The Life of the Plant

In accordance with the general conception of the life of a plant we have thus far taken it for granted that it begins and ends with the seed. Doubts, however, arise as to our right to attribute the origin, the real starting- point of the life of a plant to the seed. May we not perhaps go further back and find out its ultimate origin ? For the seed we have been describing is still a very complicated body ; we find in its embryo a com- plete little plant with practically all its parts already developed.

In order to discover this simplest starting-point of plant life we must turn to plants which are exceptions to the general rule of the typical plant with seeds and flowers, which we have just been considering. Suppose by an effort of imagination you can detach yourself for a moment from your present environment and transport yourself in thought to one of the pictur- esque landscapes of Russia, say the neighbourhood of Moscow, and suppose you try to recall your impressions of a walk down into the ravine of Kunzevo. As you

descend into the green thicket with its damp atmosphere, impregnated with many exhalations, you will notice quite a singular kind of vegetation. At every step the waving fronds of ferns grow from the floor or the slopes of the ravine, like bunches of green ostrich feathers, or the crowns of palms stuck into the soil (fig. 12). Lower down along the swampy bank of the stream, in the water itself, or in some marshy pool, you will see a brush-like mass of horse-tails crowded together here and there with

little black cones still surviving on their tips (fig. 13). Such a scene always strikes us as strange and uncommon. Involuntarily one feels that this vegetation is totally different from that left behind at the top of the ravine. This subconscious impression is no illusion. This world of ferns and horse - tails is in very truth a singu- lar world ; it is a sample of the vege- table world which used to cover our planet in by- gone geological epochs. Those ferns and horse- tails, and other plants closely re- lated to them and very common in our woods, like these dry, moss- like, creeping plants, with their yellowish cones occasionally up- raised, called club- moss (fig. 14), all these plants, I say, or rather forms related to them, used to be the prevalent

vegetation on our planet in the period when our coal-beds were formed. Coal contains the remains of whole trunks which belonged to them and the impres- sions of their leaves and fruit. These remains enable us to reproduce with the help of a certain amount of imagination the aspect of the former vegetation of our planet, the landscapes that no human eye ever looked upon. The forests of that remote period contained tree-like ferns which exist to-day only in certain moist tropical countries and in hot-houses. Our short, creeping club-moss existed then as a stately, scaly tree, Lepidodendron, whereas our humble horse- tail, which reaches the height of some dozen feet only in a few places in South America, was represented by the similar but tree-like Calamites, Equisetites, and others.

I have just used an expression which needs explana- tion, and which will naturally take us back to the main thread of our argument. I have said that the club-moss is related to ferns and horse-tails, and that all the existing forms of these plants are related to fossils. Wherein consists that relationship and wherein do these ferns, horse-tails, and club-mosses differ from coniferous and broad-leaved trees ? Some peculiarities in the life of ferns long ago attracted the attention even of unscientific people. There is a poetic fancy in Russia that ferns flower on St. John’s eve. This legend is based on the notice- able fact that ferns never bloom, never have flowers like other plants. The same is true of horse- tails and club-mosses. All these plants are known by the name of Jlowerless plants. But if they are without flowers they must be also devoid of seeds, which are usually formed from the ovules of flowers. How do they then reproduce themselves ? If we look at the under side of a fern-leaf, at the black cones of the horse-tail and the yellow cones of club-moss, we shall notice that towards maturity they all present the following general characteristic : if you shake them over a sheet of white paper you get some very fine

powder, brown or yellow in colour. This powder is composed of very minute bodies, visible only through a microscope, and so small in size that a row of them one inch in length would contain something like one thou- sand of them. Every such grain of powder can produce a new plant. Here is the so-called Lycopodium powder, yellow, soft to the touch, which falls from the cones of the club-moss (fig. 14), and is used by chemists for powdering pills. I throw a handful of this powder into the flame of a candle and the cloud of dust is illuminated

with lightning-like flashes, an effect used in former days to represent lightning on the stage. In this explosion have perished in their embryonic state millions of future plants. These microscopic bodies are called spores by botanists, and all the plants derived from them and devoid of flowers and seeds are called spore-bearing plants. Beside the plants already mentioned this class comprises mosses, water-weeds called green-slime in everyday language, and also fungi, a group which in- cludes moulds as well as mushrooms.

Thus we notice that a spore-plant, whether micro- scopic mould or tree-fern, owes its origin to an invisible grain of dust — a spore. What is this spore ? Is it not the simplest starting-point of plant life, for which we have been seeking and which we could not think that we had found in the seed ? As a matter of fact microscopic investigation shows that the spore consists of a bladder with a solid exterior, containing within it liquid and semi-liquid matter. This is the so-called cell, and it is to the cell that we must look for the simplest origin of every organism ; we are unable to split it into parts capable of independent existence ; it marks the limit of morphological analysis ; it is the organic unit. This being the case a question at once occurs to us : could we not also trace a seed back to a single cell, for surely it does not arise straight away with its root, stem, and cotyledons ? We shall have an opportunity in a subsequent lecture of proving that every seedling also starts from a single cell. We shall discover this cell in the ovule when we come to know its structure better. Hence it follows that every seed-plant or spore-plant starts its existence as a single cell. The only difference between them consists in the fact that in the case of the spore-bearing plant the cell becomes separated from the plant which has produced it ; whereas in the seed-plant the cell develops and grows into a complicated organ, a seed, and only in

that form separates itself from the maternal plant. AH that lives, be it the simplest plant or man, starts from a single cell. Some microscopic plants and even some that are visible to the naked eye preserve their unicellular condition throughout their life-time ; whereas others as they develop become more complicated in their structure and form two, several, millions of cells out of the original one. Thus every plant not only springs from a cell, but consists of ceils in all its parts. Cells are, so to speak, the bricks out of which the body of the plant is built.

This can easily be proved by very simple means. Examine, for instance, a thin slice of a ripe water- melon, and you will see that it consists of bubbles very loosely joined together and having the appear- ance of glass beads. These are cells, which generally lose their mutual coherence in the flesh of a ripe fruit and become detached. In other cases this coher- ence is not broken naturally, but can be broken up artificially. For instance, a slice of raw potato presents a compact body in which it is difficult to per- ceive a definite structure of any kind, without the help of a microscope ; but if you look closely at a boiled potato you will see quite clearly, even with the naked eye, that it consists of separate cells. Boiling water, or rather the action of steam during the process of boiling, has destroyed the coherence between the cells and liberated them. It is somewhat more difficult to separate the cells in more compact organs. But there is no organ too hard to render such a process impossible, were it even a piece of w T ood, a cherry stone, or the seed of a palm, such as Phytelephas macrocarpa, which is as hard as ivory, and is sometimes used by turners instead of it. To break up the cohesion of cells in such compact bodies we must necessarily seek the help of chemical reagents.

cohesion of cells to be convinced of the fact that vegetable matter is composed of them : if we cut very thin and transparent slices with a razor from any part of a plant we can soon satisfy ourselves with the help of a micro- scope that these are composed of cells, closely compacted together, forming what is known as cellular tissue. It is clear from what has been said that it is im- possible to become acquainted with the structure and life of plant organs without an acquaintance with the cell. As in chemistry we start the study of substances with the elements and then proceed to their combina- tions, so in botany the study of the organs of plants must be preceded by that of their elementary organ — the cell.

We have now collected enough facts to be able to make a general plan for these lectures. During its life- history the plant produces a series of organs, the external aspect of which, together with their relation towards their environment, makes it evident that they serve very different purposes and perform very different functions. It is clear that the function of the root which sinks into the soil is different from that of the green leaf which grows up into the air towards the light ; that the function of the cotyledon is different from that of the petal ; that the function of the stamen with its pollen so easily disseminated in the air is not the same as that of the ovule buried deep in the ovary. The physiologist first of all must discover the purpose of every organ, i.e. its function. Hence a twofold problem confronts him from the outset : given an organ, to find its function ; and given a function, to find the organ. Evidently he has first to study the function of the elementary organ, the cell, in its general and special manifestations. Later on, when he becomes convinced of the perfect way in which the organs fulfil their purpose and are adapted to their environment,

when he learns how necessary and well balanced is their mutual interaction, resulting as it does in the general life of the organism, he then begins to realise that his problem is not yet solved, that from behind all the particular questions there emerges the most general of problems, the question of all questions. How have all these wonderful organs combined ? how have all the organisms themselves arrived at that degree of perfection which strikes us so forcibly when we study living Nature ?

By thus including this general question among those which confront physiology, it is evident that we take our stand among those students of Nature who consider the solution of this question feasible and timely. It is notorious that there have been two schools working in the province of natural science, two parties engaged in warfare. The extremists of the one school saw in living Nature nothing but a collection, a kind of museum, of immutable living things, cast in definite fixed forms. According to them the work of the student of natural science resolved itself into an endeavour to make a general catalogue of those forms, label them and arrange them in a collection. The other school looked upon organic Nature as a vast whole which is ever changing and transforming itself. To-day the organic world is different from what it was yesterday, and to-morrow will be different from what it is to-day. The forms of life at present on our planet have derived greater perfection from less perfect ancestors by means of gradual modifications. This school has Darwin as its head, Darwin who harmonised the whole mass of accumulated evidence and gave strictly definite direction to its hitherto indefinite trend. Obviously the question as to how organs and organisms have originated and perfected themselves cannot exist for exponents of the first-mentioned theory. According to their point of view these organisms have never formed nor developed ;

they arose perfectly formed ; they were created in the same perfect form as we see them now. Only those who are convinced of the fact that organic beings are by nature capable of transformation, that they developed the one from the other, becoming more complicated or more simple as the case may be, but always improving, only those can raise the question as to how organic forms have developed and why they are so well adapted to their functions and environment. I will do my best in my final lecture to investigate the answers that science at its present stage of development is able to give to these questions ; nevertheless I should be sorry to miss this opportune occasion for demonstrating the superiority of the modem theory, if not conclusively, at least so far as to show how facts, otherwise incompre- hensible, are thereby elucidated.

In choosing and comparing certain striking examples I have tried to explain the cycle of the life-history of the plant from the point of view of the theory of meta- morphosis. Let us consider some of the facts above stated. If plants were created in final, perfectly definite forms, what purpose is to be attributed to all the transitional organs, such as petals and non-petals, stamens and non-stamens (as in the water-lily) , or to those appendages at the top of the sepals of the peony ? Taken independently these transitional organs are quite useless, since they fulfil neither the purpose of the organ from which they have developed, nor of the organ into which they are about to change (this is why they have survived only in a few exceptional cases). They are utterly incomprehensible from the point of view of individual acts of creation. But they will acquire a very definite meaning as soon as we admit the other explanation, as soon as we accept the theory that all the numberless organic forms in Nature have not been created finally nor in isolation, but have gradually developed the one from the other, becoming more or

less complicated as the case may be, but always improv- ing, i.e. adapting themselves to the conditions of their existence. Then we see in those transitional forms real stages of development, gradual steps towards perfec- tion, towards the improvement of the organ necessary to the plant. Only then will the theory of meta- morphosis, admitted by the exponents of the opposite theory, however obscure and metaphysical it may be from their point of view, acquire perfectly real and definite meaning. This metamorphosis is the expression in space of what has taken place in time. Those thick, colourless cotyledons as well as these bright perfumed petals have been derived from the origin of the common leaf, and have gradually adapted themselves to their new functions ; and those intermediate, transitional forms are nothing but the surviving formal evidences of the process of transformation. They are memorials which enable us to build up the history of the vegetable world. This is the reason of their being so precious to science. But are we entitled to affirm that the vegetable world has a history ? Geology answers in the affirmative, and we have j ust studied an illustration of the fact. We have seen that our ferns, horse-tails, and club-mosses are only degenerate descendants of former mighty masters of the soil ; degenerate forms, forced nowadays to hide them- selves in the depths of forests, or at the bottom of ravines, to escape from the aggressive denizens of the vegetable world of to-day. This means that the earth used to be inhabited by other plants, and that these belonged to the simpler spore-plants, which have receded before our more perfect seed-plants.

Hence the fact of metamorphosis, as well as many other similar facts which we shall consider later on, on the one hand, and geology on the other, prove that the plant world has a history of its own, and therefore that our question as to the origin of vegetable forms is perfectly legitimate. The physiologist’s horizon thus becomes wider and wider. After studying the life of separate organs, be- ginning with the elementary organ from which all others are formed, i.e. the cell ; after studying the general effect of the interaction of these organs, i.e. the life- history of the plant as a whole, he tries to grasp, in so far as it is accessible to him, the life of the plant world as a whole, and thus attempts to shed light on the greatest and most mysterious problem — the problem of the origin of the plant and the reason of its perfection, in other words, the problem of the harmony of the plant world.

Before we step forward, however, on this gradually rising synthetic path, we must go a little deeper in our analysis. We have dissected the plant into organs and the organs into cells, but so far we have only examined the external structure of the cell. We must peep into its interior, into the microscopic laboratory, where the innumerable substances produced by the plant are formed. We must study them and disin- tegrate them into their elements. For this purpose balance and chemical reagents will come to the assist- ance of our microscope. This study will form the subject of the next chapter.

The most remarkable fact in the life of the plant is its growth. When we analyse the phenomenon of growth we realise that it consists in the multiplication of cells. If we examine it still more closely we realise that it involves the appearance and accumulation of matter in places where it was before absent. We put an acorn into the ground and an oak appears ; we drop an imperceptible grain of dust, a spore, and a tree-like fern springs up. The question naturally arises : whence came this substance ? Evidently this question pre- supposes the conviction that matter cannot be newly created, nor disappear. This law of the non-disappear- ance, or the conservation, of matter underlies all scientific conceptions of Nature. The ancients ad- mitted that ex nihilo nil fit, but they would certainly have been in a sore quandary had they been asked, for instance, to prove that burnt matter has not ceased to exist, or to decide whence comes the substance of the plant. Only by long-continued and laborious experi- menting could the law of the conservation of matter as applied to the phenomena of plant life be demon- strated. Even in these days people unfamiliar with the results of science still believe that the growing substance of the plant is derived from the soil, whereas the error of this theory was proved more than three hundred years ago. Van-Helmont, one of the forerunners of the scientific epoch of Natural Science, one of those clear and fearless minds who steered the way for positive science notwithstanding the hampering snares of scholastic

metaphysics, at once a mystic and an ingenious experi- menter — Van-Helmont, I say, made the first exact experiment, which tended towards the solution of the problem of the origin of the substance of the plant. This experiment is remarkable not only because it is the first exact experiment in the province of plant physiology, but also because it was among the first cases in which a balance was used as a means for solving a problem in chemistry. It is well known that chemistry owes to Van-Helmont the original application of this instrument, which later on, in the hands of Lavoisier, revolutionised that science. Let us des'cribe Van- Helmont’s experiment in his own words. ‘ I placed,’ he says, ‘ two hundred pounds of earth, previously dried in an oven, in an earthenware pot and planted a willow slip in it, weighing five pounds. Within five years the willow slip weighed one hundred and sixty-nine pounds, three ounces. The pot was regularly watered with rain and distilled water. The pot was large, and buried in the soil ; and, that it might be protected from dust, it was covered with perforated tin foil. I did not weigh the leaves shed by the plant during the four successive autumns. At the end of the five years I redried the earth and found that it weighed the same amount of two hundred pounds minus two ounces, which meant that water alone had been sufficient for the production of one hundred and sixty-four pounds of wood, bark, and roots ’ ( Ortus medicinae, p. 109). This experiment proved beyond doubt that earth or rather soil cannot be considered the exclusive or even the chief source of vegetable matter. Van-Helmont saw' it in the water he used for watering the plant ; we know, however, that the plant derives its substance not only from earth and water but also from the air. Neverthe- less, Van-Helmont’s inference was perfectly correct as far as he could go.

In his day science had no definite conception of the third, i.e. the gaseous, form of matter. It is to him that science owes the first idea of gases, and even the very introduction of the word gas. Not before the end of last century 7 and the development of the chemistry of gases, could the origin of the sub- stance of the plant be fully explained. This explanation followed as a result of the investigations of the three men of science : Priestley, Ingenhouss, and Senebier.

In order to find out which of the components of this threefold medium — earth, water, and air — participate in the formation of the plant, we must know the com- position of the plant itself. Since Lavoisier, chemistry has taught us that matter not only cannot be created, but in a certain sense does not even change ; that there exist a certain number of so-called simple substances or elements, incapable of transformation one into the other. Therefore, when we find some element present in a plant, we look for it in the surrounding medium, knowing that it must have penetrated thence and could not have been created in the plant, nor produced within it from some other element.

By no means all the chemical elements are to be found in plants, and even of those which do occur, we shall mention only the principal ones, i.e. those which play a prominent part in the life of the plant. In order to get an idea of the chemical composition of a plant, we submit it to the action of a high temperature. Water evaporates first, and at a temperature a little above ioo° C. we obtain the so-called dry matter of the plant. This is the first step in our analysis. It shows that different parts of a plant contain water in very different proportions (see table on p. 43). At a higher tempera- ture we notice that the dry vegetable matter turns brown and black, and then becomes charred, until it begins to glow and burn with a flame, leaving in the end a heap of ashes, very small in comparison with the quantity of substance with which we started. Most of this substance must therefore have burned

away and volatilised. If we carry out this combustion with certain precautions and collect the volatile gases, we discover that the part of the vegetable matter which burns away consists of four elements : solid carbon and three gases — oxygen, hydrogen, and nitrogen. This combustible part, which always contains carbon, as is shown by the fact that it chars before it burns, is called the organic substance of the plant. It is called organic because it enters into the composition of all organisms. At first people thought that organic matter could be formed only in living bodies, in organisms, and that only less complicated substances, which make up dead or inorganic nature, could be produced artificially in laboratories. But this opinion has been shaken by recent progress in organic chemistry. Chemists can now produce a great number of bodies, the forma- tion of which used to be considered a mystery of the living organism. All organic substances do not necessarily consist of all four elements ; some of them are composed of three only, carbon, hydrogen, and oxygen ; or only of two, carbon and hydrogen. More- over, these same elements are combined in different proportions in different substances, so that obviously in different plants, or in different parts of the same plant, the elements will be present in different pro- portions. Nevertheless, by taking the mean of a number of analyses of various plants and of their component parts, we can form an estimate of the average elementary composition of a plant. One hundred parts of dry vegetable matter contain on an average : —

solid and gaseous elements must combine in order to produce a certain amount of vegetable matter. When we pass from the combustible organic part of a plant to study the ash, we find that a greater number of elements enter into the composition of the latter. We shall here enumerate only the principal ones, having to return to the closer study of them in our fourth lecture. The first four elements of the ash form acids, which with the four metals mentioned in the second column form salts.

When once we know of which elements a plant is composed, and knowing also that elements are incapable of transformation one into the other, u r e can say before- hand what are the sources from which these sub- stances have been derived. In the air, in the atmosphere, a plant comes into touch with free oxygen and nitrogen, and with small quantities of carbonic acid — a gas composed of carbon and oxygen — and also with very small quantities of nitrogen combined with oxygen and hydrogen. In the soil, besides the substances just mentioned, the plant comes into touch with others, which, owing to their non- volatility, cannot exist in the air ; these are salts which contain the other elements found in the plant. Some of these salts are dissolved in the water of the soil, and so form part of the liquid environment of the plant ; others exist in solid form.

So far we have only disentangled the chemical elements of which the body of a plant is composed ; or, rather, we have discovered the elements into which the substance of the plant can be broken up : for this purpose we had to destroy the plant itself, to burn it down. This elementary analysis does not, however, give us any information as to the substances or com- pounds which enter into the composition of a living plant. For this purpose another course must be fol- lowed ; and, first of all, as has been already said, we must peep into the cell, the microscopic laboratory where all kinds of matter, produced by the plant, are formed.

It is not difficult to see a cell, every part of a plant consists of them ; but to see it alive, uninjured, is easy only in such parts as consist of single cells or of single rows of cells ; such, for instance, as hairs. Many people will know by sight, if not by name, with three petals — I mean Tradescantia virginica (Spiderwort). The stamens of this flower are made conspicuous by a great number of violet hairs (fig. 15, B), each of which consists of round or oval cells, arranged in a row, like a rosary. If you detach one of these threads with a needle and place it under the microscope you will notice younger cells at the tip of it which are nearly round, whereas at the bottom the cells are older and oblong (fig. 15, C).