Popular Science Monthly/Volume 21/July 1882/Plant-Cells and their Contents
|PLANT-CELLS AND THEIR CONTENTS.|
PROFESSOR OF BOTANY, STATE UNIVERSITY OF IOWA.
A CHILD'S toy-balloon may afford us an illustration of what a naturalist might call a typical cell. We have in the toy simply a closed sac thoroughly distended by its contents, more or less perfectly spherical in shape, and affording in outline or cross-section an almost perfect circle. In the organic cell the sac is known as the cell-wall, and whatever may be inclosed by the cell-wall is called the cell-contents. A typical cell would be round, spherical, but very few cells, as they occur in nature, are perfect spheres. A cell which may be spherical at the outset may change its shape in accordance with changing circumstances, so that we may say that the form of all cells which we find united to form tissues varies with the situation which such cells occupy, and the functions of the tissues themselves. This we shall see more clearly as we go on. That vegetable tissues, as they occur in wood, pith, leaves, flowers, and fruit, are entirely composed of cells, may be easily demonstrated. All that is needed is, to take a very thin slice of any of these substances and examine with a microscope of moderate power, when the cellular structure becomes immediately apparent. So, then, all the great variety of form and color, and all the resulting beauty, which the vegetable kingdom affords, and all the varied economic value of plants, depends upon the form and contents of these little organic units—of cells. More than this: these cells are of the highest scientific interest. All the discussion of the past few years in regard to spontaneous generation and the origin of life has been a discussion of vegetable cells; and very much of all that we know about life, its activity and its mystery, has been derived from the study of the cells of growing plants. It becomes, then, a matter of some interest to know something about these cells; and, if the reader can imagine himself for a little while looking through the lenses of our microscope, it will be the purpose of this article to tell him some little of what he may see while he studies the cells of plants.
We may begin with the simplest form of plant-cells; and so, for our first experiment, let us examine a drop of brewer's yeast. Here (Fig. 1) are the cells of the famous yeast-plant, the cells which are the active agents wherever yeast is employed, whether in the beer-vat or Fig. 1.—Yeast-Cells. batter-crock. In both cases we find the cells producing fermentation: desirable in beer for the sake of the alcohol resulting; in bread, for the carbonic gas set free in form of bubbles, which, permeating the dough, make it spongy and light. But look at the shape of these cells. Little oval bodies they are, some almost round. Many are entirely isolated, so that we see a single cell may constitute the entire plant; some are linked together, as links in a chain. The attachment here, however, is not very intimate, when once the cells have attained their full size, for then each cell readily and naturally parts company with its neighbor parent, I should rather say, for the cells, as they adhere together, represent really so many successive generations, and illustrate for us one method of cell-multiplication, namely, that which is effected by budding. New cells are continually pushed out as buds on the sides of cells already in existence. The buds grow, reach maturity very rapidly, and in a very short time themselves give rise to new buds and cells. These little yeast-cells, which are not more than three or four ten-thousandths of an inch in diameter at most, are about as simple vegetable cells as we may find anywhere. Growing thus isolated from each other, hardly so much as jostling one another in life's race, there seems no reason why such cells should not be perfectly spherical, or why, so to speak, life's work should not, with them, result in a well-rounded whole.
But the yeast-plant belongs low down in the scale of life, and its simplicity of cell-structure corresponds well with its rank. For the greatest variety of form among plant-cells we must look to higher plants, though not to the highest. The Algæ, in their marine forms well known to every gatherer of "sea-moss," and in fresh-water forms familiar to ail microscopists, afford cells of almost every imaginable shape, character, and color. Here, as with the yeast-plant, a single cell ofttimes makes up the entire organism, but, while some cells are simple, others branch and divide in all directions: some simulate the stem, roots, and branches of higher plants, some are tiniest rolling spheres; some stretch away to the length of several feet, and some are microscopic specks. In Fig. 2 we have the representation of a beautiful marine alga, unicellular, and yet thirty inches or more in length.
As we ascend the scale of life we find the individual cell more subordinate to the organism as a whole, and so less complex in itself; and
yet, when we examine the cells which make up the tissues of the best plants we can find, the blooming occupants of our hot-houses, gardens, and fields, we meet with marvelous diversity, and are soon made to feel that variety of form is the law, uniformity the exception. Fig. 3 represents the appearance of a cross-section of a stem of Tradescantia. From this section we may learn not the variety of cell-forms only, but something of the manner in which every plant is developed, and something of the porousness of all cellular structure.
But let us tear off with our forceps a little shred of the epidermis of some leaf. The leaf from a petunia will do; that of the wild Jacob's ladder is better, and that of the wake-robin better still. Let us examine this little shred with our microscope, using a lens of moderate power. This is from the upper side of the leaf (Fig. 4). How delicate the cell-walls, how beautiful the pattern! Here is Nature's best attempt at uniformity. All these cells serve identically the same purpose, and, so far as we can see, might have been exactly alike. Yet, while there is similarity, no two are just alike. Let us tear off another shred of epidermis, this time from the lower surface of the
|Fig. 4.—Epidermis from the Upper Side of a Leaf.|
|Fig. 3.—Cross-Section of Tradescantia zebrina, Wandering Jew (highly magnified).||Fig. 5.—Epidermis from the Lower Side of a Leaf.|
leaf. Here (Fig. 5) we have the same arrangement and forms of cells, but more beautiful and varied outlines, and the cells are more intimately interlocked. Our magnifying power is greater, and the cells appear larger; moreover, we have before us a few cells altogether unlike any of their neighbors, little button-hole-like structures. These are the stomata of the leaf, and through these tiny mouths for the stomata are real openings through the epidermis the exchange of gases goes on between the growing plant and the surrounding atmosphere.
Let us now pass on a little further in our investigation of these plant-cells and note the contents of some of them. In our examination of the cells from the epidermis of the leaf no contents were apparent; in fact, the cells are tabular, very thin in proportion to their width, and any contents they may possess are so nearly homogeneous as to be transparent and invisible. But let us make a thin transverse section of the same leaf. Here (Fig. 6) we find cells different in shape from any we have yet seen, and evidently possessing different contents. The cells from the inside of the leaf are here seen filled with tiny green Fig. 6.—Cross-Section of a Leaf, showing the Cells containing Chlorophyl. bodies, sometimes closely packed together,sometimes scattered more sparsely. These little green bodies are the chlorophyl-granules, affording to our vegetation all the lovely tints which render charming a landscape in spring. Children of the light are these little green grains, lovers and worshipers of the sun. At all events, they appear by uncounted millions in the bright light of the open sky, become fewer and fewer in proportion as the light received by any plant is diminished, and finally disappear entirely when the plant is left in total darkness. Every one will recall the appearance of potato-stalks where growth has started in some dark corner of the cellar. Cells taken from such growth afford not a sign of chlorophyl. Botanists tell us that the petals of flowers are only altered leaves. In petal-cells, then, instead of chlorophyl-grains, we find in some cases granules of yellow, sometimes of orange. Sometimes the cell contains no such granules, but rather some colored fluid, red, blue, or purple, and then our flowers are tinted accordingly; sometimes the cells of a petal contain air only, and then the flower is white. But these tiny green grains in the leaf-cells do vastly more than simply lend their color to the foliage; they are readjusters and organizers, and perform, in those diminutive laboratories we have been calling cells, feats which the chemist strives in vain to rival. They take possession of molecules of carbon dioxide and of water, compel the binding chemical forces to relax their hold, combine again, to serve the purposes of the plant, the atoms of carbon, of hydrogen, using such part of the oxygen as may be necessary, and setting the remainder free in the open atmosphere—all this in the sunlight. The chlorophyl bodies thus work while it is day, have charge of nearly all the income of the plant, and provide in themselves for the temporary storage, of its daily accumulations, mostly in the form of starch. When the night comes, these same little factors give up at once their labors and their stores, other cells of the plant begin to work, change and transfer and change again, until all the wondrous series of vegetable products with which we are familiar (the sugars, the oils, the alkaloids, crystals of various forms and kinds) are formed and properly deposited. We might go on now to examine cells containing many of these substances, but one or two examples will suffice. Perhaps the most familiar vegetable product is starch, certainly interesting since it enters so largely into the daily food of the world. Let us make a thin section of a common potato and examine it for a moment (Fig. 7). See what a multitude of tiny spheres and ellipsoids crowd the cells! If we apply to our section iodine, we introduce the test of color. The little solids take on a bright-blue tint, and so prove themselves
starch. Now we can see why the potato forms so nutritious an article of diet. During fall and winter starch-grains, such as we have just seen, fill the cells of apple-twigs and of branches of various kinds, and form the basis for that lavish expenditure of plant-force by which our orchards and woods are made glorious in the sudden inflorescence of spring.
If we make a section of the petiole of a begonia-leaf, we may find cell-contents as remarkable as beautiful. Here are plainly crystals with their symmetrical, angular outline. Some of the mineral substances brought through the plant by currents ascending from the roots have found room in the cells of the leaf-stalk to shoot the rays of minute crystals, and here the crystals lie, sometimes a dozen jewels in a single setting (Fig. 8).
But the interest attaching to plant-cells does not culminate in chlorophyl, nor yet in starch-grains and crystals. The chlorophyl, as we have seen, owes its allegiance to the light, the starch to the chlorophyl, and the crystals to the water and the soil; but back of all this, and behind all this, though intimately united with it all, is that which owes its homage to none of these—which moves all, controls all, uses all, builds the cell-wall, and inhabits it—is, indeed, the active principle by which chlorophyl becomes efficient, by which the inorganic is lifted into forms organic, and the earth filled with the children of life, the very essence of the living cell—the protoplasm. To this protoplasm we now turn our investigation.
Twelve or thirteen years ago this word—the name, to say nothing of the thing named—would have come all but unknown to the general reader. But to-day, thanks to the continuous discussions of the last decade, the word needs no introduction. All our readers know that
protoplasm is the simplest form of living matter with which we are acquainted, is the living element of every living cell. One of the most characteristic phenomena of life is independent motion, and protoplasm more frequently reveals itself by moving. Such is the case in the cells we are now considering. In 1869 Professor Huxley set the thinking world all agog by describing, in a passage of wonderful accuracy and beauty, what he could see of moving protoplasm in the hair of a stinging nettle. Nettle-hairs and vegetable hairs generally consist either of a single elongated cell, or of a series of oblong cells arranged in a filament. Moreover, such hairs, or trichomes, are usually colorless, transparent throughout, and afford, therefore, cells admirably adapted to microscopic examination. Hairy plants are very common, so we may corroborate Professor Huxley's statements by observations made almost anywhere. Let us examine a hair taken from the evening-primrose. Here, under a magnifying power of from 400 to 500 diameters, we may see within the hair a delicate current sweeping down one side to the point, turning abruptly with slight delay, and then returning by the opposite side of the cell, leaving in the center a neutral space filled with cell-sap, across which the oppositely moving streams seem never to pass, in which they are never lost. No nucleus is present, nor any central station of power. The tiny streamlet pours on, self-guided (Fig. 9). The hairs on the young leaves of violets and on common red clover exhibit the same sort of a stream moving in much the same way (Figs. 10 and 11).
The unicellular hairs of the common morning-glory present a different phase of the same current. Here the stream is confined to the
|Fig. 9. Tip of a Hair from Evening Primrose.||Fig. 10. Tip of a Hair from Violet Leaf.||Fig. 11. Hair from Red Clover.||Fig. 12. Hair from Morning Glory, unicellular.|
cell-wall most closely, but the movement is unique. The protoplasm in its course is by turns contracted and expanded, giving to the whole current a billowy appearance, a miniature profile of rolling waters. Wave follows wave, but with no deceptive motion, for the rapidly passing granules advise us that the current is strong and real underneath waves that rise and fall but never break (Fig. 12).
The unicellular hairs found on young leaves of Verbena urticifolia, a common way-side weed, exhibit something like a nucleus at the base of the hair, from which center streams of protoplasm are constantly departing, to which they constantly return (Fig. 13). Fig. 14 shows the terminal cell of a hair taken from the petal of the purple lady's-slipper.
|Fig. 13.—Unicellular Hair of Verbena.||Fig. 14.—Hair from Petal of Lady's Slipper.|
Here the nucleus seems almost to be in the way. It is so large as nearly to close the narrow cell across from side to side, and the current appears crowded between the nucleus and cell-wall.
In the hairs that cover the common tomato-plant we may find beautiful transparent cells. In these cells sometimes the nucleus shows a vacuole, and the streams are always fine and large, but changeful as the shadows of passing clouds (Fig. 15).
But we must resort to plants belonging to the botanical order Cucurbitaceæ to find hair-cells showing greatest activity. In the hairs covering the forming bud of a common pumpkin-vine the cells are of rapid growth, with finely transparent walls (Fig. 16). Each cell has a large nucleus, which, while variable and varying, is quite constant in position, and often shows one or more vacuoles. Out to the very limits of the cell, sweeping its every corner from the nucleus as a center, vital streams go forth—streams now wide and sweeping, now narrowing and again swelling, or pouring along to join some neighboring current; now forming temporary vacuoles, now bearing on strong tide
|Fig. 15.—Base-Cells of a Tomato Hair.||Fig. 16.—Hair from Pumpkin Vine.|
particles large and small, granules of chlorophyl and what not; now branching in various directions, now diminishing to merest threads, forming and fading away, finally disappearing below the field of vision, only to reappear once more at the place of starting. The changes of movement and appearance are so rapid that no drawing can be true for more than a single moment.In studies such as these we might pass on from plant to plant, in garden, on highways, in forest and on prairie, until time should fail and patience be well wearied. The "Song of Nature" is true:
No tribes my house can fill;
I sit by the shining fount of Life,
But there are some other plants whose cells exhibit the phenomenon of living, moving protoplasm so much better than nettle-hairs or pumpkin-hairs, that I can not forbear presenting, in concluding the present article, the cells of one more plant. The plant we now select is a very common one in most parts of our country, but on account of its simple and retired habits of life is little known save to the botanist and microscopist. An aquatic plant it is, finding a home in slow-running streams, or shallow ponds whose sandy bottoms reflect the warm rays of the summer sun. Totally immersed in water, however, and so far independent of rains, our plant knows little distinction of spring and summer, and grows on vigorously until the frosts of fall are heavy enough to> seal everything under a covering of ice. If during this long, growing season we collect a sprig of Chara (for such is the name of the plant), we shall find it made up of something like a stem bearing whorls of leaves, or at least of what may pass for leaves. Let us now take one of the newest and smallest of these leaves and place it under our lens. A series of cells, you say. But through the thin wall of any cell appears again a flowing stream. Not the pale, delicate thread of silver we saw feeling its way around the cell-wall of the pumpkin-hair or tomato-hair, but a very river it seems now as it rushes on, wave after wave, up from the depths below across the field of vision and down again, over and over, or round and round, in ceaseless rotation (Fig. 17).
Now the current catches in its course this little particle, now that, hurling each along, now up, now down, now over, now under, without weariness, without hindrance, hour after hour, before us.
And now, as the stream goes on so grandly, think, for a moment, what it is at which we gaze. We call it protoplasm, but it is the current of life, the "physical basis of life"—the common bond which binds in one the whole kingdom of organic things. Think, too, of the antiquity of that stream, its lineage. The brook that "goes on for ever" is as nothing to it, for here the stream has come flowing down through ages which are to us as eternity, ever since life began on earth. The mountains have been hoary with years, and have disappeared beneath the level of the all-producing sea, but this stream is older than they. Continents have grown old, worn out, and been renewed, rebuilt from the débris of this same stream, and life has again flooded the continents, but its origin is older than they.
But now that we have before us such a fine large stream, may we not make further investigation, may we not know its mystery, the hiding-place of its power? We touch the cell with our needles, open its wall to make minuter inspection; but in an instant the charm is broken, the mystic river forgets to flow, the tiny particles settle into unbroken peace.
And close their crystal veins;
And where the glittering current flowed,
We are permitted to look in and see how the work of life goes on, but we can as yet go no further. We may explain. We may say it is all the result of chemical forces, and doubtless chemical forces are working there; but such explanation demands an explanation. Does chemical action renew itself? Chemical action is one thing, chemical action perpetuated and controlled by life is quite another. We may say, life is the property of protoplasm, or we may reverse the statement and say that protoplasm is that form of matter which manifests the phenomena of life, but that is as far as we can go. The streamlet hemmed by the narrow walls of the cell of any plant is to us a boundary. On one side the line, peace unbroken, eternal fixity, rest, of a world whose chemical forces acted once and for ever; on the other, the vast procession of life begins, rises before us, spreads away in variety, activity, in beauty, in wonderfulness, incomprehensible.
- Illustrations from drawings by C. H. Dayton, Mary McBride, and the author.