The Life of the Plant

1 Another curious effect of a purely mechanical influence upon the growth of tissues has been demonstrated. The movements of the trunks of trees caused by the wind (as was shown at the beginning of the Can we not go a step further now and explain the very mechanism of growth — why it is that a cell grows, and how it is that under the influence of external agents growth takes this or that direction ? The fundamental mechanism of the growth of cells is explained by botanists in the following way. Owing to chemical changes in the albuminous substances of the protoplasm, there appear substances which absorb water very greedily ; drops of watery fluid, 1 the so- called vacuoles, appear in the cell. These vacuoles blend

into a general vacuole which drives the whole proto- plasm back against the cell-wall (fig. 63, x, Vac .) , so that the protoplasm assumes the form of a bladder, called the primordial utricle (fig. 63, 1 , Pr. u.) . Into this vacuole, as into our apparatus in fig. 46 under the influence of substances dissolved in it, there will pass osmotically larger and larger quantities of water ; and since a cell nineteenth century by a famous English scientist, Knight, who also discovered the effect upon growth of the force of gravity), as well as the subjection of the growing parts to artificial strain (as has been demon- strated by some contemporary German botanists), lead to an increased development of mechanical tissues. Thus mechanical tissues develop most strongly when they are most needed.

is like a whole bladder, this increasing volume of cell- sap will press on the wall and oblige it to stretch — to grow. The protoplasm meanwhile will go on producing on its external surface adjacent to the cell-wall fresh quantities of cellulose. That cellulose is formed by the protoplasm is proved by the following neat experiment. We cut a living cell under the microscope, and let part of the proto- plasm out into water. This protoplasm, like any other liquid in a free state, assumes a spherical form, and in a short time produces cellulose on its surface, transforming itself into a new cell. It is most curious that only those portions of protoplasm which contain a nucleus possess this property of forming new cells. There is a similar close connexion between the growth of cells and the nucleus. During the normal growth of cells a close relation is observed between the protoplasm and nucleus and the formation of cellulose. During this process the walls are thickened either uniformly all over their inner surface, or only at certain parts of it, and this depends on the relative positions of the proto- plasm and the nucleus.

On the strength of what has been said, the process of growth comes to this. A mixture of substances, called protoplasm, consisting mainly of albuminoids, in break- ing down and undergoing chemical transformation, on the one hand, gives rise to substances that dissolve in the cell-sap and osmotically attract water — hence the increase of the vacuole and the stretching of the cell- wall ; on the other hand, owing to the same breaking down of the protoplasm cellulose is formed, i.e. the material with which the walls of a growing cell are built. If our explanation is correct, on reversing the conditions we shall get a phenomenon contrary to growth, i.e. instead of an increase a decrease in the size of the cell, and particularly of the vacuole and the protoplasmic bladder distended by it. This conclusion is in fact supported

by the following simple experiment. If the increase in the volume of cells and, therefore, of whole organs depends on the supply of water attracted by the solution in the vacuole, by surrounding the cell or the organ instead with a solution that will draw the water out of the vacuole we shall cause a decrease in its volume. Let us actually observe under a microscope a living cell in a solution of sugar or salt more concentrated than the cell-sap. We shall notice that the volume of the cell will decrease (fig. 63, 1 and 2), and when the wall consisting of solid matter cannot reduce itself any longer the protoplasm will loosen itself from the cell-wall, and owing to its surface tension will follow the further decrease of the vacuole (fig. 63, 2), and in the end shrink into a regular little ball (fig. 63, 3). This shrinking of the protoplasmic bladder is termed for the sake of brevity plasmolysis. Evidently we may con- clude that the fundamental mechanism of growth is the converse of that of plasmolysis. 1 Even if we have no microscope at hand we can easily prove the truth of what has been said. Let us take the succulent stem of any herbaceous plant, measure its length exactly, and place it in a solution of common kitchen salt containing five per cent, of salt. After a short time let us measure its length again. We shall notice that the stem has considerably shortened, which means that here we have a phenomenon the reverse of growth. Suppose we transfer the stem back into water. It will regain its former size, and go on living and growing. Therefore this experiment gives us an opportunity for verifying our explanation of the mechanism of growth, without injuring the stem in any way.

1 We have indicated this symbolically with arrows in fig. 63. As has been already said, the wall, consisting of solid matter, is unable to contract as completely as the primordial utricle. C means cellulose; Pr. u . — primordial utricle ; Vac. — vacuole. somehow arrested ; for instance, if during the process of absorbing water the cell should lose it by evaporation, the cellulose formed from the protoplasm will go on forming on the inner side of the undistended wall and cause it to thicken. This thickened wall, in its turn, yields still less to the pressure of the sap, and retards the growth of the cell still further. Thus we may explain the above-mentioned fact that checking the growth of straw goes hand in hand with the thickening of its cell- walls. We notice at the same time that this retardation in the growth of organs and the simultaneous thickening of their cell -walls must happen whenever there is a deficiency of water. Probably the inhibitive influence of light upon growth, which we have already studied, depends upon the fact that plants evaporate more water in the light ; and therefore the pressure of sap upon the cell-wall, which causes growth, will not be so great as when there is an abundance of water, in the shade or in the dark. If the phenomena of heliotropism may thus be connected with the phenomena of the evaporation of water, perhaps we may also explain in the same way the particular case of the transmission of the effects of heliotropism in Darwin’s experiments upon the seedlings of cereals. You remember the energy with which these organs exude water at the tips — from which we may conclude that these same tips give off water vapour at a similar rate. This loss of water must be made up by the lower girdle of growth. Hence one-sided illumination will cause one-sided transpiration and growth, as the re- sult of which there will be a curvature towards the light.

We have already seen that the growth of tissues can be retarded by direct mechanical pressure (as in the growth of the wood) ; now we notice that it can be also retarded by the weakening of the internal pressure, and this retardation is accompanied in both cases by greater thickening of the cell-walls. Under the influence of a one-sided action of external factors, the growth of whole tissues will be irregular, causing the curvature of whole organs. But similar unequal growth may also be manifested in different parts of one and the same cell, according to the distribution of the protoplasm and the nucleus, as we have already seen. It has, in fact, been noticed that whenever single cells curve, accumula- tions of protoplasm become noticeable on the concave side. This side is probably more thickened, and offers greater resistance to the osmotic pressure of the sap, and gets less distended. We may add that it is not the thickness of the cell-walls alone which figures in the phenomena of growth : the chemical and physical pro- perties of the cell-wall may vary, and make it more or less resistant and elastic. It has been proved that a certain ferment exists which softens the cellulose of the cell-wall. The local appearance of this ferment can influence the direction of the growth of the cell and its external outlines. Perhaps these facts will furnish us in their turn with a clue to the explanation of another of Darwin’s observations which we mentioned before — his observation concerning decapitated roots. Very possibly during the cell’s earliest period, when it is consequently lying close to the root-tip, there is an irregular distribution of the protoplasm and the nucleus, etc., which may result in a subsequent irregular growth and a curvature of the whole organ.

Only now can we fully estimate the endless variety of the effects of external conditions upon the growth of organs, tissues, cells, or even parts of cells, and the com- plicated combinations all these phenomena may present when taken together ; and only now can we realise what a mistake is made when, instead of trying first of all to find out these possible explanations, people make up their minds that in the phenomena of growth are to be seen the results of some psychical, almost conscious, activity of the plant.

But we must now return to our cress and asparagus. You may have noticed, while I have been talking, how the patch of light has been steadily creeping up the wall ; it is no longer at the tenth division, but some- where near the fortieth. This is because the mirror has passed from the position mn to that of rs ; so we have seen for ourselves how the plant grows. At the same time the root-tip of the cress has long ago moved beyond the circle traced round it with the pencil, and has considerably elongated. This means that we can observe not only the result of this process, not only its immediate cause, i.e. the growth and division of cells, but that we can also grasp the very process, i.e. the very movement that we call growth. 1

We have thus performed the first part of the task we set ourselves at the beginning of this lecture. But what of the second part ? Can we hear how the plant vegetates ? Can we, for instance, make the plant tell us by means of sounds of some kind how it thrives ; whether it is hungry or satisfied ? The following ex- periment will show us that we can. A plant is being grown in an artificial soil 2 under a glass bell, with its flange carefully ground (fig. 64, A). We know, however, that one of the most important sources of a plant’s life is the carbonic acid of the atmosphere. How can we guarantee a continual source of car-

1 In order to ascertain how much the plant has really grown, we have only to measure the distance between the axis of the pulley and the wall. The real increase in growth, as has been already said, will be as many times smaller than the visible transposition of the image of light as is this distance greater than half the diameter of the block. Evidently, if people at the back of this large audience can be shown with the help of this apparatus the growth of a stem during the interval of an hour or even half an hour, the observer standing nearer will be able to notice the displacement of tlife beam of light during the interval of one minute. In fact this method permits of our observing, minute by minute, the increase in length of the stem ; and of seeing the movement, so to speak, as if it were the movement of a minute hand on a clock.

bonic acid to the plant under the glass bell ; and. still more, how can we learn that the plant actually uses it ? We know that the activity of plants and animals is diametrically opposed with regard to the atmospheric gases. Plants absorb carbonic acid and give off oxygen ; animals absorb oxygen and give off carbonic acid. Hence, if we placed an animal under the glass bell along with the plant both would thrive together. But we can substitute an apparatus for the animal (fig. 64, B) , which, so far as the exchange of gases goes, will act precisely in the same way as an animal would. It will be in a certain sense an artificial animal. This is how the apparatus is arranged. A liquid greedily absorbing the oxygen of the air is poured into a glass

basin. 1 A glass jar is placed in the middle of the basin ; through the cork of the jar the tube of a glass funnel is passed right down to the bottom, and also another tube twice bent. An acid is poured into the jar, while a ball of marble or chalk is placed in the funnel. Let us see what happens to the apparatus when it is her- metically covered by the bell A. There is air under the bell ; therefore there is also oxygen. This oxygen will be absorbed by the liquid in the basin ; as a result the volume of air (a) under the bell will decrease ; the pressure of air under the bell will diminish ; and, if it diminishes, the small volume of air ( b ) contained in the jar, above the acid, will begin to expand, and, pressing upon the acid, will oblige it to rise in the tube and to appear in the funnel (i.e. the level of the acid in the beginning at m will be now at n, fig. 64, B). But here it will meet the ball of marble or chalk, and will oblige it to give off its carbonic acid. 2 This carbonic acid will replace the oxygen absorbed, and will go on being given off until the former pressure is established under the bell A ; then the air in the jar ( b ) will fall back to its original volume ; the acid at the same time will go down from a again to m, and everything settle down until the liquid in the glass basin, after having absorbed a fresh quantity of oxygen, upsets the balance again between the air under the bell (a) and the air of the jar ( b ). Was I not right in calling this apparatus an artificial animal ? It breathes ; it absorbs oxygen and gives off carbonic acid, and almost in the same quantities. The plant is placed on a tripod in the upper part of the bell ; it will use the carbonic acid given off by the apparatus, and will itself in its turn give off oxygen,

1 In order to absorb oxygen in presence of carbonic acid Saussure’s well- known mixture can be used consisting of iron filings, flowers of sulphur, and water ; as well as a solution of cuprous chloride (Cu Cl) and kitchen salt, or a solution of chromous chloride (Cr Cl*). Phosphorus (dangerous in summer) may also be used. that will be absorbed by the liquid in the basin of the apparatus B. To sum up : between the plant and the apparatus B there will be a similar circulation of matter as between a plant and an animal. The plant under the bell will be provided with a periodical automatic source of carbonic acid. The plant will thus be supplied with carbonic acid until the little marble ball is entirely dissolved, a matter of days and weeks. Without coming into contact with the marble, the plant is thus able to nibble it all up, and use its carbonic acid for food. 1 But in order to know whether every- thing is right under the bell, i.e. that carbonic acid is ■being given off and decomposed by the plant, the apparatus B is supplied with the following adjustment. Into the twice bent tube C (shown in greater detail in fig. 64, C) a drop of mercury is introduced. Evidently whenever, owing to the expansion of the air b (fig. B), the level of the acid in the funnel rises from m to n, the level of mercury in the tube C will likewise rise from m' to n' (fig. C). Two insulated conductors joined to a common electric bell are introduced into the open end of this tube. One of the conductors is always immersed in the mercury, the other stops a little higher up, and comes into contact with the mercury only when the latter rises to n'. The moment this happens the electric circuit is closed, and the bell rings. It is pretty easy to regulate the apparatus in such a way that the giving off of carbonic acid will happen only when there is very little of it left under the bell, and that the bell will always ring whenever carbonic acid is given off. Just as the giving off of carbonic acid ceases when the acid returns to its former level m, so the bell ceases to ring when the drop of mercury returns to its former position and opens the circuit. But if, owing to some

1 By weighing this little ball from time to time we can determine approximately the amount of carbonic acid absorbed by the plant in so many days or weeks. defect in the apparatus, carbonic acid should not be given off in time, the bell will go on ringing without stopping. I am sorry not to be able to show the apparatus at work, for reasons I have already explained more than once : its action depends on the giving off of oxygen by the plant, and this process takes place exclusively in daylight. But I can nevertheless give you an idea of how it works. If we rapidly cool the air under the bell (a) its volume will decrease as it would have done in consequence of the absorption of oxygen ; and this decrease in volume, provided that the apparatus is in good working order, must be followed by the same result — the giving off of carbonic acid by the marble ball and the ringing of the bell. In order to quickly cool the air inside the bell, I have sprinkled it with some ether. The air has cooled, has decreased in volume ; the column of acid has moistened the marble. The marble has effervesced in giving off carbonic acid, and the bell instantly rings. But the effect of the momentary cooling ceases ; the usual order is re-established — and the bell stops ringing.

Thus every time the plant is threatened with a lack of carbonic acid, the latter is given off by the apparatus B, and this is accompanied by the ringing of the bell. If the apparatus gets out of order, if carbonic acid ceases to be given off — the bell rings without ceasing. If I had asked you a few minutes ago whether it were possible to oblige the plant every time it is hungry, every time, it is merely threatened by hunger, to inform us of the fact by ringing a bell, you would probably have considered it an untimely joke. And yet this is exactly the significance of our apparatus. It is kept working by the activity of the plant, by its power of decomposing carbonic acid and giving off oxygen. We take advantage of this faculty in order to oblige the plant to inform us from time to time by a short ringing of the bell when its feeding was going on successfully, and by beating

an alarm, calling for help, whenever it was threatened by hunger. In a word we oblige the plant to let us know by means of conventional sounds how it thrives. We can now give a positive answer to the question raised at the beginning of this lecture : we can not only see but even hear how the plant vegetates. The experiments by which we have studied the various stages of the process give us at the same time a clear idea of the contrivances to which we must have recourse in investigating Nature. We are not content with the passive part of the observer, but enter into a struggle with her, during which the experi- mental art offers us a whole range of tools and methods. The plant is dumb, it does not answer our questions — we oblige it to write ; it cannot talk — we oblige it to ring ; somehow or other we obtain from it an answer to the question raised. It is vain to look upon this experimental art, as many do, as almost a mechanical activity, something inferior to abstract thought. The mistake was made even by the great Goethe himself. In distinguishing a certain duality, a certain discord between the two ways of investigating truth, between theory and experiment, he made Faust speak of Nature in the following way : —

Certainly it was neither lever nor press that extorted the mystery from Nature — it was the far-sighted medita- tion and the stubborn will of the investigator that have done it. The tool for investigation is as much the product of a creative mind as the theory confirmed by it : it is the very same thought in palpable form. Is it not strange that while one might have seen for centuries in almost any old city of Europe 1 collections of those disgraceful instruments of torture by which man vainly tried to extort truth from his fellow creatures, it was only in 1876 that for the first time the idea occurred of collecting in one place the glorious implements man has used during three centuries in his struggle with Nature, in the course of which he has triumphantly come to wring from her one great truth after another. 1 Is it not strange still to hear the naivete with which people wonder that educated folk can choose some frog or blade of grass as a subject of study; or their open lamentations that the study of Nature, by engrossing the human mind with material subjects, diverts it from higher problems, makes it narrow and causes it to degenerate. In so saying they frequently cast sad glances on the past, as if the human mind used then to be concentrated exclusively on topics more worthy of its attention. Is such a reproach justified ? Is it true that natural sciences narrow the mind, degenerate it ? Is it true that they are less worthy of man's attention than other sciences, when, as we have just persuaded ourselves, these natural sciences occasionally give man access to regions where in the good old times only the immortal gods were given to tread ?

1 In 1876 there was an exhibition in London of instruments and apparatus used in the experimental sciences, with an interesting historical section devoted to apparatus that had served famous scientists in their investigations. The nutrient substances spent in the building up of the solid parts of the plant reach their final destination in the phenomena of growth. Thus the life of a plant resolves itself into nutrition and growth, A plant feeds in order to grow, grows in order to feed, i.e, to enlarge the surface of its food-absorbing organs. These two conjoint processes may last a very long time ; in some plants they last even thousands of years ; yet they always reach a limit, though as a matter of fact we are unable to explain the necessity for such a limit, or to understand why one and the same vegetable organism should not exist for an indefinite length of time. Let us imagine a plant that produces surface runners, like those of the strawberry, or underground stems, so-called rhizomes, like those of the couch-grass ( Triticum repens): these new parts will spread out and cover an ever wider area ; old parts will die away, and consequently the connexion between them and the young parts will break : they will separate, but nevertheless they will continue to be parts of one and the same plant, which, while destroyed at one end, will go on growing at the other. Or let us take another example from among trees : a well-known Indian fig-tree, the banyan tree, produces adventitious roots from its outstretched branches. These roots reach the ground, thicken, and form pillar-like supports to the branches, furnishing them at the same time with necessary food. In this way a single tree may cover whole acres of land. The main trunk may get destroyed here also ; but I do not think this fact would prevent branches which have

taken root from continuing their existence for an indefinite length of time. The capacity of the plant for reproduction is not limited to such spreading ; it is also manifested in another way. Whole parts of plants, such as stems with leaves, may acquire a special form and then free themselves from the plant that has produced them ; such are, for instance, the little bulbs formed in the angles between the leaves and stems of lilies, and also the tubers that appear on the underground stems of potatoes, in which we can recognise branches only changed in form. We may consider all the plants which spring from these organs as individualised ramifications which have separated from one and the same plant, in consequence of its own rapid spreading. It might seem that these and similar methods of so-called vegetative reproduction are quite sufficient to make the life of a single plant secure for an unlimited length of time ; but matters turn out otherwise. It happens that vegetable life cannot be infinitely prolonged in one direction ; it is bound from time to time to interrupt its course, to ascend again to its source, so that, starting once more from the very beginning, as a single cell, it may retrace the same course in the same order of con- tinuity. In a word, we notice in the life of plants, as well as in the life of animals, a necessary succession of generations, and in each generation an invariable sequence of different stages of development, that we call age. Moreover, it happens that for this periodical renovation not one, but two beings must participate in the formation of a new organism. This is the phenomenon of sexual reproduction. At all levels on the organic ladder, beginning with the weed and ending with man, it presents one and the same phenomenon, which is the blending into one of two beings, two lives, ultimately two cells.

This inference, that in order to maintain vegetable life, periodical sexual renovation is necessary, is proved by the fact that, apart from the very lowest repre- sentatives of the vegetable kingdom which stand, so to speak, on the threshold of the organic world, we do not know a single vegetable group that maintains its existence exclusively by means of a vegetative process, by asexual reproduction as it is generally called ; not one that does not also go through the process of sexual reproduction.

Let us see under what form this phenomenon takes place in the vegetable kingdom. At first, the existence of two sexes was noticed only in some plants, mainly in those that had flowers ; hence the name of Phanerogams given to them by Linnaeus in the eighteenth century in contradistinction to the Cryptogams. To-day the name Cryptogam has lost its meaning, because phenomena of sexual reproduction have been discovered in all classes of plants, with the exception of the very simplest organisms, where it probably does not exist.

The idea that this process must take place in the flower, and that the fruit and seed, i.e. a young embryo-plant, are the result of this process, ori- ginated long ago ; but as a definite scientific theory it has not more than two centuries of history behind it. The idea must have been suggested by such plants as have two kinds of flowers distributed on different individuals. Such are many trees, e.g. the willow, the aspen, the juniper, and also hemp. All these plants produce two kinds of flowers : those that bear the fruit and the seed, and those that possess only stamens, which do not transform themselves into fruit, but are necessary in that they contribute to the formation of fruit in other ' flowers. The first plant which attracted man’s attention in this connexion was probably the date palm. At all events we read that people in the markets of Babylon and the Arabs of later days used to sell the male flowers

of this palm that they might be hung by the purchasers among the female flowers to pollinate them ; for this plan was seen to be followed by more abundant crops of fruit. We call those flowers female that contain a pistil which after the petals fall transforms itself into fruit ; by male flowers we understand those that contain only stamens which produce the fertile dust or pollen and die when the flower withers. It is not, however, in all plants that the male and female, the staminate and pistillate, flowers are distributed on separate individuals. In many cases they grow on one and the same plant, as in the birch, the oak, the pine, and in maize ; while in the considerable majority of plants stamens and pistils

are found together in one and the same flower, i.e. the flowers are herma- phrodite. Such is the flower shown in fig. io. Let us consider the part played by Fig. io. stamens in the for- mation of the fruit. A stamen, as we already saw in our first lecture, is in its most perfect form a more or less well-developed filament, to the end of which are attached two oblong sacks, which split longitudinally and shed a kind of dust, as a rule yellow in colour. Every such grain of dust represents a cell, commonly spherical in form, with a double wall ; the external layer is thick and generally very elaborate, while the inner layer is thinner and simpler. The external layer has usually some pores closed with lids which can spring open on occasion.

The pistil in its simplest and most regular form looks like a bottle (figs, io and 65, on the right-hand side). Its large ovary, the inside of which is hollow, contains ovules ; one, several, or even a great number as in the poppy. The elongated part, the style, is occasionally traversed by a canal ; but in most cases the style is solid, though its tissue is porous and spongy, its cells not coming into close contact with each other but having spaces between them. This style ends at its apex in a broadened part called the stigma, which may be flat like a button, delicately branched like a feather, or in- deed of almost any shape. The surface of the stigma is generally covered with short hairs and secretes a sticky fluid. The ovule enclosed within the cavity of the ovary, if split longitudinally, presents the following structure : the central part, the so-called nucellus, is surrounded by a double wall, through one end of which a canal passes right down to the nucellus. This canal may be directed upwards, or downwards as in fig. 65. In the part of the nucellus near the canal, a very big cell attracts our atten- tion. This cell has been called the embryo-sac (fig. 65), because Hk .. it is here, as we shall see later on, Jj

Such in general terms is the struc- ture of these two flower organs — the stamen and the pistil. Their most essential parts are the ovule, which has to undergo fertilisation, and the pollen grain, by which the process of fertilisation is effected. In order that fertilisation may take place, the pollen grain must first of all reach the surface of the stigma, to which it easily adheres by reason of the hairs and sticky fluid. The means by which this end is achieved in Nature will be studied later on ; in artificial breeding, in horticulture, it is found convenient to convey the pollen to the stigma with a brush. What happens next ? The distance between the surface of the stigma and the ovule is still great. In what way does the pollen affect the latter ? This question preoccupied botanists for a long time, and gave rise to a series of more or less fruitless suggestions. It was supposed that the pollen grains sink down to the ovary ; that they burst on the stigma and let out their contents which reach the ovule ; even that they act from a distance by means of some kind of emanation. Not one of these hypotheses proved true when exact microscopic investigations at last solved the problem.

Whenever a pollen grain reaches the stigma or a suitable liquid, say a solution of sugar (but not water, in which it generally bursts), it begins to germinate, the inner wall protrudes through one of the pores in the external wall as a tube (fig. 65, in the centre), into which the contents of the grain are conveyed. The tube goes on growing at the apex and reaches a considerable length. As it grows forward, it often dies behind. These pollen-tubes push their way down through the porous tissue of the style (fig. 65, right-hand side), sometimes for a long distance, as in the cactus, for instance, where the style is several inches in length. Once inside the ovary, the pollen-tubes reach the entrance of the ovule, make their way to the nucellus down the canal and gain entrance to the embryo-sac (fig. 65 right-hand side ; at the bottom of the left-hand side is the upper part of the nucellus, showing the pollen-tube applied to the embryo-sac). The pollen-tubes reach the canal of the ovule almost haphazard ; but the accident happens

fairly frequently, because the number of pollen grains which penetrate into the ovary is generally considerable. Observations have, however, been made under the microscope which point to the fact that pollen-tubes are definitely attracted towards the tissue of the stigma or style, if it lies sufficiently near to them. This tissue seems to attract them even after it has been killed by boiling. Meanwhile events are occurring in the nucellus. I n the upper part of the embryo-sac three little cells are formed out of its protoplasm. These cells have no cellulose wall, and are therefore only spherical lumps of thick protoplasm each with a nucleus. One of them is called the egg-cell or ovum, because it is the actual origin of the future embryo of the plant. This is therefore the primary cell we were looking for ; from which, as we mentioned, every plant arises, be it a spore-bearing plant or a seed-plant. 1 The egg-cell is enclosed in the upper- most part of the embryo-sac so that the tip of the pollen- tube, on reaching the nucellus down the canal of the ovule, comes into close contact with the egg-cell (fig. 65, at the bottom, on the left-hand side).

Recent investigations have proved that the act of fertilisation really consists in the passage of a nucleus, from the end of the pollen-tube, through the softened and dissolved wall of the tube into the embryo-sac (the wall of which also becomes softened or dissolved), and its fusion there with the nucleus of the ovum. It is curious to notice that half of the chromatin substance involved in this process, i.e. half the total number of rods, belong to the male and half to the female nuclei respectively, so that the first nucleus of the embryo, formed by their fusion, contains chromatin from both paternal and maternal organisms. This fact explains very obviously why the properties of the parent plants are blended in their posterity. We shall soon see that in the flowerless