The Life of the Plant

Are we entitled to infer from this that light does not influence growth ? Not in the least. A very simple experiment will prove how considerable this influence is. If we let cress seeds germinate in two pots filled with exactly similar soil, placing one of them in the dark and leaving the other in the light, the difference will not be long in showing itself. Cress grown in the dark will sometimes be ten times longer than that grown in the light ; but its stems will be slender and unhealthy, and

1 For instance, the considerations generally brought forward by botanists concerning the growth of the root-tip in mercury are far from being convincing, two different phenomena having been confounded in these experiments: growth (of the whole organ in length) and curvature (depending only upon the difference in growth of the upper and lower sides of the curving part). A root exerts its pressure first of all because it grows, and it is quite obvious that this pressure caused by growth has nothing to do with the weight of the root, any more than the weight of the flexible rod (fig. 54, IX d) when overcoming a certain resistance and at the same time bending passively downwards,

many of them will droop. Cress grown in the light will have short but healthy, thick and turgid stems. This means that light is not without an influence upon the growth or rather the elongation of stems ; but this action is not an accelerating or favouring of growth in length, but on the contrary" an inhibiting of it. The influence of light is not limited to this retarda- tion of growth. If we have a plant in a room so that it receives light always from one side, we shall see that its young growing stem will bend over and turn to the light, as we generally say. Evidently we have no right to attribute any attractive power to the rays of the sun, and there is no need to have recourse to such a futile hypothesis. By comparing the two experiments just described we can infer what is the explanation of this phenomenon — of the turning of stems towards the light. Light retards the growth of stems, but a one-sided illumination will not act with the same effect on both sides of the stem — the front will receive all the light and the back will always remain in the shade. The front will consequently grow a little more slowly than the back, and the result will be the turning towards the light. In a word we have here a case contrary to the action of gravity. The force of gravity accelerates growth on the side turned towards the centre of the earth — the stem withdraws from it. Light retards growth on the side turned towards its source — the stem turns to it. The name heliotropism has been given to this phenomenon.

But if light retards the growth of stems, does it not follow that plants must grow chiefly during the night ? This question has been raised many a time, and has been settled in various ways. These contradictions must not puzzle us, because the point itself is a very complicated one, and the observation of growth at such short intervals requires rather delicate methods of investiga- tion, which science has come to possess only recently. In fact, apart from some rare cases, 1 growth in length during an interval of ten to twelve hours is not con- siderable enough to be easily observed, if experimental methods had not stepped in to help us where our sense organs appear to fail. Let us see what are the methods science possesses for demonstrating longitudinal increase in growth, which, owing to its insignificance, escapes immediate observation. We turn for the purpose to the microscope, i.e. we can magnify the object of observation, or use another method which will demonstrate in a magnified form not the plant itself, but simply its motion called by us growth. The most convenient microscope for the purpose is the so-called 1 solar microscope/ which makes it pos- sible by means of sunlight, or some other sufficiently strong artificial source of light, to throw the image of an object consider- ably magnified upon a screen, as we shall now proceed to do with the root- tip of germinating cress. Once on the screen we shall draw a pencil line round the image and leave the root growing (in water), and return to it at the end of the lecture to see that Fig. 55. it has succeeded in growing con-

siderably in the interval. Mean- while, here is a figure showing (fig. 55) the successive outlines of a wheat root, observed about every five minutes for an hour. 2 1 Such are, for instance, the shoots of the bamboo, and the inflorescences of Agave mentioned in one of the previous chapters, which grow several inches a day ; such are also the spiral stalks of ValUsneria , a plant known to all lovers of indoor aquariums (see chapter viii.) 2 Fig. 55 shows the successive outlines of a wheat root, projected by means of a microscope and a magic lantern.

The sensitiveness of the method as demonstrated in this instance leaves nothing to be desired, but it would be less convenient for investigating the growth of larger organs or whole plants ; in such a case we have to use the second of the methods indicated, i.e. instead of magnifying the growing organ itself, we magnify only the motion of the growing parts. We use for the purpose an apparatus, the main part of which consists of an index fixed to the axis of a small pulley (fig. 56, I, a). Over the pulley a silk thread is thrown to one end of which a small weight ( b ) is attached, and to the other a small hook (c) made of thin wire. Catching the apex of any stem with the hook (a small wound as the result of the prick will not do it any harm) we let the weight hang freely on the other side of the pulley, and pull down the silk thread. Let us now suppose that our stem has grown a little ; what will be the conse- quence ? In growing, the stem will slightly loosen the silk thread, and the weight will fall as far as the plant has grown ; at the same time the silk thread lying close to the pulley will oblige it to make a turn of correspond- ingly insignificant magnitude owing to the friction it exerts upon it. With the pulley the index will also turn ; but its point will naturally trace a much wider course, so that an imperceptible movement on the part of the tip of the growing stem will cause an already very considerable movement of the index point. This, latter movement will exceed the former as many times as the length of the index exceeds half the diameter of the block. In our apparatus half the diameter of the block is equal to two millimetres, the length of the index is twenty-centimetres, i.e. a * hundred times as much ; therefore every increase in the length of the stem will be shown by a movement of the point of the index a hundred times as great.

The advantage of this instrument is obvious. We may fasten behind the index a circular scale with divisions, like those on a clock, and read what division the index points to from time to time. But we can do something better still : we can turn this apparatus into an automatic one, and make the plant itself record the progress of its own growth during the different hours of the day. For this purpose let us apply the point of the index to a cylinder, the axis of which is set in motion by clock-w r ork, so that it makes a complete rotation in twenty-four hours in the

direction indicated by -— jxp - — , r j l j the arrow. In order that on it will be almost hori- zontal (as is a ... n in fig. 56, II). On the other hand, if the arrow moves very rapidly in comparison with the rotation of the cylinder, it will trace an almost vertical line (a . . . m) . If the arrow moves at a moderate rate the line will be oblique : the steeper its inclination the quicker must the movement of the index have been ; the more the line slants, the slower the movement of the index. A glance at the line a, b, c, d, e, f shows that the stem grew rapidly from a to b, slowly from b to c, and so on. Knowing how long the cylinder was revolv- ing, we can tell also to what hours of the day the rapid growth and the slow growth respectively correspond, and are able to trace the influences controlling such acceleration and retardation. The plant, so to speak, writes down its own impressions, as we have already said.

The sensitiveness of the indications of this apparatus depends upon the length of the index. It is incon- venient in many ways to use a very long index ; and therefore, when we wish to have a very sensitive apparatus such as will enable us to observe growth during exceedingly short intervals of time, during one minute, for instance, or, as in the present case, to demon- strate this phenomenon of growth to a large audience, we have recourse to another method. We use some- thing intangible instead of the index — a ray of light, to which we can give any desired length without technical inconvenience. For this purpose, instead of the index we fix to the axis of the pulley a small mirror (fig. 57, inn). Light coming from a lamp or a candle placed in front of this mirror will be reflected and form a bright spot somewhere on the wall. The

little hook at the end of the silk thread is fixed as before to the plant, and naturally every increase in growth which results in even the minutest rotation of the pulley, and the mirror connected with it, will cause a considerable displacement of the spot of light. If the index magnified the amount of growth a hundred times, the apparatus with the mirror will magnify it many thousand times, or, generally speaking, any de- sired number of times, since the magnification only depends on the distance of the mirror from the wall. In order to be able to judge more conveniently, as well as more exactly, of the displacement of the patch of light, large divisions are marked on the wall. Let us notice the mark at which it stands at present — it is exactly on the figure io — and let us leave the plant (which happens to be a shoot of asparagus) to grow in peace, and then return to it at the end of our lecture.

Possessing such perfect means for observing growth, botanists have been able to study a whole range of questions concerning these phenomena. Thus for instance, the reason for the above-mentioned fluctua- tion in the rate of growth with night and day has been investigated. In order to settle this question it was necessary to realise that light is not the only condition that influences growth ; it is affected also by the amount of moisture, and especially by temperature.

By growing plants in the dark and a constant amount of moisture, and varying only the temperature, we find that they grow more quickly at a high and more slowly at a low temperature. If we make the temperature rise and fall alternately, we shall notice that the index of the recording apparatus described above will draw upon the surface of the cylinder a line similar to the line a, b, c, d, e,f (fig. 56) , where every more steeply inclined part of the line will correspond to a warmer interval and every more sloping part to a colder one. This means that heat acts in an opposite way to light : while light retards

growth, heat on the contrary accelerates it ; which as a matter of fact was known very long ago to gardeners, who base upon it their method of forcing plants, ac- celerating or retarding their grow r th in order to make them develop in readiness for a certain date. Evidently the question as to the time when the plant grows most is not as simple as it seemed to be. It is dark in the night but also colder ; it is light in the day-time but also warmer. It is difficult to say beforehand which of the two influences will preponderate in any case ; the only obvious thing is that growth must be most energetic during a dark and warm night, and least energetic on a bright and cold day.

We have brought forward the simplest explanation of the phenomena of heliotropism, i.e. of the inclination of stems towards the source of light ; but many people are not satisfied with it, because together with the general phenomenon of bending towards light , there are also cases, comparatively rare it is true, of bending away from light ; or, as it is sometimes stated, amid the preponderating phenomena of positive heliotropism there are some rare cases of negative heliotropism. This objection, which has obliged many botanists to give up the explanation just brought forward, may be easily re- moved on the strength of the more recent discovery that one-sided heating can cause phenomena similar to those of heliotropism, and called thermotropic phenomena. Evidently the result of thermotropism will be quite the opposite. Heat accelerates growth — therefore the heated part will grow more quickly and the organ will bend away from the source of heat. But sunshine acts at the same time both as light and as heat ; it is obvious, therefore, that according as the one or the other influence predominates the organ will bend either towards or away from the light. We have just mentioned that the production of different effects attributed to a single factor may depend either upon a difference in the pro-

parties of the organs or upon the complexity of the apparently simple factor. Here we have, it appears, an example of the latter case. The way in which botanists conceived the dependence of growth upon external influences became of necessity considerably more complex after the brilliant and always original investigations of Darwin. He showed that the point affected by the external influence may not coincide with the place where its effect is mani- fested. Thus, for instance, the force of gravity appears to act chiefly on the root-tip, although its effect is manifested by geotropic curvature in the region of most vigorous growth, lying some distance from the tip. We come to this conclusion because rootlets, the tips of which have been cut off, hardly ever curve until a new root-tip is produced. Stems appear to behave differently ; but on the other hand some shoots show a similar behaviour in relation to light, a phenomenon which is not observed in roots. Thus, for instance, the tip of an oat seedling, and, especially, of the seedling of canary grass, are remarkably sensitive to light. If we cover these with caps made of tinfoil, the helio- tropic curvature generally observed in the lower parts is considerably weakened.

These facts were sufficient to make some botanists think that in the root-tip and at the tip of the seedlings of cereals there existed special sense-organs, which communicated their impressions to the growing parts in some unknown way and caused them to curve. We shall see presently that there is no reason whatever for presupposing any such sense-organs or nerves in plants ; here we need only say in passing that there is no reason whatever for such an explanation of the facts just described until other simpler explanations have been exhausted ; and this condition, for all scientific explanation, is as yet far from being fulfilled in the sphere we are now engaged in studying.

Having now grasped in general outline the influence of the chief external agencies, light, heat, and the attraction of the earth, let us try and study more closely the very essence of the phenomena in question. So far we have investigated the plant as a whole ; but the life of the plant is the sum total of the life of innumer- able cells. Let us see how the development of cells is related to the general phenomena of growth. We know that every cell in the course of its life-time increases in size, changes its shape and the structure of its walls — in a word, grows. We also know that whatever be the size of the plant, it commences as a single cell and eventually contains millions of them. It is evident that the growth of a whole plant depends upon two phenomena, the growth of single cells and their reproduction.

It is now necessary for us to catch a glimpse of how these phenomena of the growth and reproduction of cells, which underlie the growth of the plant as a whole, take place. We must choose for the purpose appropriate material, an organ or organism in which we can study a living cell without disturbing it. We find such appro- priate material in the filamentous weeds that form the main part of what is generally known as 1 green slime.’ If we place one of these green filaments under a microscope, we shall see that it consists of a single straight row of cells. Fig. 58 shows at the top such a cell with the green matter we have called chlorophyll, which gives plants their green colour distributed in a very characteristic way. Here it forms green bands with toothed edges, twisted spirally round the inner surface of the cell-wall : hence its Latin name of Spirogyra. Apart from this peculiarity the cells of Spirogyra do not differ in any way from the ordinary type of a cell. The same wall of cellulose is found in

them, and within it the same protoplasm and cell-sap; while in the middle, like a spider in its web, we find a nucleus connected with the wall by very fine strands of protoplasm. We can observe such a filamentous weed in a drop of water under the microscope for several hours, and even . days, and submit it during that time to different temperatures, and to varying illumination. In this way we see, for instance, that in the absence of light cells grow or rather elongate more quickly than in the presence of light. Light has a similar retarding influence on the multiplication of cells ; at all events under normal conditions, this pro- cess takes place mainly and sometimes even exclusively at night. Formerly investigators had to arm them- selves with much patience : they either had to sit up part of the night to observe one and the same cell, or else to lay aside, in spirits of wine, specimen filaments every hour or so, and subsequently observe the succes- sive steps of the process in these different cells. 1 Nowa- days the same result may be reached in a far easier way : when a vessel containing the weed is placed for the night in a cool place like a cellar, the process of multiplication is arrested till the next day, so that in this way we can always take advantage of the more pleasant hours of the day-time instead of the less pleasant hours of night for observing the process in the same cell.

The process itself is very simple. It consists in the division , the splitting into two of the contents of a single cell. This happens in the following way : when a cell has reached the stage of development at which it divides, two outgrowths from the wall appear projecting into the cavity of the cell (fig. 58, C, D). The cells under investigation are cylindrical in form. By making them rotate round a longitudinal axis by 1 Under continual artificial illumination division also takes place in the presence of light. Evidently the light does not interfere directly with division, but rather promotes the activity of the cell in another direction.

go on observing the same cell we shall notice that this girdle will grow deeper and deeper into the cell, dividing its contents in half. About this time two nuclei are observed instead of the single central one. In the end the girdle joins up completely in the middle, forming a continuous partition across the cell. Two cells are thus formed out of the one, each with its own nucleus protoplasm and chlorophyll band, and separated by a partition of the same cellulose that forms the external walls. A similar process of division takes place in each of the newly-formed cells after it has reached maturity, and so on. In the end a whole series, a whole filament, of cells is formed out of a single cell.

Such is the process of division in the simplest cases observed, but in most cases it takes place a little differently. Scientists were struck by the fact that during the growth of tissues in the highest plants, one has never the chance of observing the above-mentioned gradual ingrowth of a new partition into the cell cavity. The partition seemed to appear almost in- stantaneously, but on closer investigation they found out that here also it is formed gradually, though in a little different way. This became apparent when due attention was paid to a part of the cell we have already mentioned more than once, and upon which we shall now dwell. This is the cell nucleus, to which whole volumes have lately been devoted. In Spirogyra we do notice that the process of cell division is preceded by the division of the nucleus, but in most cases the con- nexion between the two processes is closer still. The division of the cells is preceded by a series of changes which take place in the nucleus with invariable uni- formity, and, what is very curious, are almost identical in the cell-division of plants and of animals. Under the microscope two component substances can be easily differentiated in the nucleus ; the one is easily stained with different colouring matters, the other is not. Some time before division takes place, the easily stained substance — chromatin — has the ap- pearance of a bundle of tangled threads (fig. 59, A). Later on these threads break into short pieces, which group themselves (as is shown in fig. 59, B on the right) in the equatorial plane of the nucleus. Later still these pieces, of which there are usually a definite number, split into halves and are drawn towards the poles of the

nucleus, which by this time has assumed the configura- tion of a spindle, with a striped appearance (these stripes are not stained, hence the substance which forms them is called achromatin, fig. 59, C). When the chromatin has gathered at the poles and become bunched up together, so that we have already two nuclei (fig. 59, D) and division of the nucleus is complete, the division of the cell begins. Again in the equatorial plane of the spindle, there appear minute grains (fig. 60, 1) which blend later on into a kind of plate consisting of the same substance as the cell- wall, i.e. of cellulose (fig. 60, 2) . As it forms, this plate pushes itself against the cell-wall, and we see a partition dividing the cell into two parts (fig. 60, 3). Each of the newly-formed cells has a nucleus of its own, and starts an independent existence, grows, and on reaching a certain dimension divides in its turn.

We had before a general explanation as to why the plants elongate more quickly in the dark. This depends on the fact that both the elongation of the cells and their division — the two phenomena which determine the growth of organs as a whole — take place more energetic- ally in the absence of light. But can we not find out the approximate cause of this inhibitive effect of light ? There are investigations which do to a certain extent explain this phenomenon, and which at the same time are connected with a question of great practical import- ance. It was long ago known that stems grown in the dark appear to be more watery and to have less tension in their tissues. They are in general less turgid, as is easily seen in cress grown in the light and in the

dark. But a difference similar to that noticed in these extreme cases (i.e. between stems grown in the light and in total darkness) could be shown — to a smaller degree, it is true — in plants grown respectively in the shade and in a bright light. The suggestion has been made that some such effect might explain the laying of crops which, as we saw, people have vainly attempted to explain by want of silicon (see chapter iv.). This supposition might well have been based upon the fact that for the most part it is cereals which are sown very thickly that are badly laid, while this never happens with thin crops, however strange it may seem at first sight. The following' method was used in order to shade the plants of a crop artificially. One or more plants were surrounded with a drain-pipe ; as soon as

the straw rose above it, a second pipe was added on the top of the first, and so on. It is clear that under such circumstances the plant received light only from above, being always shaded at the sides. As was expected, these conditions produced very high, weak, and flexible straw. On submitting this straw and normal healthy straw to a comparative microscopic investigation, the following difference was observed : the former had per- ceptibly longer cells, but the walls were con- siderably thinner than those of the latter, the cells of which were shorter and had thicker walls. It follows that light does not, so to speak, retard growth, but apparently only modifies its direction. Instead of growing in all directions, the cell -walls thicken. The same difference has been observed when healthy straw is compared under the microscope with laid straw, as may be seen by comparing the transverse sections of two pieces of straw, one of each kind, in fig. 61 . All the cell -walls of the normal one (on the right- hand side) are much thicker, and the lower rows (the external layers of the straw) are even so thick that the cavity is reduced almost to a point. (The lines in fig. 61, joining the cavities of adjoining cells, are the pits.) As well as having their walls thus thickened, the cells of the straw of a normal specimen are con- siderably shorter in longitudinal section. Therefore it is in a too rapid elongation of the straw, together with an insufficient thickening of the cell-walls caused by mutual overshadowing in a thick crop, that we must

look for the real reason of the laying of crops. This bad effect can be prevented by sowing the seed more thinly, or by drilling. Then every plant will get sufficient light for its normal development. These facts lead us to contradict one more current idea about the growth of the plant. We have already seen that growth does not always imply increase in matter — during germination the increase in volume is accompanied by a loss of substance. We now see that growth does not always imply the elongation or expansion of an organ, because sometimes growth can show itself in a different way — in a thickening of the cell-walls. Strictly speak- ing, we see that a plant grows only when its cell-walls grow, whether in length, in breadth, or in thickness.

But it is not only light which may modify the form of cells. Occasionally this is also possible as the result of strictly mechanical causes. We have seen in our last lecture that so-called annual rings are noticed on the transverse section of the trunk of a tree (fig. 44, III). These rings are most clearly marked in fir-trees — in a pine, for instance. The reason for such rings appears to be connected with the periodical cessation of the vegetative processes during the winter. Neverthe- less, if in spring, i.e. after the winter’s rest, new rows of quite similar cells were added to the cells deposited the previous autumn there would be no difference whatever between the adjoining layers ; the boundary between them would be imperceptible, and they would all blend into one continuous mass. The layers are clearly marked only because the wood formed in spring is distinctly different from the wood formed the previous autumn. Even with the naked eye we may see that two parts can be differentiated in every annual ring : the spring or early wood which lies nearer the pith, and the autumn or late wood which lies nearer the bark ; the former is looser and therefore lighter in colour, the latter is denser and consequently darker. We notice

this alternation of dark and light layers in every splinter, even in a match. The microscope betrays the immediate reason for this difference. This figure (fig. 62) shows a small piece of pine wood, the transverse section of a match. Across the middle of it runs the boundary between two annual layers. 1 In the lower part lie some summer and all the autumn cells, say of last year ; in the upper the spring cells of the present year. The sharp transition from the autumn to the spring cells is easily perceived : the former have a flat shape, thick walls, small and narrow cavities ; the others are almost square, have thin walls and large cavities. For a long time botanists could not account for this change in the form of the cells laid down at different seasons of the year, until they conceived the idea that the fact must depend on the mutual pres- sure, the mutual tension of the tissues. We made

1 The transverse line dividing the section into two parts-— to the right and to the left — is the medullary ray (see previous chapter). acquaintance a little while ago with phenomena of longitudinal tension, i.e. of tension depending on the unequal elongation of different tissues in a stem. Similar inequalities of growth can and do also take place in the transverse direction. The bark is con- tinually compressing the wood which grows more quickly than itself, and in its turn it is continually stretched and strained by the latter. This is proved by the longitudinal fissures that generally appear in the bark owing to the internal pressure of the wood. It is not difficult to see that the smoothest bark is usually in a taut condition, and therefore presses on the wood. If we make a longitudinal cut with a knife, we shall see that the wound will gape ; or, better still, if we cut off a ring of bark and replace it immediately (without giving it time to dry) in the same place, we shall notice that the edges will no longer fit and no effort will bring them together again. This means that the bark presses like an iron ring on the growing wood, and the more the latter develops the stronger is the pressure it has to overcome. Evidently this pressure will continually increase, and in autumn reach its maximum. Owing to this increasing pressure the cells of the wood will become increasingly fiat. The truth of these statements is fully supported by experiment. If the pressure on the wood is artificially increased at the beginning of spring by an iron ring being girt round it, cells similar to the autumn ones will appea'r at those places through- out the year ; whereas, on the contrary, if the pressure of the bark is weakened in summer and autumn by means of longitudinal cuts, we shall find in those places throughout the year cells similar to those found in the spring. Such, therefore, are the changes of form in the cells, which can be caused simply by mechanical means . 1