Beginnings of Plant Life
( Originally Published Early 1900's )
ALTHOUGH the origin of life is practically a sealed book to us, the processes of life are fairly well understood. Living organisms may be roughly classified as plants and animals. Plants are storers of the sun's energy, and animals reconvert this stored energy into other forms. The plant is thus a fuel former, and the animal a heat engine. Let us consider the matter in detail. Of the flow of radiant energy that is for ever being poured out in such apparently wasteful profusion from the surface of the sun, a minute portion falls upon the earth and warms it. When it falls upon a growing leaf, it does wonderful work. It breaks the bonds that weld together the molecules of combined carbon and oxygen. The welding force is ten times as strong as the most tensile steel, yet, in these wonderful leaf laboratories solar energy sets the atoms vibrating and tears them asunder, liberating the oxygen into the atmosphere. Then, without permitting the welding force to act, chlorophyll—the green pigment helps the living matter, in some way not as yet understood, to build the dissevered and other atoms into complex groups. In these the atoms wait, as it were, with folded arms, ready when the time comes to again seize upon their partners, and, as free gaseous molecules, to resume their dance in the sunlight.
The ways in which plants and animals obtain and utilize their food are sometimes very complex indeed, and sometimes very simple. One of the single-celled organisms—the amoeba—comes into contact with something it can digest; slowly the substance is engulfed or enfolded; then, when all the nutritive material has been utilized, the living cell rejects the useless remainder. Almost as simply the green particle floating in water absorbs its necessary salts, absorbs its carbonic acid, in the sunlight liberates oxygen, and builds up the complex out of the simple, just as the giant tree in the forest does. The sharp line that, with complex organisms, divides the animal from the vegetable, loses distinctness as we descend in the scale of complexity, until it becomes hard to tell which is animal and which is vegetable. Even the fact that green plants are able to utilize carbonic acid as a source of carbon-supplies, does not always help us, for there are animals, with chlorophyll, and there are plants, like mushrooms, which are not green. Many an animal remains fixed in its habitat, especially during one stage of its existence; many simple plants, such as Volvox globator, spin and travel in the water as full of apparent animal vitality as almost any animal. But it is only with the lowly organisms that this fusion of characteristics occur; the contrast is marked enough higher up. The plant is characteristically a reducing agent, the animal is as characteristically an oxidizer; the plant stores potential energy, the animal converts this energy into heat and motion; the plant absorbs carbonic acid, decomposes it, and liberates oxygen; the animal uses the liberated oxygen and recombines it with carbon, and this burning develops heat and gives the power of the animal just as truly as the combustion of the fuel of an engine develops heat which is partly converted into mechanical work.
When we burn a heap of plant rubbish it almost disappears—as gases—into the atmosphere, leaving a mass of light-colored ashes. All that has burned away the plant may again get from the air, but the salts that form the ash constituents it must get from water in the soil. To do this it sends out roots with delicate root-hairs near their growing-points, and, by a peculiar action, these hairs absorb the saline water. From the roots the salt-laden water passes up the stem of the plant into the leaves, and thus reaches the place where the chlorophyll is spread out in thin layers, and exposed to the action of sunlight. As we go far back in time we seem to see Nature making apprentice efforts—spreading the chlorophyll as a covering to vegetable masses. Gradually more and more perfect methods were acquired, the chlorophyll surface be-came thinner and thinner, then—to produce contact with much air, so as to get at much carbonic acid—the parallel-veined plants (or Monocotyledons) divided their leaves into thinner and thinner pennants to wave in the sun-shine. But Nature, becoming more and more economical of material, began to reticulate the sap-bearing tubes into the complex net of the Dicotyledons; then placed the leaf on a thin stem, the petiole, so that the bannerets might flutter as freely in the air as the long streaming pennants of the grasses and palms. With many minor families, flowering plants may be grouped, broadly, as net-veined and parallel-veined. These differ in the number of their seed-leaves (Monocotyledons, with one; Dicotyledons, with two), in the structure and growth of their stems, and in many other ways. The early vegetation of the earth was flowerless, and for a long time there were no insects
two facts that must be taken together, since we know that in the majority of our flowering plants the insects carry from blossom to blossom the fertilizing golden dust or pollen which converts possible seeds into real seeds. In earlier days, the plants had not "unfurled their colored flags, to lure winged creatures to their nectar stores."
And now let us consider the function of the flower. To make the beginning of a new life—whether of plant or of animal—it is required, save with very lowly creatures, and in exceptional cases, for instance when buds are set free, that there be a union of two elements of different kinds; a sperm-cell or male element, and an egg-cell or female element. Failing union, both die. The male elements of flowering plants are contained in the pollen-dust formed in the stamens of the flower; the female elements or egg-cells lie within the ovules or possible seeds inside the seed-box or ovary, in the central parts of the flower, which are called carpels. Now, it is better for evolution that these male and female elements should come from different organisms. Hence, in pine trees the cones bearing the egg-cells are at the tips of the branches, while everywhere throughout the tree cluster the pollen-bearing growths; and so numerous are the pollen grains, that, in the season, when some conifers are struck, the mass of golden dust is wafted by the wind like a cloud of solid sun-shine from the tree; so abundant is it that the very ground grows yellow where it falls. Among the many wind-driven wanderers some find egg-cells, and the possible seeds or ovules turn into real seed, that is, a young life or embryo develops within. But in the flowering plants this carrying of the fertilizing dust is in most cases the work of insects, who, working for their own ends, serve that of plant multiplication at the same time. Many naturalists believe that all the beauty of form, all the brilliancy of coloring, all the wonders of perfume, and the sweetness of nectars, have been evolved to attract in-sects, so that the male and female cells of different blossoms of the same kind may be brought together, and loveliness and perfection result from the variation in the offspring, due to cross fertilization.
We may look upon a flowering tree as the highest development of vegetation. It consists of root, stem, branches, and leaves; at a certain season some incipient leaves are modified and become flowers, and parts of these flowers—the stamens and carpels—produce the pollen dust and the ovules, within which, as we have said, there are the male elements and the female elements. The roots hold the tree in its place and supply it with water and salts ; the stem is a means of communication, and uplifts the leaves in the air; the leaves use the energy or power of the sunlight to convert the raw materials furnished by water, earth, and air into complex stuffs which go to build up living matter. Part of the manufactured sap of the leaves is carried back to the roots for their nourishment, part goes to thicken the branches and stem, and part supplies the flowers and fruit.
The flower is the organ of reproduction. When perfect, it consists of four whorls of modified leaves. The first whorl is the calyx or cup. It consists of protecting, steadying leaves or sepals, generally green, but sometimes colored, as in the Fuchsia and in the singular Bougainvillea. The next whorl is the corolla, and it is usually to this that the flower owes its beauty. Its parts—called petals—have the function of attracting insects, and they often protect the more essential parts within from rain and wind, and from unwelcome visitors. The next whorl consists of stamens; rod-like leaves which bear on their tips the anthers, often hanging delicately poised on light filaments. Within the anthers is formed the fertilizing dust or pollen which contains the male elements. The central organ, the pistil, has at its base the seed-box or ovary, containing the ovules, each of which again contains an egg-cell or ovum. At its tip is a sticky body called the stigma. When a pollen grain falls upon this adhesive surface it is caught, and it begins to send out a filament that grows and travels through the style—which connects the stigma with the ovary, or seed-box —till it reaches an ovule, which it enters by a small opening. When the male element from the pollen tube unites with the female element or egg-cell within the ovule, the new life begins; the ovule has become a real seed, it contains a young life from which a plant similar to the parent may grow, again to repeat the process. That cross fertilization may take place, an insect visits the flower, attracted by the perfume or the color, which indicates that a certain nectar is hidden there. While at-tempting to sip the sweet juice the insect comes into contact with the pollen, and the little visitor carries this pollen to the pistil of the next flower which it investigates; thus the male elements of one flower fertilize the egg-cells of another. It is interesting to note that many insects, like bees, keep for a time to one kind of flower, for, of course, the pollen of the wild thyme would be of no use if landed on the stigma of the broom.
In flowerless plants, which usually live in water or in damp places, the male elements are actively moving bodies like those of almost all animals, able to seek out for themselves-and in some cases to find—the egg-cells of their kind. This is so in club-mosses, ferns, horse-tails, mosses, liverworts, and in many seaweeds and fungi. In the strange flowering plants called Cycads, such as the Maiden Hair Tree, similar actively moving male elements come out of the pollen-tube, and make for the egg-cells, thus removing one of the most notable differences between flowerless and flowering plants.
Most complex, most wonderful, are the ways in which the structure of flowers and the structure of insects are suited to one another. They fit like hand and glove. They present us with miracles of adaptation, especially in the case of some rare orchids, and the rare insects that visit them. But not alone is the flower surprising. Everywhere there is the extraordinary, unconscious ingenuity, if we may say so, by which plants compete with one another, vying with each other in their efforts to absorb the sunlight, to procure the soil-water, to mature and scatter their seeds, having stored therein nutrient material for the early stage of their germination; then, by prodigal production of such seeds, to secure a chance that every available spot shall be the bed for the germination, growth, and perfection of similar progeny. Yet, though plants present such profusion of wonder, they are far outdone by the organisms it is their duty to provide food for. When we come to consider the perfection of skill, the surprising mental qualities, and the marvellous emotions of the higher animals, we are confronted by facts, each one a miracle. As Walt Whitman said, the mouse can out-stagger sextillions of infidels.