The Life of the Plant

roots, using for the purpose the same method as we used in determining the course of the rising sap. Let us make a circular cut in the rind right down to the cambium, as is shown in fig. 51, and place our branch in water for several weeks. We shall notice that the roots will appear this time not at the lower part of the stem, but at the upper edge of the circular cut ; in cutting across the rind we have evidently barred the way for the nutrient sub- stances which moved down the stem. It follows that the circular cut in the rind, which does not hinder the ascent of the sap from the root, com- pletely stops the sap moving in the opposite direction. This means that whereas the sap rising from the root passes by way of the wood, the sap coming from the leaves passes by way of the rind.

The truth of this inference is also proved by another experiment. Let us choose a branch of some plant, on which fruit has only just begun to set, and let us cut a ring in the bark between the fruit and the near- est leaves : the fruit will cease to develop. Thus the circular cut in the rind, which separates an organ, such as the root or the fruit, from the leaves it feeds on, will deprive that organ of the very possibility of development. We have therefore proved beyond doubt that the nutrient substances serving to build up the

various organs of the plant move through the rind. But the rind, as we have already seen, presents a complicated structure ; we differentiate in it the primary and secondary rind ; by way of which of these two systems does the nutrient sap move ? Again let us repeat our girdling experiment ; but this time we carefully cut only through the external part, the bark proper, being careful not to injure the secondary rind, i.e. the bast of the vascular bundles. We get results similar to those of the former experiment, i.e. roots are formed at the base of the branch. This proves that the movement takes place by way of the secondary rind. Let us venture one step further, and determine by which elements of the secondary rind this sap moves. We know that they are mainly two in number : bast fibres and sieve-tubes. A comparison of the mere form of these two elements makes it probable that the latter fulfil the functions we are considering, because the fibres have very thick walls with almost no cavities, whereas the sieve-tubes have broad canals, communicating with, each other by means of open pits, through which not only liquid and semi-liquid substances but even minute grains of starch can pass. This probability changes into certainty after the following experiment. We take an oleander branch and manipu- late it in the same way as we manipulated the willow branch in our second experiment, i.e. we cut off a complete ring of the bark right down to the cambium. A wholly unexpected result follows. Roots are formed not only at the edge of the cut, but also at the base of the branch — it follows that the nutrient substances pass otherwise than by way of the rind. This apparent contradiction is fully explained when we learn that the stem of the oleander shows a deviation from the typical structure of the stem above described.

Besides the sieve-tubes in the bark, bundles of these elements are also found in the pith ; and it is these which, in spite of the circular cut in the bark, convey the sap to the lower part of the stem. Thus all these four obvious experiments with willow and oleander branches gradu- ally and systematically reduce the circle of possible suggestions, and in the end point decidedly to the sieve-tubes as the course along which the nutrient sub- stances of the plant spread — the plastic substances, so called because they serve the purpose of building up new parts in the plant.

Recent investigations as to the distribution of the latex-tubes in a leaf lend a certain colour to the suggestion that they likewise serve as a very convenient course for the movement of the nutrient sap. This suggestion is based upon the fact that they are generally found in the immediate neighbourhood of the green tissue of the leaf where nutrient substances are manufactured. This suggestion is supported by the observation that the loss of latex exhausts some plants.

After having traced the course of the movement of the sap from the leaves, we have still to find out the causes which set it in motion. Again, for the last time, the key to the puzzle is found in diffusion — the word which like a constant refrain is repeated every time there is any question as to the absorption or transloca- tion of matter into the plant from the external medium, or from one part of the plant to another. According to the laws of diffusion, matter when dissolved apparently flows especially to places where it is changed into an insoluble form, either being deposited in store for the future, or simply spent in the building up of the solid parts of the plant . 1 The nutrient substances are deposited all along the system of vascular bundles. Cells surrounding these bundles are generally very rich in starch, and occasionally also in crystals and other matter. We have observed the storage of nutrient substances in the endosperm of the seed ; similar

storage, only in much larger quantities, is also found in other parts of the plant. It is deposited, for instance, in the pith, in the medullary rays, in a word, in the fundamental tissue of the stems. In the pith of the sago-palm, starch is stored in quantities which can be measured by hundreds of pounds ; potato-tubers also store starch, the beetroot an abundance of sugar, cabbage-heads or turnip roots the most varied nutrient substances ; lastly, in the fleshy leaves of the afore- mentioned Agave , sugar is stored up during many years. In fact, there is scarcely any vegetable organ which may not become the receptacle and store-house of nutrient substances. These stores are either used up the next year after they are deposited, as is the case with the beetroot or cabbage, where the stores are spent on the development of the stem and flower organs in the second year of the plant’s existence ; or else they are accumulated during many years, as is the case with the sugar in the leaves of the Agave , which is eventually spent in the formation of a huge branching inflorescence bearing the flowers and fruit. In every case storage is only a temporary, transitory destination of nutrient substances : their final destination is reached only when they are entirely used up in the formation of new parts of the plant, of new organs, new cells, i.e. when they contribute to its growth. Thus after having studied the phenomena of nutrition, in the sense of absorption, digestion, and translocation of food, we can pass in our next chapter to the study of the phenomena of growth.

In the folklore of some northern people the gods and diviners are endowed with a faculty for not only seeing but even hearing the grass grow. In the present chapter we shall investigate the question whether the eye and ear of a simple mortal can ever develop such acuteness as to see and hear the growth of a plant. Let us begin by settling in what sense we are going to use this term. By growth, in the narrow sense of the word, we shall understand the increase in bulk of the plant, which takes place as a result of the transformation of the assimilated food-substances into the solid skeleton of its structure, consisting mainly of cell-walls. Thus, although growth necessarily presupposes nutrition, these two processes are not bound to take place simultaneously. Growth can also take place under conditions which make nutrition for the time impossible, as in the absence of light. These two processes may in fact be carried on in different places and at different times. Growth is usually most active in the youngest parts of the plant, which develop at the expense of the activity of organs already developed and serving mainly for the purposes of nutrition. These two main functions of vegetable life, nutrition and growth, are sharply separated in time, particularly in those cases enumerated in our last lecture, where growth takes place at the expense of abundant stores of food, often the accumulation of many years. We have already seen that during germination the increase in bulk of the seedling does not depend on a correspond- ing addition of matter, but is accompanied by a

continual and important loss of substance, owing to respiration. Let us begin our survey of the phenomena of growth with the moment the little root and stem emerge from the germinating seed, when the one seeks, so to speak, to escape from the light as quickly as possible and buries itself in the ground, while the other stretches up into the air to meet the light. The first question that ought naturally to arise in our mind when w r e investigate this phenomenon, but probably one which seldom occurs to people, so accustomed are we to the fact, is : Why do the root and stem grow in different directions, the one into the soil, the other into the air ; the one downwards, the other upwards ?

This point has perplexed scientists very much, and even now it cannot be considered settled in all its details. In seeking the cause of this phenomenon scientists very naturally turned to light and the moisture in the soil. It has been pointed out that stems grow towards the light and roots away from it ; and consequently it has been said that light must be looked upon as the external force which conditions the direction of growth. But this theory is pretty easily proved to be untenable.. The direction of the organs in question will be the same in the absence of light ; moreover, if seeds are sown in a sieve suspended above a window so that they are lighted only from beneath, after passing through the layer of earth the roots will equally pass through the holes of the sieve and continue to grow towards the light, while the stems will grow upwards and therefore away from the light. The other theory that the direc- tion of the root is determined by the moisture of the soil is disproved by an experiment, in which germinating seeds are surrounded by moist earth or embedded in a wet sponge. The degree of moisture in such cases is uniform, and yet the direction of the root and stem will be as before, perpendicular.

Thus the main direction of growth of root and stem bears no permanent relation to light or moisture ; it is only their position with regard to the horizon that is per- manent : the root grows always downwards, the stem up- wards ; in other words and speaking more strictly, since this phenomenon is observed all over the globe, at the an- tipodes as well as herewith us, the root is directed towards the centre of the globe, and the stem away from it. This very constancy of direction points to the fact that the force to which it is due must be gravity, i.e. the attraction of our planet. This can be proved by experiment. If this direction of the parts of the plant depends upon the force of gravity, then by eliminating its action, we shall arrest the phenomenon itself ; by modifying the force we shall modify the phenomenon also ; finally, by substituting for this force another, which acts in a different direction, we shall change the direction of the phenomenon correspondingly. But how shall we actually do this ? how can we remove any body on the surface of the earth from the earth’s attraction ? How can we arrange matters so as to prevent the plant from having a top and a bottom ? It is clear that we cannot do it literally. We can, however, make gravity act at short intervals in opposite directions and thus counterbalance its own influence. For this purpose let us fix a germinat- ing seed to the rim of a rotating wheel (e.g. a wheel set in motion by electro-magnetic power). If the wheel moves in a horizontal plane (as is shown in fig. 52, A 1) it will certainly not prevent the root from growing down- wards and the stem upwards. But if the wheel moves in a vertical plane (fig.

52, B 1), or, what comes to the same thing, if we fix the germinating seed to the minute hand of a clock, it is clear that at every half-turn the position of the root and stem will change ; in the end a top and a bottom, a right hand and left hand side will cease to exist for the seedling ; the continual action of the force of gravity in any one direction will be prevented. Experiments performed under these conditions have proved that the root and the stem may be made to assume any desired position, and generally to keep growing in the direction in which they have been fixed (see fig. 52, B 1).

So far we have supposed the wheel to rotate slowly, but yet quickly enough to prevent the seed from .remain- ing a long time in the same position with regard to the horizon : now let us make it rotate more quickly. In this case a centrifugal force will be set up, just as when we quickly twirl in the air a heavy body of some kind at the end of a rope. This force acts, as it were, from the centre to the circumference, as is easily seen by the following experiment. A ring is placed on a smooth spoke near the axle of a wheel which rotates horizontally. As soon as the wheel begins to rotate at a moderate rate the ring begins to slide down the spoke until it touches the rim of the wheel. Therefore when this centrifugal force acts upon bodies it compels them to move in the direction away from the centre towards the circumfer- ence of the wheel. It is clear that this force cannot remain without influence upon germinating seeds. In fact if we make the wheel B rotate fairly rapidly we shall observe that rootlets and stems will assume a certain

definite position : the rootlets will grow in the direction of the force, i.e. away from the centre, while the stems will stretch towards the centre of the wheel (fig. 52, B 2). Now let us see what will be the effect of a wheel rotat- ing quickly in a horizontal position. Obviously the con- ditions will be different here from what they were in the case of the vertical rotation. There the influence of the force of gravity was completely neutralised and the centrifugal force alone was directive. When the wheel rotates horizontally, on the other hand, both forces act. The force of gravity alone would have induced the root to point in the direction indicated by the arrow n (fig. 52, A 2). Centrifugal force alone would have caused it to point in the direction indicated by the arrow m. With both forces acting simultaneously it must necessarily assume an intermediate position, as is shown in the figure— a position that will be nearer to the horizontal the stronger the action of the centri- fugal force, i.e . the larger the wheel and the more quickly it rotates. Experiment fully confirms this hypothesis.

Thus the direction of the different parts of the plant depends upon a force tending towards the centre of the earth. By neutralising the action of that force (as on a slowly rotating vertical wheel) we destroy its influence. By the action of another force (as in the experiment with the wheel rotating quickly in a hori- zontal position) we modify the effect accordingly. But only one force is known to us which corresponds to these facts, and that is the force of gravity, i.e . the attraction of our planet. Finally, we can cause similar phenomena by substituting centrifugal force for the force of gravity (as in the experiment with a wheel rotating quickly in a vertical position) . We shall then see that the two organs tend in the corresponding directions, i.e. the root in the direction of the action of the force, the stem in the opposite direction.

It follows that the attraction of the earth is the force which determines the permanent direction of the growth of the stem and root. But it is one thing to indicate the force to which a phenomenon is due and quite a different thing to explain just why and how this force acts in that way. As a matter of fact it would be quite easy to under- stand that the force of gravity would make the root grow towards the centre of the earth ; but how are we to understand that, under the influence of the very same force of gravity, the stem tends on the contrary away from the centre of the earth ? This is, nevertheless, exactly what does happen. It is not only that a stem placed vertically continues to grow in this direction, but even a stem laid horizontally bends sharply upwards. Here is a small seedling of cress, which some hours ago was placed flat on a glass plate (a, b, fig. 53). Its little stem has turned up as you notice, has reached the posi- tion of n from that of m. Here is some cress, grown up on a piece of felt. First the felt lay horizontally, later on I placed it on its edge, then successively upside down, on the other edge, and horizontally again. In this way the

stems have changed their position four times with regard to the horizon and after having de- scribed a whole circle, and twisted themselves into a knot they continue to grow upwards. away from the direction of its action. How is this to be explained ? We must, of course, always keep in view not the stem only, but also the root. Only such an ex- planation can be considered satisfactory as will explain both the reason why the stem rises and at the same time the reason why the same thing does not happen to the root. The desired explanation must consist in a difference of structure of some kind between the stem and the root, because we cannot admit that one and the same force will act on exactly similar bodies in a different way.

Let us see what explanation we can find for the growth upwards of stems. In order to do this we must begin by studying a curious property of vegetable organs, a phenomenon knowm as the tension of tissues. Let us cut a long piece from the middle of a young growing stem, as is shown in fig. 53, p , where the shaded part represents the epidermis and cortex, and let us moisten it with water to prevent it from drying up ; then let us split it longitudinally into two halves with a sharp knife. Both parts will immediately curve as in fig. 53, r . This curvature can only be caused either by the outer side of each part becoming shorter or the inner side longer than before, or both at the same time. At all events we come to the conclusion that in the undivided section the external and internal parts are in a mutually strained state ; the one stretches the other, and is itself restrained in its tendency to elongate by the resistance the other offers to extension. We can prove this fact by making two slits instead of one, and separating the external tissue in two parts and disengaging from between them the middle tissues (fig. 53, s). We shall now actually see that the middle part will stretch and become longer than it was before at p , while the external parts will shrink and become shorter than they were at p . It is clear that the inner parts of the stem tend to elongate, but being opposed in this tendency by the external parts stretch these instead. This mutual tension of tissues plays a very important part in the life of a plant ; it is to this property that delicate and succulent stems owe their rigidity.

Tissues consisting of very thin cell -walls and liquids would not by themselves be rigid. It is only when cells become overfilled with liquid till their walls distend, and the inner tissues press upon the outer ones and themselves are compressed, that an organ becomes turgid, and does not easily bend over or droop as do fading stems in which, owing to an insufficiency of water, the tension of the membranes in individual cells as well as the mutual tension of the tissues is weakened.

Let us make a comparison, rather rough it is true, but one which will give us a general idea of what takes place in the plant during its growth. I hold a glove in my hand. Its empty fingers hang down. I breathe air into one of the fingers and seize it near its base, — now it can preserve the vertical as well as the horizontal position without drooping, or bending. This finger filled with air represents to a certain extent a cell over- filled with sap or a stem, the external parts of which remain stiff under the outward pressure of its more rapidly growing inner parts.

Let us now discover the relation between all that has so far been explained and our original question : Why is it that a stem laid in a horizontal position curves upwards by itself ? Whilst a stem remains in a vertical position, the force of gravity acts uniformly on all its parts ; but as soon as we place it in a horizontal position, the conditions change. Owing either to a stronger current of the nutrient substances or to other causes, the lower part will grow and stretch more quickly than the upper. We already know that the tendency of the inner part of the stem to stretch is continually checked by the resistance of the epidermis outside. But in the horizontal stem the lower half of this inner part will grow more quickly and will at the same time stretch the epidermis unequally — stretching the lower and nearer part more strongly than the upper and more

distant part . 1 Moreover, the lower epidermis itself will grow more quickly than the upper, and consequently will more easily yield to stretching. This explanation is also supported by the fact that upward curvature in a lying stem only takes place in the part which grows most rapidly ; in parts where growth has already stopped such a phenomenon is impossible. Therefore by placing the stem in a horizontal position we cause in it unequal and unsymmetrical growth ; the lower side gets ahead of the upper, the stem curves and rises. But this method of argument may appear unconvincing ;

in that case it can be enforced by a direct experiment. Let us take two similar stems, leav- ing the one to grow vertically and obliging the other to grow horizontally by pushing it through a narrow glass tube where it will not be able to curve. After a certain time let us split the latter stem into an upper and a lower half. The moment we do so the upper part will shorten and the lower elongate, and if we compare them with the length of a vertical stem we shall see that the

1 It is quite clear that the resistance exerted by the upper half of the epidermis against the tendency to stretch will be stronger because it acts so to speak on the longer arm of the lever, whereas the lower acts on its shorter arm (see fig. 54, II., c'). upper half of the horizontal stem is shorter and the lower longer than the vertical stem, as was to be expected. What is true of a horizontal stem can be applied also to one in an inclined position : as soon as a stem deviates from the perpendicular, the force of gravity by causing increased growth of the lower side of the stem brings it back to its vertical position.

Now we understand why it is that owing to the force of gravity the stem curves in a direction con- trary to the direction of this force. But a question arises : Why is it that the same thing does not happen to the root ? You see now that our closer investigation of the phenomenon has reversed the question. At first we thought it quite natural and comprehensible that the root should grow along the line of the force of gravity, and incomprehensible that the stem should grow in a contrary direction ; whereas now we under- stand why the stem grows exactly as it does, and find it difficult to understand why it is that the root grows differently. Let us turn for an explanation of this apparent contradiction to the following model. Let us imagine two wooden discs (fig. 54, I., c and d), joined by means of springs to a cross bar ( b ) . Two pliable india- rubber rods (a, a) joined by a transverse handle (e) are passed through holes in this bar. Their ends press against the centres of the discs c and d. If we push these rods in the direction of the arrow, we shall separate the coils of the spirals and bring the two springs into a state of tension. The india-rubber rods in our figure are meant to represent the rapidly growing axial parts of organs, while the compressed coils represent the slower growing external tissues of these organs, distended by the growth of the inner parts. This movement demonstrates sym- metrical growth, and the tension of tissues resulting from it. Let us now try to show by means of the same figure unsymmetrical growth, such as is caused by the action of the force of gravity, in which the lower part

of an organ grows more quickly. We do this by mak- ing the points of contact of the india-rubber rods with the discs lie this time near their lower edge instead of at their centres (fig. 54, II). On pushing in the handle as before, we notice quite a different result ; while the lower spiral elongates in a straight line, or even hangs down a little (d') under its own weight, the upper one curves upwards ( c ') more or less sharply. This result is easily explained by the construction of the model. The springs are chosen purposely of varying elasticity : the upper one of much thicker wire exerts a stronger resistance to the movement of the bar than the lower one of thinner wire. We infer that the un- equal, unsymmetrical pressure manifests itself in a visible curve only when there is a certain degree of mutual tension between the parts. Evidently the same can be applied to growth. Unequal and unsymmetrical growth will be followed by a sharp and perceptible curvature of the organ only when this organ attains a certain degree of turgidity owing to the mutual tension of tissues. But does a young growing root actually show such tension of tissues as we saw in the stem ? A glance at such a root will convince us of the absence of such tension inside it. If we hold a stem horizontally it will not bend, nor droop, whereas a root will frequently hang down like a stem already withered. If we study the structure of the epidermis in the stem and the root, we shall find a further difference which has long attracted the attention of anatomists.

The epidermis of the stem consists of cells with thicker walls, and moreover is covered with a special membrane which it is difficult to moisten with water, and which is very elastic ; on the other hand the epidermis of the root consists of cells with thinner walls, it easily absorbs water, and therefore is more easily stretched, and less elastic than the epidermis of the stem. If now we make a direct experiment similar to the one we made with the stem, we shall see that there is not the same tension in the root as in the stem. If we split a length of root into two halves (like the stem in fig. 53, r), no curvature will be noticed in these halves ; if it be cut into three parts (53, s) no elongation of the middle part, nor shortening of the outer part, • will be noticed. To sum up, the root has none of the tension of tissues peculiar to the stem ; its external parts grow as quickly as the inner ones. This is also evident from one more property of the root : a young root generally elongates more quickly than a stem, and this is why it does not show any tension, which is nothing but restrained growth.

Thus if the force of gravity does not cause the upward curvature of the growing root-tip, this is partly explained by the absence of a mechanical condition necessary for the purpose: it lacks the corresponding tension of tissues. Our model shows how, given merely a difference in the structure of two organs, we may get entirely opposite results from a similar action of the force of gravity. It is useful to remember this whenever physiological facts are discussed. If one and the same external factor causes different effects in different organs, we must admit either a difference in the pro- perties of the organs or a complexity in that same factor. This second alternative is impossible with regard to the force of gravity ; but the difference in the tension of tissues is certainly not the only possible difference between the properties of the stem and root.

Our explanation would be perfectly satisfactory could we further prove that the lower half of a root lying in a horizontal position grows more quickly than its upper half, as is the case with the stem; and that in spite of this, owing to its own weight, it passively bends down like our spiral d\ II in fig. 54. Some experiments seemed to prove this, but doubts have arisen in connexion with them, because contradictory results have been subsequently obtained, so that the question as to the mode of action of the force of gravity upon the root must be considered as still open . 1 Further on we shall see that this question becomes considerably complicated, and that in order to explain the phenomenon we must take into account not only the structure of entire organs, or of the tissues that form them, but also the details of the structure of the cells forming these tissues.

Now let us see what other external conditions influ- ence the phenomena of growth. In looking for the causes which determine the natural, vertical direction of growth of both the stem and root, we have proved that it does not depend on the light. Further, we have seen that growth is possible even when light is com- pletely absent : potatoes and turnips put out long shoots in the total darkness of cellars. The same can be proved by experimenting with any seed or shoot ; they all will grow in the dark.