that drops of water have decreased at c, while air bubbles have correspondingly increased. at right angles to section A. This will give us the profile of the pit, i.e. its section (C, D, E), which will explain everything to us. It happens that the common walls of the two adjoining cells form here a cavity shaped like a lens, the margin of which corresponds to the outer ring of the pit (b). 1 This cavity seems to be formed, as it were, by two watch-glasses, perforated in the middle with round apertures (a in fig. A). These apertures lie one below the other, so that their out- lines blend into a single inner ring (a). In fig. B both circles are seen, owing to the fact that we are looking at them obliquely. The lens-shaped cavity is divided into halves by a veiy thin membrane stretched across it, the centre of which is thickened like a disc (C) . The edges of this disc, generally visible through the glass-like transparent cell-wall, give the impression of the middle ring ( c in A and B). Having realised the arrangement of these pits, and knowing that the air and water in them are under tension like that produced by a suction pump, their significance is easily understood. They are valves — most perfect valves. When the suction pressure in the vessels is not great, water passes through the thin membrane (as shown by the arrows at C). The resistance of these membranes to the movement of water is insignificant : if we let fall a drop of water upon the upper cut end of a long branch, the exudation of a similar drop from the lower cut end follows almost instantaneously. But these thin membranes might easily break under pressure such as we have observed in vessels. The membrane then curves, and the disc applies itself to the one or the other aperture according to the direc- tion of the pressure (as indicated by the arrows at DandE). Thus the bordered pit is shown to be a very conveniently arranged double valve, adapted to a variety

1 Dotted lines are drawn to show the corresponding parts of all the of pressures in vessels. These valves secure the uniform distribution of water in plants. Having seen that water moves through the vessels, and also the reason for this movement, we must try and discover its rate. We proceed as follows : we put the cut end of a branch of the plant to be examined into water which contains a small quantity of some substance the presence of which is easily detected in a plant ; thus, if we cut the stem across at intervals after some time, we can discover the height to which the water had time to rise during the experiment. The very greatest height ever reached in this way is approxi- mately seven feet per hour.

Evaporation from the leaves, which constantly draws water from the aerial parts of the plant, is therefore the principal reason for the absorption of fresh quantities of water from the root into the stem. But, one must ask, why do we ascribe this function of evaporation to the leaves and not to the stem ? The anatomical structure of the stem gives us the answer to this question. It is only at a very young stage that the stem has a skin or epidermis like that of the leaves ; this very early dies, splits, and falls off, while under- neath it, in the part of the stem called the primary cortex, there is formed a corky tissue. It is called cork because it is strongly developed in a certain species of oak, where it forms the material out of which bottle corks are manufactured. The structure as well as the external appearance of this tissue may vary very much : thus, for instance, in the cork oak it forms a con- tinuous layer several inches thick ; while in the birch- tree it is only a thin and scaly bark. In all cases, however, it has the same general property, namely, that it is impermeable to water, and thereby forms a kind of impervious covering on the stem guarding it from unnecessary or even harmful evaporation. It is a curious fact that this cork tissue spontaneously

appears when a vegetable organ is wounded, that is, just when it becomes exposed to abnormal evaporation, and thus puts an end to this unhealthy condition. Thus, for instance, we have only to wound any vege- table organ and thereby lay it bare and leave its inner tissues unprotected, and in a short time we shall see the skin of cork tissue forming over the wound. Thus the root drives water into the stem, the stem carries it along to the leaves, the leaves evaporate it into the air. It is only in the conjoint and uniform fulfil- ment of all these functions that the activity of the plant will be completely normal. The balance is upset when the plant evaporates more than it absorbs — then it withers ; the balance is also upset if the plant has no time to evaporate all the water it absorbs — then it begins to exude it in the form of drops, such as we notice on the blades of grass on warm damp evenings when, owing to the saturation of the atmosphere with water vapour, evaporation from the leaves has almost ceased.

We pass now to the investigation of another movement of the nutrient substances, tending not towards the leaf, but from the leaf towards all parts of the plant, including the root. The fact that there must be such a movement is evident a priori , because the organic matter out of which all the parts of the plant are built up is formed in the leaf ; the fact that it actually exists is clearly proved by the following simple experiment. Let us cut off a willow branch and place it in water. After a few days or weeks a kind of excrescence appears round the lower cut end of the branch, and from this excrescence there spring little roots. Evidently these rootlets must have been formed at the expense of matter obtained from the leaf, or else of matter which was already on the way from it, i.e. in the stem. Let us try to show the way by which it has come down to the newly formed