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The Wonders of Scientific Discovery by  Charles R. Gibson
Table of Contents


 

 

DISCOVERIES IN BOTANY

Is it a dry subject?—The wrong kind of botanist—The vegetable soul—An interesting experiment—The cell theory—Large artificial cells—How the cell grows—A demonstration of cellular action—The growth of seeds—Artificial soils—The death-blow to "vital force"—Where does the plant get nitrogen?—Nitro-bacteria—The plant and carbon—Importance of the leaf—Taking the respirations of a leaf—Priestley's experiment concerning carbonic acid gas—What makes the plant green?—Male and female trees—Sexes of flowers—How plants are reproduced—Fossil plants—Evolution

[145] BOTANY is looked upon by many young people as a particularly dry subject. Indeed, the great collections of dead and dried flowers seem to them to be quite symbolic of the nature of Botany itself.

A Russian professor has put the matter very neatly in the following words: "It is not, I think, much beside the mark to say that the word 'botanist' still calls up in the minds of many even well-educated people not conversant with Science one of two pictures. Either they expect in the botanist a tedious pedant with an inexhaustible vocabulary of double-barrelled Latin names, sometimes most barbarous, who is able to name at a glance any kind of plant, the type of botanist who bores one to death, and is certainly incapable of inciting any interest in his subject; or, on the other hand, 'botanist' depicts the somewhat less sombre figure of the passionate lover of flowers, who [146] flits like a butterfly from one bloom to another, admiring their bright colouring, inhaling their perfume, singing the praises of the proud rose and the modest violet. These are the two extreme types associated in the minds of so many people with the word `botany.'"

Possibly some readers in coming to the title of the present chapter have even hesitated to begin it, deeming it to be necessarily a dry and uninteresting subject. It certainly requires an enthusiast to make a great collection of dried specimens of all the flowers in a particular locality. Such work often takes the form of a competition, with or without the encouragement of a prize. The hobby may become quite fascinating as the collection accumulates, but the Latin names are soon forgotten and the interest in the collection soon lost if the collector's chief aim has been merely to collect a greater number of specimens than any other competitor. Botany is of real interest only to those who study the science of the subject.

The Ancients studied Botany not as a science, but in the hope of finding new medicinal properties with which the physician might cure the sick. The why and wherefore of the growth, the nourishment, and the reproduction of the plant were all explained by the simple assumption that the plant possessed a soul. The possession of a mysterious soul satisfied the inquiries of the Ancients in many departments of Science. When they rubbed a piece of amber and found that it attracted light objects towards it, the explanation was that the amber possessed a soul. So in the botanical world there was a vegetable soul which counted for much with the Ancients. They endeavoured to discover the seat of this soul in plants, and believed it to be at the junction of the root and stem.

[147] Although the subject of Botany has been existent for thousands of years, the Science of Botany is only a few hundred years old. People began to inquire where the substance of the plant really came from. It was obvious that the huge amount of material which grew out of the ground came from somewhere, and it was natural to suppose that this transformed matter came out of the soil itself. In order to discover if this really were the case, a botanist made the following experiment.

He took a very large earthenware pot, and having dried a quantity of earth so as to drive out all the moisture, he weighed the exact quantity required to fill the large pot. This happened to be two hundred pounds of dry earth. Having filled the pot, he then planted in it a slip of a willow tree, which plant weighed exactly five pounds. He took the precaution of protecting the top of the pot with perforated tin-foil so that none of the soil could be blown out of the pot, nor could any be added. Having placed the pot in the ground, he watered the plant regularly. He watched it flourish for five years, by which time it had reached considerable dimensions. He then weighed this five-year-old tree, and found that what had weighed originally only five pounds now weighed one hundred and sixty-nine pounds three ounces. After deducting the original five pounds there remained one hundred and sixty-four pounds three ounces of vegetable matter to be accounted for. He then weighed the earth contained in the pot, taking care to dry it as nearly as possible as he had done five years previously. Ile found that there was practically no difference whatever, the earth still weighing two hundred pounds all but two ounces, and this after a lapse of five years. It was perfectly clear that the soil had not been transformed into [148] vegetable matter. The only other source seemed to be the water which had been supplied at regular intervals, and without which the plant would have died. This appeared to the people of that time (about 1600) to be the only alternative; the atoms going to build up the plant must have been transformed out of the water. But the inquiry once started went on until, with the advance of Chemistry and the introduction of the microscope, it was discovered exactly how a plant does grow. When the chemist was able to tell the botanist of what elements his plants were composed, it became easier to discover from whence these materials came.

Salts were found in the soil, and these salts were compound substances, containing some of the chemical elements of which the plants were composed. It became clear that the sap of the plant could carry these elements from the soil to the different parts of the plant. It soon became apparent also that the leaves played an important part in the nourishment of the plant.

The chief elements composing the plant are carbon, hydrogen, oxygen, and nitrogen. We know that the chief constituents of the air are oxygen and nitrogen, while carbon and hydrogen exist in the atmosphere in very small quantities, but all these four elements are contained in the compound salts existent in the soil. How, then, does the plant absorb these, and how does it grow?

We had no clear understanding of these things until the discovery of the "cell." This discovery has been mentioned already in connection with the discoveries concerning our own bodies. Just as our bodies are the aggregate life of a myriad of individual microscopic cells, so is the body of the plant. We have seen that a cell is a tiny speck [149] of protoplasm surrounded by a cell wall. We know that the cells grow and divide, thus multiplying and producing the phenomena of growth, but how does the cell itself grow?

About forty years ago a chemist made an interesting experiment in which he produced large artificial cells. He took a solution of tannic acid and placed a drop of a syrupy solution of gelatine in this. Immediately there was a chemical action between the surface of the gelatinous drop and the tannic acid, and a colloidal wall or skin was formed around the drop. It was observed that the gelatinous contents of this artificial cell continually drew water through the outer skin, thus increasing the size of the cell. The following simple laboratory experiment should make this action quite clear.

Suppose we have a glass vessel containing a colourless solution of iron salts. We have another solution containing tannin, and this also is colourless, but if we mix a little of these two solutions together they become black, and might serve as a writing-ink. Indeed, a good black ink is a tanno-gallate of iron, a small quantity of gum being added to retain the precipitate ini solution. Suppose wte have a glass vessel with a partition wall dividing it into two compartments. If we fill one compartment with the solution of tannin and the other with the iron salt solution, they will, of course, remain colourless so long as they are kept separate. But suppose the partition were made of a collodion membrane, such as is used sometimes for making experimental balloons, and which has been used also for making the film on photographic plates. We should find now that the compartment containing the tannin solution would become black, and from our previous experiment we [150] should know that some of the iron salts had found their way into the tannin solution. It is evident that these iron salts have penetrated the collodion partition, and it is also obvious that the tannin solution cannot penetrate the partition, for the compartment containing the iron salts remains colourless, whereas it too would become black if the tannin could reach it.

Suppose we try to represent a magnified cell by filling a collodion bladder with a colourless solution of tannin. There is, of course, no leakage through the solid wall of the bladder. We place this giant cell in a clear solution of iron salts and immediately we find the liquid in the bladder turns gray and then black. From this result we know that iron salts have passed through the bladder wall into the model cell, causing the tannin solution to become black. We notice again that there is no passage of tannin out of the bladder into the surrounding liquid. It is a one-sided operation. We find that this penetration by the iron salts continues until all the iron salts have been transferred from the outer vessel to the inside of the collodion bladder.

Now we are in a position to realise the action of a cell in the root of a plant. We can picture the cell absorbing salts from the soil, we can think of chemical actions thus stimulated within the cell, the productions of which cannot pass back through the cell wall. There is of necessity an accumulation of matter within the cell; the cell grows. When it reaches its maximum size it divides in two, and each of these parts becomes an individual cell, and in turn grows and divides.

This action of the cells is not confined to the roots. The cells forming the green parts of the plant are exposed to the atmosphere, from which they absorb carbonic acid gas.

[151] When this enters the cell it combines with water in the cell and forms carbohydrates, which accumulate gradually. A vegetable cell has been described as a trap which lets things pass easily one way, but does not let them out again. We have noticed that this action takes place in the roots and also in the green parts, and of the latter the leaves give a large working surface. By means of an ingenious apparatus we can actually record the rate at which the leaves of a plant are absorbing the carbonic acid gas from the atmosphere.

Those of us who have seen green cress seeds sown upon the bare back of an earthenware pig or other object, or upon a piece of damped felt, may wonder where the cells get nourishment in such cases, there being no soil laden with salts. Such experiments serve a useful purpose if they impress us with the fact that the seed is practically independent of the soil. The seed contains the necessary chemicals, which are stimulated into action by the water. The cells of the seed absorb oxygen gas and exhale carbonic acid gas; in other words, the seeds breathe. By the time the seedling has used up all its internal store of nourishing matter it has its roots and leaves ready to absorb nourishment from the salts in the soil and from the gases in the air.

In an earlier chapter we saw that the soft soil is composed of decayed rock, and we have no difficulty in realising that the growing plant cannot look to the soil itself for nourishment. We have seen that this fact was discovered by the old chemist who grew a willow slip in a fixed quantity of soil. It is clear that so far as the roots are concerned, the food of the plant is in the substances contained or entangled in the soil. But how can we discover which of all these elements are necessary for the growth of the plait?

[152] A series of experiments was made by placing two exactly similar plants in similar soils. By abstracting one element after another from the soil of one plant, and by comparing the growth of the plant placed in this depleted soil with the similar plant grown in the original soil, it was soon discovered which elements were absolutely necessary to the growth of the plant. Hence have followed all the practical applications of artificial manures.

Recent experiments have made this matter very clear, for it has been found possible to grow plants in artificial soils composed of sand, crushed pumice-stone, glass beads, and similar materials, so long as the necessary chemical substances are added. In this way a most unfertile soil is converted into a very fertile soil. By such means it is easy to discover the exact results of different chemical substances.

We have seen how the Ancients got over many difficulties by assuming the presence of a vegetable soul. Even so late as the beginning of last century it was a common practice to pass over any process, which was difficult to explain by physical or chemical laws, by attributing the phenomenon in question to the effects of "vital force." But it soon became apparent that this too was a mere assumption, used in order to help the botanist out of a difficulty. We have seen that the growth of a plant is due to ordinary chemical and physical laws, and that we need no longer introduce a vital force or a vegetable soul to account for the phenomenon of growth.

As nitrogen gas forms about four-fifths of the whole atmosphere, and as nitrogen is one of the important elements in the composition of a plant, one might suppose that the leaves would absorb the necessary nitrogen from [153] the surrounding atmosphere, but fifty years ago it was discovered that this was not the plan of Nature. The cells of the leaves are incapable of absorbing the nitrogen gas, and it is only when nitrogen is combined with other elements in compound substances in the soil that the root cells can absorb it, and thus obtain the necessary supply of nitrogen. The leaf used to be considered a rather unnecessary appendage to a plant, and it took botanists a long time to realise the great importance of the leaf in the obtaining of food for the plant.

About the close of the nineteenth century it became apparent that one way in which nitrogen is made ready as a food for the plant is by means of certain microbes in the soil. It was not an easy matter to discover what microbe was the active agent, but after many experiments and much careful study it was discovered that two classes of microbes were at work. One of these nitro-bacteria has the power of constructing nitrites from ammonia, while the other class combines oxygen with the nitrites and thus forms nitrates. Once discovered, these two microbes were distinguished easily again, as the first was of a globular or oval shape, and the second one of a slender rod-like form and among the smallest known bacteria.

Experiments show that a plant may live in a soil from which it cannot possibly obtain any carbon, and as nearly one half of the whole composition of the plant is carbon, it is evident that this most important food must be obtainable by means of the leaves. Although these experiments do not prove that the plant is incapable of absorbing some carbon by means of its roots, it is obvious that the leaves must be the chief gateway.

By collecting the gas given off by the leaves of a plant [154] when exposed to light, we find on testing it that it is oxygen gas. We have been familiar, no doubt, from childhood with the fact that a plant inhales carbonic acid gas and exhales oxygen gas, this being exactly the converse of what we ourselves do. In the case of the plant what happens really is that it absorbs carbonic acid gas, which is composed of one part of carbon and two parts of oxygen, and the carbon enters into the composition of the plant, while the oxygen is freed and escapes. A century ago the Rev. Dr. Joseph Priestley, the famous pioneer chemist, made a very simple experiment which proved this action of the leaves of a plant. The experiment was as follows.

Taking a glass bell-jar, he placed a lighted candle in it, along with a little water to dissolve the smoke, and in this way he soon had the bell-jar filled with carbonic acid gas. This was his simplest means of combining all the oxygen of the enclosed air with the carbon of the candle. As soon as all the oxygen had disappeared the candle would cease to burn. A lighted taper put into the jar would be extinguished immediately. Having prepared the carbonic acid gas, known to him as "fixed air," he placed some fresh green leaves in the gas. He then exposed the bell-jar and its contents to sunlight for some time in order to stimulate the action of the leaves. He found that when he then introduced a lighted taper into the bell-jar the combustion was increased instead of the flame being extinguished as before; indeed, he now had a bell-jar of oxygen where only cabonic acid gas had been present at the outset. The leaves had absorbed the carbon and freed the oxygen.

It was discovered that this transformation of carbonic [155] acid gas into oxygen takes place only in cells containing a substance known as chlorophyl, which material gives the green colour to the leaves and stems. The microscope reveals green chloroplasts within the cell. It is these chloroplasts stimulated by the Sun's rays that decompose the carbonic acid gas absorbed by the cells. What about those plants which possess no green chlorophyl? Such plants or fungi, of which the mushroom is a well-known member, can exist only in soil where organic matter is present already.

It is stated sometimes that it is a mistake to think of a plant's breathing as being the converse of that of an animal, and that the plant really absorbs oxygen and gives off carbonic acid gas just as an animal does. It is necessary to recognise that there are two distinct functions. There is a genuine breathing of oxygen and exhaling of carbon dioxide; a true "respiration." There is also that very important function by which the green parts of the plant assimilate carbon dioxide, throwing off the surplus oxygen, and this "photosynthesis" is a special property of plants. The "respiration" is of interest as being common to all living things, both animal and vegetable.

When botanists endeavoured by means of the microscope to see the cells multiplying and producing actual growth it was discovered that this phenomenon took place for the most part at night during the absence of light. At first the experimenters sat up at night to make such observations, but later it was found that if the plant were placed in a cool cellar during the night, its cell division was arrested, and could be examined the following day. In the domain of arithmetic it would be paradoxical to speak of multiplication being due to division; we might [156] multiply the number of parts, but there would be no increase. In the case of the cell multiplication there is in the first place a division, giving in itself no increase, but we have seen how each cell grows by absorption; and it is this which produces actual growth. What really happens is that when a cell has reached its maximum size a partition forms within it, a regular dividing wall, and gradually the cell parts in two, each half becoming an independent cell, just as we saw was the case with certain bacteria. In this way a single cell becomes a complex plant.

The Ancients were aware that there were male and female trees in such species as the date-palm, the juniper, and some others. But more than two centuries ago it was discovered definitely that there were males and females among plants. It was believed that this was the case only with flowering plants, but now we have discovered that all plants have two sexes, except in very simple organisms, which multiply by simple subdivision.

But all plants are not distinctly male and female, for in many cases the male and female flowers grow on the same individual plant, as in the birch, the oak, the pine, and in maize. In the greater number of plants there is not even this distinction of male and female flowers, for the male stamen and the female pistil are present in one and the same flower.

We are all familiar with the appearance of the male stamens; the graceful upright filaments with their small sacs at the points. We may have seen these little sacs burst and shed a yellow dust, which is one of the fertilising elements. But how is the one element to be carried to the other? It is well over a century since botanists first noticed that many insects acted as carriers of the male [157] element in one flower to the female in another. When this became known man recognised that the bright colours of the flowers, their sweet perfumes, and their honey had not been created for his special benefit, but in order to attract the insects to the flowers and thus cause them to act as carriers in the process of fertilisation. In this one subject there is a world of interesting detail. The flower is shaped to suit the form and habits of the visiting insect, and details are so arranged that the insect is bound to touch the stamen of one flower, and when it reaches another flower the same part of its body must touch the stigma on the sticky surface of which it leaves the fertilising element, and from whence this pollen element can extend tubes to reach the other element within the pistil.

Those plants which have neither highly coloured flowers, nor scent, nor honey, are dependent upon the wind acting as the carrier, and how often does the horticulturist or amateur gardener wish that the wind would do its planting of weeds in other ground than that which has been set aside for flowers. In the wind-fertilised plants there is not the wealth of beauty associated with the insect-fertilised plants.

Just as we find a clear case for Evolution in the animal world, so do we find convincing evidence of this in the vegetable world. An examination of fossil plants shows that the farther back in the Earth's history we can get, the simpler were the forms of plants. In the most remote periods we find only water-weeds, then mosses make their appearance, then horsetails and club-mosses, which were so abundant during the time in which our coal-fields were laid down. All these are spore-plants; they do not reproduce in the manner described in the preceding [158] paragraphs. They have spores attached to the fern leaves, and these small spores are wafted away by the wind, and under certain conditions these simple cells are developed into new plants. The seed plants were evolved from these at a later date, the first transformation being the cone-bearing trees, and then the flowering seed plants, which are so prominent in the world of to-day. If it seems marvellous that the complex plants to-day have been evolved from the simple plants of past ages, it must seem even more remarkable that the beautiful flowers, with all their details of petals, stamens, and pistils, must have been evolved from a simple leaf.

Before leaving the subject of Botany, which has been so very briefly touched upon in the present chapter, it will be of interest to consider one of the newest developments—"Growing crops and plants by electricity."

Although no special chapter has been devoted to Zoology, which along with Botany forms the subject of Biology, it is not because of any lack of interesting detail in Zoology. The reason is that the whole field covered by the discoveries in Science is so very wide, and something of Zoology has been included already in the chapter dealing with the living creatures of long ago, and in the chapter of Evolution entitled "Whence came Man?"




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