IT has been emphasized that life is only known to us in the form of individuals, and we turn now to concrete examples of unicellular plants and animals which present, in relatively simple form within the confines of a cell, an epitome of all the fundamental life processes which .we shall later have occasion to review in their complex expressions in the higher animals and plants.
Unicellular green plants are distributed all over the world and adapted to a great variety of conditions. We find them, for example, forming green coatings on the bark of trees, scums on puddles and ponds, or being blown about as dust by winds. Of the many hundreds of species we select Sphaerella lacustris because it can readily be obtained and kept for observation, and because its life history has been carefully studied.
A single Sphaerella is invisible or barely visible to the naked eye, but, like many another microscopic form, it makes up in numbers for the small size of the individual, and sometimes gives a stagnant pool of rain-water a bright green or red color. Sphaerella has a complicated LIFE CYCLE, or series of forms which it assumes under different conditions, chiefly environmental. We shall take as the initial stage for description the so-called DORMANT FORM which may be assumed when the water in which it has been living dries up. In this condition the organism consists of a spherical mass of protoplasm near the center of which is a rather large nucleus.
The protoplasm, which appears greenish or reddish for reasons to ; be discussed later, is enclosed within a distinct, rigid cell wall. This has been secreted by the cell and is composed of cellulose, a carbohydrate which is especially characteristic of plant cells. It is evident that the organism is a single cell.
Sphaerella in this phase is able to withstand unfavorable conditions for several years at least. All the life processes of the protoplasm are reduced to the lowest ebb; so low that it is difficult to demonstrate any chemical change whatsoever going on. Life in a dormant condition is not peculiar to Sphaerella, but is quite a characteristic phase in the life of many animals and plants, being most familiar to us in the case of plant seeds, some of which are known to retain their vital ity for nearly a century under proper conditions.
When dormant specimens of Sphaerella are placed in rain water in the sunlight active life shortly is resumed. The cell wall swells up and the protoplasm within divides twice, with the result that four smaller but otherwise essentially similar cells, known as SPORES, take the place of the original cell and are set free by the rupture of its wall. The four daughter cells soon become more or less oval in outline and secrete cellulose walls of their own. The cell walls do not fit closely about the body of protoplasm, termed the PROTOPLAST, but are separated from it by a liquid-filled space, or vacuole, except where cytoplasmic strands extend through the vacuole to the wall. But a more remarkable change occurs at the same time two long slender cytoplasmic strands are developed from the more pointed end of the cell, and these,
Life history of Sphaerella lacustris
A, b, c, d, asexual cycle; a, w,x, y, z, sexual cycle, a, dormant cell enclosed within ' a protective cyst wall which has ruptured to allow the enclosed protoplast to escape; b, division of the protoplast to form four spores (c) each of which grows, develops two flagella, and assumes the typical 'adult' free living form of Sphaerella (d). This may divide many times, but each cell eventually assumes the dormant form (a) again. Under other circumstances the protoplast from the dormant form may divide until 32 or 64 small cells(w) are formed. These make their escape and are gametes (x) since they fuse in pairs (y). The composite cell resulting from fertilization is a zygote (z) which soon forms a cyst wall and assumes the dormant phase passing through the cellulose wall, extend for some distance into the surrounding water.
These threads of cytoplasm, or FLAGELLA, lash vigorously and pull the cell rapidly through the water. The activity of the flagella is one expression of a fundamental property of protoplasm, CONTRACTILITY, which is exhibited in its most specialized form in the muscles of the higher animals.
Returning now to the life-history of Sphaerella. The four free-swimming individuals, which have arisen from the parent dormant phase, may each divide many times so that instead of four there may be, before long, thousands of flagellated cells, all direct lineal descendants of the original resting cell.
If this number seems high, one only has to determine how many cells there would be at, say, the twenty-fifth generation by raising 2 to the 25th power. Sooner or later, however, these active cells withdraw their flagella and again become dormant forms.
But Sphaerella is still more versatile. Now and then, probably influenced by environmental conditions, the protoplasm within the wall of a spherical dormant form divides rapidly into 32 or 64 relatively small cells which, when set free, are termed Gametes. These differ structurally from the active form already described chiefly by the absence of the prominent cell wall and vacuole. But it is the behavior rather than the structure of these small cells which is characteristic. After swimming about for a time by means of their flagella, they come together in pairs, the two cells of a pair completely fusing nucleus with nucleus and cytoplasm with cytoplasm to form a single cell, or zygote, with four flagella. Soon the individual absorbs its flagella and, secreting about itself a heavy cell wall, enters upon a dormant stage with the characteristics and potentialities described above.
It is clear that the various forms which follow one another arise by cell division in every case, though this is interrupted once by just the opposite process the complete cytoplasmic and nuclear fusion of two distinct cells to form one cell.
This is the process of Fertilization, an expression of a fundamental phenomenon of protoplasm at the basis of sex and sexual reproduction, which we shall consider at length later.
Such is the history of Sphaerella. It is apparent that the sequence of diverse forms which arise from one another constitute a life cycle, and although each individual cell in the cycle is a Sphaerella, nevertheless the plant called Sphaerella lacustris comprises all the forms assumed. From one viewpoint we may look upon the cycle as forming an individual of a different or higher order an individual the component cells of which are separate.
METABOLISM IN SPHAERELLA
We now turn our attention from the structure and life history of Sphaerella to the point it was chosen especially to illustrate the metabolism of green plants. It may appear to the reader that a tree or shrub might with more propriety be taken as the example of a typical plant, but, since the fundamental distinction between animals and plants is chiefly a question of metabolism, there are advantages in studying it in a single cell, where one's attention is not dis tracted by root, stem, and leaf.
Since Sphaerella lives, grows, and multiplies in pools of water exposed to sunlight, it is to this environment that we must look for the materials which it turns into protoplasm, and the energy by which it makes the transformation. And further, since the organism is enclosed in a cell wall, its income and outgo of materials must be in solution in ordei to pass through.
Plant Food Making
In short, Sphaerella takes materials from its surroundings in the form of simple compounds, as carbon dioxide, water, and mineral salts, which are relatively stable and there fore practically devoid of energy, and, through the radiant energy of sunlight, shifts and recombines their elements in such a way that products rich in potential energy result. Sphaerella thus exhibits the prime diagnostic characteristic of green plants the power to construct its own foodstuffs.
This pigment, which is segregated in special cytoplasmic bodies known as CHLOROPLASTIDS, gives to Sphaerella during its active phases and to the foliage of plants in general their characteristic green color. The chlorophyll arrests and transforms a small part of the energy of the sunlight, which impinges upon it, in such a way that the protoplasm can employ this energy for food synthesis.
The first great step in the constructive process is a com bination of carbon with hydrogen and oxygen to form a carbohydrate. Sphaerella gets these elements from carbon dioxide and water by a process of molecular disruption. We know that when charcoal, for instance, is burned, carbon and oxygen unite to form carbon dioxide, and energy in the form of light and heat is liberated. Obviously Sphaerella must employ an equal amount of energy in separating the carbon and oxygen of carbon dioxide; that is, in overcoming their chemical affinity. And this kinetic energy which the plant employs is then represented in the chemical potential which exists between the oxidizable carbon and free oxygen it is a rough approximation of the formula of chlorophyll. A slight chemical modification of chlorophyll results in hematochrome, which gives at certain times the reddish tinge to Sphaerella.
potential energy. Thus the plant in sunlight is continually separating the carbon from the oxygen of carbon dioxide.
The oxygen is liberated as free oxygen while the carbon which has been separated from the oxygen is combined with molecules of water to form carbohydrates grape sugar (glucose) and fruit sugar (fructose).
The conventional equation for this reaction is:
6 C0 2 + 6H 2 O = C 6 H 2 O + 6 O 2 (carbon dioxide) (water) (glucose or fructose) (free oxygen)
It should be emphasized, however, that the processes involved are by no means so simple as is implied above; but since there is little conclusive data in regard to the details, the equation as stated affords a formal explanation which is adequate for the present discussion.
The first great step in food synthesis, the formation of a sugar, having been accomplished, the green plant trans forms the sugar and stores it as starch for future use as fuel or as the basis of further synthesis. Starch is the first visible product of photosynthesis.
We have seen that the characteristic of proteins as com pared with carbohydrates (sugars, starches) is the presence of nitrogen, and this element must be added to the CHO basis already constructed as the next step toward protein synthesis. The green plant not only can, but must employ nitrogen in such simple combinations as nitrates, and this is a fact of prime importance, for typically, as will appear later, animals and most colorless plants require nitrogen in more complex combinations.
By the addition, then, of nitrogen to the carbohydrate basis a very simple nitrogen compound, such as an amine (e.g., asparagine = C4H 8 N 2 O 3 ), is built up, which may be transformed into a protein by the ad dition of sulfur and other elements secured from sulfates, phosphates, etc. Again, we do not know how this is done, or, after it is done, how the protein becomes an intrinsic part of the living material itself. So we attribute it to synthesizing Enzymes. These are chemical bodies which are only known as products of living protoplasm and are the activating agents (catalytic agents) for chemical transformations in which, however, they themselves take no integral part.
The chemical composition and constitution of enzymes is undetermined.
Sphaerella thus takes the raw elements, so to speak, of living matter and by the radiant energy of sunlight, which its chlorophyll traps, constructs carbohydrate, protein, protoplasm. In other words, the green plant is a synthesizing agent, building up highly complex and unstable molecular aggregates brimming over with the energy received from the Sun.
So the green plant, whether Sphaerella or Elm, manufactures its own food and itself! But, as we have said before, protoplasm is always at work to live is to work and this means expenditure of energy, the same energy which chlorophyll has secured for the plant and stored away in its food. In other words, the food must be oxidized in order to release the energy, and for this the plant must have available a supply of free oxygen. Sphaerella obtains this oxygen dis solved in the water and, incidentally, in sunlight, from that liberated through photosynthesis.
The process involved, for the sake of simplicity, may be represented by the equation:
C 6 H 12 6 + 6 O 2 = 6 CO 2 + 6 H 2 O
which, it will be noted, is the reverse of the equation for photosynthesis. This intake of free oxygen by the cell and outgo of carbon dioxide and water, the chief products of combustion, is known as Respiration.
It is an interchange of gases between the living matter and its surroundings which is not only characteristic of Sphaerella and all green plants, but of all living things. Plants breathe just as truly as animals, though the active life of most of the latter requires a more or less elaborate respiratory apparatus in order that an adequate gaseous interchange may be effected with the necessary rapidity.
Thus the green plant may be regarded as a chemical machine for the transformation of energy the radiant energy from the Sun into lifework; the matter and energy which enters, forms, and leaves the organism obeying, to the best of our knowledge, the fundamental laws of matter and energy of the non-living world.
We have now obtained some idea of one living organism, Sphaerella lacustris, a green plant reduced to the simplest terms a single cell provided with chlorophyll. And we have seen that this chlorophyll is the key to the photo synthetic activity of the green plant. In other words, the expression ' green plant' does not refer specifically to the color of a plant (in some cases it may appear red, as in Sphaerella under certain conditions), but to the fact that there is present a complex pigment functionally similar to chlorophyll by virtue of which the plant is a constructive agent in nature. It has the power to manufacture its own foodstuffs from relatively simple compounds largely devoid of energy and, in particular, is able to utilize nitrogen in the form of nitrates.
We pass now from the essentially constructive agents in nature to the chiefly destructive; from the collectors of energy to the energy dissipators; from the green plants to animals and to ' colorless ' plants.