The Life of the Plant

two following tests serve to distinguish carbonic acid from other gases. If carbonic acid is passed through lime water, i.e. through water in which quicklime has been dissolved, this clear solution becomes clouded with a milky precipitate of chalk, i.e. calcium carbonate, a compound of lime with carbonic acid. I take a flask (fig. 22 A) into which two bent tubes are introduced through the cork ; one of them goes right down into the lime water, while the other, the shorter one, ends above the surface of the liquid. I start by taking the shorter

tube (a) into my mouth and inhaling air through it. The external air enters through the other long tube, and passes in bubbles through the liquid, which remains transparent. I turn the vessel, take the end of the long tube ( b ) into my mouth, and exhale the air ; the air repasses in bubbles through the liquid which immediately becomes turbid. In order to prove that the white precipitate at the bottom is really chalk, and that it contains carbonic acid, I add a few drops of vinegar — the precipitate dissolves with effervescence, and these effervescing bubbles of gas are nothing but the carbonic acid which I have just exhaled.

Another test for carbonic acid is as follows. All caustic alkalies readily absorb carbonic acid. I take a glass tube with some carbonic acid in it, close its open end with my finger, and sink it into a vessel containing an alkaline solution. When I take away my finger, which has been blocking the mouth of the tube, the liquid rushes into it and fills it. The carbonic acid contained in it has disappeared, i.e. it has been absorbed by the liquid. With these means at our disposal for detecting the presence of carbonic acid, we can now return to the question : do germinating seeds breathe ? We have answered already one part of the question ; we have seen that germinating seeds cannot get on without oxygen, and that they absorb it ; we have now to show that they exhale carbonic acid, in exchange for the oxygen they inhale. For the sake of clearness I will give another form to the experiment. A stream of common air is driven into this intermediate vessel through an aperture indicated by an arrow (fig. 22 B). (How this is done does not concern us here, as it is only a technical detail having nothing to do with the main point of the experi- ment.) The air passes in bubbles through the solution of caustic alkali, depositing in it those traces of carbonic acid which are always present in the air, especially in a room where so many people are breathing. From this vessel the stream of air, now deprived of carbonic acid, divides into two parts, passes (as indicated by arrows on the figure) through two vessels (c and c'), and, emerging from each, bubbles through the lime water in the funnels (b and V) at the top of them. The vessels are identical, a stream of the same air is driven into both of them ; but there is this difference, that the one marked c contains a layer of living germinating hemp or bean seeds, while the other contains similar seeds previously poisoned with corrosive sublimate. The air in passing through the apparatus continually

plays over the surface of the seeds, and then passes through the liquid in the funnels b and V. You notice already the difference which manifests itself : while- the liquid maintains its transparency in the left funnel, it becomes turbid and turns milky white in the right one, and within a short time a considerable layer of chalk is precipitated. Evidently air after passing over a layer of living, germinating seeds contains carbonic acid. Seeds, therefore, absorb oxygen and give off carbonic acid. We have now to show that these two processes are correlated, i.e. that carbonic acid is given off in place of

oxygen which has been absorbed. This can be demonstrated by the following experiment, which at the same time allows us to judge of the energy with which this process of respiration proceeds. A narrow glass bell a (fig. 23) is divided into two parts by wire gauze ; some germinating hemp seeds are scattered in the upper part, while in the lower, which is closed with an india- rubber stopper, a small beaker is placed containing a solution of caustic alkali. The upper aperture of the bell is likewise corked with an india-rubber stopper through which passes a bent manometric tube containing a column of coloured liquid and

This stop-cock is open for the pur- pose of keeping the air inside and outside the apparatus in equilibrium. Whenever this stop-cock is closed the manometer and falls in the right one, so that its level will very soon reach c on one side and c' on the other. The meaning of this experiment is obvious : the seeds in the upper part of the vessel give off carbonic acid, which as we know is greedily absorbed by caustic alkali, some of which is contained in the beaker below them; conse- quently, there is a decrease in the volume of air in the whole apparatus, manifested by the rising of the column of liquid in the left limb of the manometer. This experi- ment proves that carbonic acid appears in exchange for another gas, absorbed by the seeds ; because, if carbonic acid were only added to the air enclosed in the apparatus, one of two things would happen : either the volume of air in the apparatus would increase, or else it would remain unaltered (on the supposition that the whole of the carbonic acid produced is absorbed by the alkali). The decrease in volume depends on the absorption of oxygen by the seeds ; in place of this oxygen an equal volume of carbonic acid is generally given off, and this is absorbed by the caustic alkali. Hence, this decrease of volume serves both to measure the quantity of oxygen inhaled and of carbonic acid exhaled. The column of liquid rises so quickly that I shall have to open the stop-cock ( b ) several times during the lecture in order to keep the coloured liquid from running over into the bell. This continual motion of the liquid in the manometer demonstrates without further explanation the invisible, inaudible, and yet fairly energetic breath- ing of the seeds.

The latest investigations prove that the formation of diastase, a ferment already familiar to us, is apparently closely connected with respiration. When seeds, already swollen in water, were enclosed in a vessel filled with hydrogen instead of air, they never developed, nor was any diastase to be discovered in them ; whereas, when seeds of the same kind were left in contact with air they sent out shoots containing diastase. Thus it is that we come to an understanding of one of the im-

mediate results of the respiration which awakens plants to life. Respiration as such explains the continual loss of dry matter, the fact which called our attention to the relation of the seed to the air. Respiration is a slow and continual combustion of the carbon and hydrogen of an organic substance, and as a matter of fact if we compare the analysis of a seed with that of the seed- ling which has grown out of it, the decrease in dry weight has to be put down precisely to those elements, whereas the quantity of nitrogen shows no change.

Having proved that a process of respiration takes place in the germinating seed similar to that in the animal organism, we may now go a step further and ask : is not this process followed in the vegetable organism by the same results as in the case of the animal organism ? Respiration, being as a matter of fact slow combustion, keeps up the temperature of the animal, warms it ; will it not warm in the same way a young germinating plant, providing it with the heat necessary for its development ? This question brings us to the considera- tion of the third of the three conditions on which germination depends, i.e. to the consideration of the effects of heat.

Even without exact experiment we notice by general observation that seeds become perceptibly warmer during germination, evidently owing to respiration. Long ago it was noticed that during the malting process the heaps of germinating barley grain get so warm that the rise in temperature can be detected without a thermometer, simply by plunging one’s hand into them. It has been noticed also that rotten seeds spontaneously burn, although here to normal vital processes there are added processes of decay which are due to the activity of certain micro-organisms. In more exact experiments,

in which micro-organisms were excluded as far as possible, a rise of forty, fifty, and even more degrees above the surrounding temperature has been observed. Apparently this heating process is of advantage to growing seedlings, because numerous observations made by farmers and also more exact experiments by botanists have proved that the rapidity of germination, i.e. of the first appearance of the rootlet and the further growth of the embryo, depends directly upon the temperature ; and, moreover, that for different plants there are different limits at which germination is arrested. For a great number of plants we can determine the lowest and the highest temperatures at which germination will begin ; between these two limits the rapidity of growth increases up to a certain temperature, beyond which it begins to decrease. In this way we distinguish three temperatures : the minimum and the maximum, forming the two limits of possible germination, and then the optimum, at which the process is most successful, i.e. the most rapid. Our cereals, for instance, generally begin to germinate at about 35°-40° Fahrenheit, and the higher the temperature the quicker the development ; but beyond 66° -68° Fahrenheit the process becomes slower again, and almost stops at ioo°. For a long time it was supposed that at 32 0 F., i.e. at the freezing-point of water, all active life, and consequently germination also, were impossible ; but recently the interesting discovery was made that seeds can germinate even in ice. The experi- ment was as follows : a small cavity was hollowed in a piece of ice, seeds were placed in the hollow and covered with another piece of ice ; the whole block was then placed in a box surrounded with a layer of ice two feet thick. Two batches of seeds were treated in this way, one in January and another in March, and kept in a cellar. Two months afterwards, i.e.

in March and May respectively, seeds of the most various plants, such as wheat, rye, beans, cabbage, mustard, were found germinating ; their tiny rootlets were piercing through the ice. This strange, unexpected but quite authentic experiment, as well as similar facts concerning the blossoming of some alpine plants, which flower even in the snow, are probably to be explained by the fact that heat is generated by the respiration of plants and is capable of melting ice in the immediate neighbourhood of the plants in question. Life was formerly considered impossible at 32 0 F., among other reasons, because water should freeze at that temperature ; but this is a mistake, for it is known that water may not freeze even at a temperature below 32 0 ; for instance, it does not freeze even at io 0 F. in very narrow capillary tubes.

We see, then, that germination (and, generally speaking, this is true of all vital processes) can take place only within the narrow range of temperature between the limits 32 0 and 104° F. These limits do not, however, apply to resting seeds. Owing to their dryness they are able to bear without injury much greater extremes of temperature. When thoroughly dried seeds can be exposed on the one hand to such high temperatures as 25o°-320° F., and on the other hand to very low temperatures, obtained by means of liquid air, without losing their capacity for germination. A resting seed is thus distinguished by possessing to a remarkable degree the power of enduring great extremes of temperature, and this endurance constitutes one of its important attributes.

Such is the significance cf heat, the third factor by which germination is conditioned. We must not, however, suppose that the accelerating effect of heat, the repressing influence of cold, and the fact of limits of temperature, constitute a peculiarity of living organisms as such. On the contrary, we know that the majority of the chemical and physical processes at work in it depend on the temperature. With an increase of temperature, diffusion and the conduction of liquids are

accelerated in narrow capillary vessels ; with an increase of temperature, too, diastase acts more rapidly upon starch. But, we might ask, if physical and chemical processes are accelerated by a rise in temperature why is it that a further rise in temperature depresses the vital activity of the seed ? Why is it that an intermediate, most favourable temperature exists at all ? Is this really a peculiarity of the living organism ? In the meantime no such supposition is necessary. We know that if heat furthers some chemical processes, which contribute to the acceleration of vital processes, it also brings about other effects with which life is incompatible. Albuminoids, for instance, such as enter into the com- position of protoplasm, the foundation of every living cell, coagulate like the white of an egg at about *144° F., and probably begin to change even before that tem- perature. Obviously, if a rise in temperature causes at the same time processes both favourable and unfavour- able to . the activity of the seed, that activity will be most energetic at a certain mean temperature when the salutary and the noxious effects of temperature present the most advantageous combination. Thus we see nothing in the effects of temperature on the germinat- ing seed that would induce us to give up the physico- chemical explanation of phenomena taking place under its influence.

There is one more aspect of the life of seeds, an aspect as yet unfortunately far from being explained, and that is the difference of degree to which different seeds preserve their vitality, i.e. their power to germinate. Some seeds preserve it for years, or even centuries. Others can germinate only within a few days after being separated from the parent plant, and then lose the capacity very quickly ; such are, for instance, coffee beans and willow seeds. Others, again, can germinate only after a considerable lapse of time. In this last group we find most of the stone fruits. Here also closer

investigation will probably enable us to discover the immediate cause of these phenomena. As a matter of fact the property of conserving during many years the capacity for germination should not strike us as strange. If a seed does not contain the necessary water, or is isolated from atmospheric influences by its membranes, or in some other way one or other of the conditions for chemical changes is therefore not fulfilled, it is difficult to imagine what influence time can have, once the possi- bility of mechanical injury is removed. There are indubitable facts which prove that seeds taken out of a herbarium, in which they had lain for more than a hundred years, germinated very successfully. Another illustration frequently cited is the so-called ‘ mummy wheat ’ which remained in Egyptian tombs for more than a thousand years ; but this case is not authenti- cated. The capacity for remaining in a dormant state during so many years without losing the power of reviving is not the exclusive property of seeds alone. Many of the lowest, microscopic animals, when in the condition of a dry powder, can be kept for years in that state, and revive again as soon as they are moistened with water. The other extreme, the loss within a few days of the capacity for germination, is harder to explain. The facts here seem rather to bespeak some special vitality in the seed which is gradually lost. Yet such facts are not beyond explanation nor without analogy elsewhere. Coffee beans, for instance, have their nutrient material stored up in great part as cellulose, which makes their endosperm hard and horny. Very probably the solubility of this endosperm varies greatly in course of time, since cellulose is able to undergo such changes even outside the organism.

Freshly precipitated cellulose, or cellulose kept in a moist condition, easily dissolves in a certain reagent, but the same cellulose when dried and forming dense and horny masses becomes almost insoluble. Possibly some- thing of the kind happens in coffee beans, so that only the fresh beans contain cellulose capable of passing into a soluble state. As to the last category of seeds, those that sometimes require several years to germinate, such as the seeds of stone fruits, the mere mechanical obstacles they have to overcome may be regarded as one cause of such a delay. Indeed germination can frequently be accelerated by making incisions in the hard wall of the seed.

Let us now sum up all we have found out about the phenomenon of germination, and let us try to define the general characteristics of this unique period in the life of the plant. The period of germination is unique because of the fact that during its course the plant does not require any external supply of food ; it subsists on the food stored up in the endosperm or in the cotyledons. Three factors are indispensable to arouse the seed to activity ; these are water, the oxygen of the air, and heat. Water acts in a twofold way : mechanically and chemically. Mechanically, it causes the seed to swell, tears off its membranes, and supplies the energy necessary for over- coming the resistance of the surrounding particles of the soil ; chemically, it first dissolves the various ferments, and then with the help of these the insoluble substances stored up in reserve. These solutions flow into the embryo and are spent in its growth, in which they are once more transfoi'med into an insoluble or hardly diffusible form.

Respiration, apparently a function of every organism, every living thing on the earth, with very few excep- tions which will be mentioned later, manifests itself in the germinating seed. At the same time it account for the loss in dry weight and the rise in temperature, noticed in germinating seeds. In this way the seedling, in spite of its increase in bulk, not only shows no increase in substance during that period, but on the contrary expends its substance. During germination it is only transformation which takes place, and not assimilation. Consequently, we have arrived at the conclusion that the phenomena of the assimilation of nutrient substances and the phenomena of growth do not always take place simultaneously ; and the period of germination may be best characterised by saying that it is a period of growth without assimilation.

This period in the life of the plant may be more or less compared to the period of education and develop- ment in man. Both plant and man are incapable of independent and productive activity before they reach the close of that period. They exist at the expense of stores carefully accumulated by the preceding generation, though man can scarcely be said to gain by the comparison ; on the contrary, it is the plant that sets an example worthy to be followed. On the one hand plant-parents, after having accumulated a modest inheritance, do not seek to secure an idle and careless existence for their children ; they simply endow them with what is strictly necessary for their development and well-being ; and, on the other hand, plant-children do not dissipate their inheritance : they spend it on their proper development and the generation of energy indis- pensable for the first struggle for life that awaits them.

At the close of this period we find the plant already provided with organs well formed and quite ready for use. It is very curious that certain plants pass the whole period of germination without separating from the parent plant. Such is Rhizophora Mangle , which grows on the shores of the tropical seas, on strips of land generally covered at high tide. The seeds of this plant germinate in the fruit and form long, heavy, and sharp- pointed roots while still growing upon the maternal plant. After having reached a certain stage of develop- ment * they break away, and falling vertically stick by

means of their roots into the mud, and continue their existence without any interruption whatever. logical division of labour manifests itself in the plant. From the general physiological point of view the plant represents two more or less well-developed surfaces — the surface of the root and that of the leaf — adapted to its corresponding twofold environment ; both these surfaces are connected through the intermediate organ, the stem. Such is the general physiological scheme of the plant, a scheme that manifests itself very early, even at the lowest stages of plant life. There are weeds, for instance, which consist of single cells, but which have parts analogous to leaf, root, and stem. Here is a weed (Botrydium ) , not uncommonly found on flooded plains, consisting of a green, round head and a colour- less branching base, something like a root (fig. 24 to the left), by which it is fixed to the soil. This is only a simple bladder, but in it we notice already two physio- logically differentiated parts. Another more striking example is a weed found in the seas of warm countries (fig. 24). A specimen of this sea-weed ( Caulerpa ) picked up at Puzzuoli, in the bay of Naples, when straightened out on a sheet of paper, covers an area as large as the palm of the hand, and sometimes even larger. This giant cell, one of the largest in the entire organic world, presents expanded parts remarkably similar to green leaves, stems, and colourless rootlets. The stems spread along the bottom of the sea, the leaf -like laminae grow up vertically, while the rootlets bury themselves in the ooze ; and this is nevertheless one and the same cell, having one continuous cavity. 1

Such dissimilar organs as the root, leaf, and stem must evidently fulfil absolutely different functions ; and therefore in our subsequent study of the life of the plant, we must study separately the life of these organs. There is, however, one more question to be settled : when does the period of germination come to an end, and the independent life of a plant begin ? As a matter of fact the period of germination ends the moment the 1 Fig. 24 represents two species of Caulerpa — the upper one the common Caulerpa prolifer a , found in the Mediterranean,

stores of food become exhausted ; the independent life of the plant begins when the activity of the leaves becomes apparent ; but in order that this may happen, the leaves must be exposed to the action of light, in the absence of which they will never become green, but will remain yellow and unhealthy. Thus we have finally one more peculiarity of the period of germination ; during its course the plant is completely independent of light, and does not require it ; this is why that period can be passed in total darkness, underneath the soil. But the independent life of the plant starts with the first ray of light which falls on its leaf ; it then begins to develop new organs at the expense of surrounding inorganic compounds instead of from food stored up within itself. The decrease in weight diminishes and soon passes into an increase. Assimilation of matter begins.

We saw at the end of our last lecture that during the period of germination the young plant does not exhibit the most characteristic feature of vegetable life, increase in mass. On the contrary, in spite of a visible increase in volume, it loses continually in dry weight, burning away part of its substance in respiration. It is only when the organs become individualised and begin to fulfil their natural functions, the root sinking into the soil and the stem with the leaves stretching into the air towards the light, it is only then that the full and independent life of the plant manifests itself in the real assimilation of matter from the external environment.

We know already what kind of matter is thus assimi- lated : it consists of the twelve elements already enumerated. We have now to settle a further question : From what part of the environment does the plant obtain these various substances — from the soil, water, or air ; and what are the means by which they enter the plant? We shall learn at the same time which of the two organs, the root or the leaf, is to be considered the nutritive organ of the plant, or whether they both serve that purpose, each in its own way.

Let us begin with the root, since the problem seems simpler in connection with this organ. Scarcely any one has ever doubted the fact that the root serves the purpose of nourishing the plant, and it is fairly easy to prove that at least one form of nutrient substances the root. Thus, for instance, substances that enter into the composition of ash cannot exist under normal conditions in a gaseous state ; it is for this reason that after complete combustion they remain in the ash and do not volatilise with the rest of the vegetable matter. Consequently, all that is found in ash must be looked for in the soil ; and therefore we conclude that such, substances enter the plant through its root. As to the other elements, however, they may be in the soil as well as in the air. We remain as yet uncertain with regard to them : we cannot tell, until we get clear evidence from an experiment, whether they are obtained from the soil or from the air, and so whether they pene- trate into the plant through its root or through its leaves.

Let us turn our attention first of all to the root ; let us see what it absorbs from the soil, how it absorbs it, and why it absorbs precisely that which is indispensable to the plant. But before we begin the study of the root and its functions, we must become acquainted with the environ- ment in which it manifests its activity, and cast a passing glance at the soil itself and its composition. Every kind of soil which is covered with vegetation has two classes of components, which are sharply differentiated ; these are the combustible and non- combustible, the organic and inorganic substances. The organic part, which is merely the residue of decayed plants, gives the soil its black colour. This black matter, which is burnt away when soil is calcined, is called humus. Even the blackest soils, however, the real moulds, contain comparatively small quantities of humus, rarely more than xo per cent. Soil which has been thoroughly calcined and therefore deprived of decaying organic matter no longer appears black, but yellow or red. The mineral parts of the soil which remain after calcining, and are in bulk its main constituents, can be

divided into three groups according to their degree of solubility. One part of them, the smallest, dissolves in water ; the second, larger part does not dissolve in water, but dissolves in acids ; and, lastly, the third and by far the largest part does not dissolve either in water or in acids. These three degrees of solubility correspond roughly to the three degrees of accessibility of those substances to the plant. The substances of the first group which dissolve In the water of the soil are appar- ently easily accessible to the plant ; the substances of the second group are less accessible ; while the sub- stances of the last group are totally inaccessible to the plant, unless in the course of many years they are partly transformed into substances belonging to one of the first two groups.