Chloroplast

“ By careful focusing it is easy to show that the protoplasm is confined to a thin layer lining the wall, the central cavity of the tube being filled with cell sap. In the protoplasm are numerous elongated chloroplasts (cl.) . and a larger or smaller number of small, shining, globular bodies (ol.) . These latter are drops of oil, and, when the filaments are injured, sometimes run together, and form drops of large size. No nucleus can be seen in the living plant, but by treatment with chromic acid and staining, numerous very small nuclei may be demonstrated. ”

“ The organic cell contents include plastids, starch grains, mucilage, inulin, sugar, hesperidin, alkaloids, glucocides, tannin, resin, and oils.
CHLOROPHYLL
The chloroplasts of the higher plants are green, and they vary somewhat in size, but they have a similar structure and form.
Chloroplasts are mostly oval in longitudinal view and rounded in cross-section view. Each chlorophyll grain has an extremely thin outer wall, which encloses the protoplasmic substance, the green granules, a green pigment (chlorophyll) , and a yellow pigment (xanthophyll) . ”

“ Making a thin cross-section of the stem, and magnifying strongly, we find bounding the section a single row of epidermal cells (Fig. 94, A, ep.) whose walls, especially the outer ones, are strongly thickened. Within these are several rows of thin-walled ground-tissue cells containing numerous small, round chloroplasts. The innermost row of these cells (sh.) are larger and have but little chlorophyll. This row of cells forms a sheath around the ring of fibro-vascular bundles very much as is the case in the horse-tail. ”

“ Chlo"ro*plast (?) , n. [Pref. chloro- + Gr. &?; to mold, form.] (Biol.) A plastid containing chlorophyll, developed only in cells exposed to the light. Chloroplasts are minute flattened granules, usually occurring in great numbers in the cytoplasm near the cell wall, and consist of a colorless ground substance saturated with chlorophyll pigments. Under light of varying intensity they exhibit phototactic movements. In animals chloroplasts occur only in certain low forms. ”

“ In the chloroplasts, especially if the plant has been exposed to light for several hours, will be found numerous small granules, that assume a bluish tint on the application of iodine, showing them to be starch grains. If the plant is kept in the dark for a day or two, these will be absent, having been used up; but if exposed to the light again, new ones will be formed, showing that they are formed only under the action of light.
Fig. 60.—A, B, young antheridia of Funaria, optical section, × 150. C, two sperm cells of Atrichum. ”

“ If it were possible to determine the amount of chlorophyll or glucose produced, these could be used as satisfactory measures of response. As it is, they can only be determined approximately by counting the chloroplasts or starch grains. The arrangement of the chloroplasts can not furnish the measure sought, since it does not lend itself to quantitative methods, and since the relation to light intensity is too inconstant. Hesselmann (l. c., 400) has determined the amount of carbon dioxide respired, by means of a eudiometer, and has based comparisons of sun and shade plants upon the results. ”

“ We have the surplus sugar, for instance, deposited as starch in the chloroplasts themselves. The manufacture goes on very actively so long as light shines upon the leaves, and we find towards night a very great surplus stored in the cells. This excess of manufacture is one of the features of plant life, and is exhibited, though in various degrees, by all green plants. The accumulated material is made to minister to the need of the plant in various ways; it may be by increasing the bulk of the plant, as by the formation of the wood of the trunk, branches and roots ”

“ We have the formation of numerous mechanisms which have arisen in connexion with the question of food supply, which may not only involve particular cells, but also lead to differentiation in the protoplasm of those cells, as in the development of the chloroplastids of the leaves and other green parts.
The inter-relations of the members of a large colony of protoplasts such as constitute a tree, demand much adjustment. Relations with the exterior are continually changing, and the needs of different regions of the interior are continually varying, from time to time. ”

“ The position and movement of chloroplasts have been the subject of repeated study, but the factors which control them are still to be conclusively indicated. Light is clearly the principal cause, although there are many cases where a marked change in the light intensity fails to call forth any readjustment of the 133plastids. The position of air-spaces as reservoirs of carbon dioxide and the movement of crude and elaborated materials from cell to cell frequently have much to do with this problem. ”

“ Vines suggests that the carbohydrates are secretion products of the chloroplasts, derived from decomposition of previously formed proteids. The food substances are usually quickly translocated, those that accumulate being changed to starch, which appears in the cells almost simultaneously with the sugars. The chloroplasts perform photosynthesis only in light and within a certain range of temperature, varying according to climate. This is the only way in which a plant is able to organize carbohydrates. All plants without a chlorophyll apparatus, as the fungi, must be parasitic or saprophytic. ”

“ Others thrive best in the subdued light of a dense forest. One curious little moss grows in caves where there is almost no light at all. It is equipped with a plate of cells forming a battery of lenses capable of focusing the scattered light on the bodies especially concerned in carbohydrate formation. These are the chloroplasts which contain the mysterious substance, chlorophyll, which acts as a catalyst for action of sunlight on carbon dioxide and water. ”

“ These are for the most part destitute of a firm cell wall, but are imbedded in masses of gelatinous substance like many Cyanophyceæ. The chloroplasts are smaller and less distinct than in Protococcus. The cells are here oval rather than round, and often show a clear space at one end.
Fig. 12.—Chlorococcum, a plant related to Protococcus, but the naked cells are surrounded by a colorless gelatinous envelope. A, motionless cells. B, a cell that has escaped from its envelope and is ciliated, × 300. ”

“ The green tissue of the leaf is much more compact than in the fern we examined, and the cells are more nearly round and the intercellular spaces smaller. The chloroplasts are numerous and nearly round in shape. ”

“ While the composition of chlorophyll is not surely known, iron is certainly one of its constituents, as plants deprived of iron lose their green color. It also is known to contain oxygen, carbon, hydrogen, and nitrogen, but merely to catalog what we know about its make-up does not tell us that it is a living green substance and that sunshine sets it in motion. Just exactly how light acts on chlorophyll no one really knows; we merely know that it does so act and that the result is one of the marvelous secret processes of nature, perhaps like the secret of life itself forever hidden from man. ”

“ A plant that has been grown in darkness from the seed will soon die, although for a time the little seedling will grow very tall and slender. Why? Light favours the production of chlorophyll, and the chlorophyll is the agent in the making of the organic carbon compounds. Sometimes chlorophyll is found in buds and seeds, but in most cases these places are not perfectly dark. Notice how potato tubers develop chlorophyll, or become green, when exposed to light.
Photosynthesis.—Carbon dioxide diffuses into the leaf; during sunlight it is used, and oxygen is given off. ”

“ The action of chlorophyll in the life of a plant is important, as it breaks up the carbonic acid gas taken in by the plant into its two elements, oxygen and carbon, converting the carbon with the water in the plant into starch, and giving back the oxygen to the air. Light is indispensable for the production of chlorophyll; without light plants become bleached or etiolated, a circumstance utilized by the gardener to produce a blanched appearance on certain vegetables. ”

“ In any case we have here presented to us the strange and startling fact that the remarkable arrangement of protoplasmic matter and chlorophyll, which enables the vegetable cell to perform, with the aid of solar light, the miracle of decomposing carbon dioxide and water, and forming with them woody and corky tissues, had already been introduced upon the earth. It has been well said that no amount of study of inorganic nature would ever have enabled any one to anticipate the possibility of the construction of an apparatus having the chemical powers of the living vegetable cell. ”

“ An ample pipe-system extends from root, through stem, to leaf, but it does not communicate directly with the openings at either end. All material, whether liquid or gaseous, absorbed or given out, has to run the gauntlet of the living cells, which are jealous watchmen, and allow only selected substances to pass through them. The crude building materials and food materials are assembled in the leaves, where in cells spread out to the light the chlorophyll is massed. Under the microscope, the chlorophyll is seen to be located in minute granules embedded in the semifluid contents of the cells. ”

“ Chlorophyll grains lie just behind the skin cells in all parts of the plant that look green. The cells they lie in are often long with their short ends towards the skin. Leaves contain several layers of chlorophyll cells. The inner ones are not long like the outer ones, and do not contain so many chlorophyll grains. In the illustration, a, a represent the upper and lower skin and b the cells containing chlorophyll. The under side of a leaf usually has fewer chlorophyll grains in its cells, for the light is not so bright there, and chlorophyll needs plenty of light. ”

“ The chlorophyll-grain is not simply immersed in protoplasm but is permeated throughout its substance by a protoplasmic network or sponge-work apparently continuous with the protoplasm around; or, according to Sachs, consists of protoplasm holding chlorophyll-particles in suspension: the mechanical arrangement facilitating the chemical function. The resulting abstraction of carbon from carbon-dioxide, by the aid of certain ethereal undulations, appears to be the first step in the building up of organic compounds—the first step in the primary anabolic process. ”

“ In white light, the bacteria will be seen to precipitate themselves upon the grains of chlorophyl, to get the oxygen disengaged from them. They thus constitute a very delicate reagent. To witness the effect of the different colors, we let the microscopic spectrum fall upon a filament of conferva, or a transverse leaf-section. The bacteria will collect upon the plant in the red, at the point of maximum absorption, next in the blue, and the density of the population will nearly follow the absorption-curve of the coloring-matter. ”

“ Typical plants have chlorophyll; they are able to feed at a low chemical level on air, water, and salts, using the energy of the sunlight in their photosynthesis. They have their cells boxed in by cellulose walls, so that their opportunities for motility are greatly restricted. They manufacture much more nutritive material than they need, and live far below their income. They have no ready way of getting rid of any nitrogenous waste matter that they may form, and this probably helps to keep them sluggish. ”

“ A plant supplied with pure carbon, hydrogen, oxygen, and nitrogen, phosphorus, sulphur, and the like, would as infallibly die as the animal in his bath of smelling-salts, though it would be surrounded by all the constituents of protoplasm. Nor, indeed, need the process of simplification of vegetable food be carried so far as this, in order to arrive at the limit of the plant's thaumaturgy. Let water, carbonic acid, and all the other needful constituents be supplied except nitrogenous salts, and an ordinary plant will still be unable to manufacture protoplasm. ”

“ The variations in depth of color of foliage during different growing conditions, from a pale yellow when conditions are unfavorable and growth is slow to the rich dark green of more favorable conditions, is a familiar phenomenon. Whether this change in pigmentation is the result of an adjustment of the plant protoplasm, so that it can absorb a more highly actinic portion of the light, or is a direct effect of the lack of conditions favorable to chlorophyll-production and active photosynthesis, has not yet been determined. ”

“ The great majority of conspicuous plants are, as everybody knows, green; and this arises from the abundance of their chlorophyl. The few which contain no chlorophyl and are colorless are unable to extract the carbon which they require from atmospheric carbonic acid, and lead a parasitic existence upon other plants; but it by no means follows, often as the statement has been repeated, that the manufacturing power of plants depends on their chlorophyl and its interaction with the rays of the sun. ”

“ Pure protoplasm is influenced equally by all rays: that associated with chlorophyll is affected by certain red and violet rays in particular. These, especially the red ones, bring about the dissociation of the elements of the carbonic acid, the assimilation of the carbon, and the excretion of the oxygen.' [12]
It is this vigorous life-activity ever at work in the leaves, the roots, and the sap-cells, that builds up the plant, in all its wondrous beauty of bud and foliage, flower and fruit ”