Douglas Houghton Campbell

“ If we watch any bright-colored or sweet-scented flower for any length of time, we shall hardly fail to observe the visits of insects to it, in search of pollen or honey, and attracted to the flower by its bright color or sweet perfume. ”

“ 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. ”

“ We have already examined in some detail the structure of the macrosporangium or ovule. In the full-grown ovule the macrospore, which in the seed plants is generally known as the “embryo sac,” is completely filled with the prothallium or “endosperm.” In the upper part of the prothallium several large archegonia are formed in much the same way as in the pteridophytes. The egg cell is very large, and appears of a yellowish color, and filled with large drops that give it a peculiar aspect. There is a large nucleus, but it is not always readily distinguished from the other contents of the egg cell. ”

“ If we compare the structure of the sporogonium of a moss or liverwort with the plant bearing the sexual organs, we find that its tissues are better differentiated, and that it is on the whole a more complex structure than the plant that bears it. ”

“ Comparing this now with the development of the sporogonium in the bryophytes, it is evident that the young fern is the equivalent of the sporogonium or spore fruit of the former, being, like it, the direct product of the fertilized egg cell; and the prothallium represents the moss or liverwort, upon which are borne the sexual organs. In the fern, however, the sporogonium becomes entirely independent of the sexual plant, and does not produce spores until it has reached a large size, living many years. ”

“ The protoplasm contains oval masses, which are bright green in color; but the forward pointed end of the cell is colorless, and has a little depression. At this end there is a long vibratile protoplasmic filament (c) , by means of which the cell moves. There is also to be seen near this end a red speck (e) which is probably sensitive to light. A nucleus can usually be seen if the cell is first killed with an iodine solution, which often will render the flagellum (c) more evident, this being invisible while the cell is in motion. The cells multiply by division. ”

“ Starch is composed of carbon, hydrogen, and oxygen, and so far as is known is only produced by chlorophyll-bearing cells, under the influence of light. The carbon used in the manufacture of starch is taken from the atmosphere in the form of carbonic acid, so that green plants serve to purify the atmosphere by the removal of this substance, which is deleterious to animal life, while at the same time the carbon, an essential part of all living matter, is combined in such form as to make it available for the food of other organisms. ”

“ In its visits from flower to flower, the insect is almost certain to transfer part of the pollen carried off from one flower to the stigma of another of the same kind, thus effecting pollination.
That the fertilization of a flower by pollen from another is beneficial has been shown by many careful experiments which show that nearly always—at least in flowers where there are special contrivances for cross-fertilization—the number of seeds is greater and the quality better where cross-fertilization has taken place, than where the flower is fertilized by its own pollen. ”

“ Occasionally in the same plant that bears tetraspores, but more commonly in special ones, there are produced the sexual organs, and subsequently the sporocarps, or fruits, developed from them. The plants that bear them are usually stouter that the non-sexual ones, and the masses of ripe carpospores are large enough to be readily seen with the naked eye.
If a plant bearing ripe spores is selected, the young stages of the female organ (procarp) may generally be found by examining the younger parts of the plant. The procarp arises from a single cell of the filament. ”

“ By making a series of longitudinal sections with a sharp razor through the top of the plant, and magnifying sufficiently, it is found to end in a single, nearly hemispherical cell (Fig. 23, B, S) . This from its position is called the “apical cell,” and from it are derived all the tissues of the plant. Segments are cut off from its base, and these divide again into two by a wall parallel to the first. Of the two cells thus formed one undergoes no further division and forms the central cell of an internode (y) ; the other divides repeatedly, forming a node or joint (x) . ”

“ If the spores are kept in water for a few days they will germinate, bursting the outer brown coat, and the contents protruding through the opening surrounded by the colorless inner spore membrane. The protuberance grows rapidly in length and soon becomes separated from the body of the spore by a wall, and lengthening, more and more, gives rise to a green filament like those we found attached to the base of the full-grown plant, and like those giving rise to buds that develop into leafy plants. ”

“ The cells form a flat disc, the outer ones being generally provided with a pair of spines.
New individuals arise by internal division of the cells, the contents of each forming as many parts as there are cells in the whole colony. The young cells now escape through a cleft in the wall of the mother cell, but are still surrounded by a delicate membrane (Fig. 11, E) . Within this membrane the young cells arrange themselves in the form of the original colony, and grow together, forming a new colony. ”

“ When nearly ripe, the division walls of the canal cells are absorbed, and the protoplasm of the lowest cell contracts and forms a globular naked cell, the egg cell (D, o) . If a ripe archegonium is placed in water, it soon opens at the top, and the contents of the canal cells are forced out, leaving a clear channel down to the egg cell. If the latter is not fertilized, the inner walls of the neck cells turn brown, and the egg cell dies; but if a spermatozoid penetrates to the egg cell, the latter develops a wall and begins to grow, forming the embryo or young sporogonium. ”

“ In many flowers, the stamens, as they ripen, move so as to place themselves directly before the entrance to the nectary, where they are necessarily struck by any insect searching for honey; after the pollen is shed, they move aside or bend downward, and their place is taken by the pistil, so that an insect which has come from a younger flower will strike the part of the body previously dusted with pollen against the stigma, and deposit the pollen upon it. ”

“ Each leaf is traversed by two fibro-vascular bundles of entirely different structure from those of the ferns. Each is divided into two nearly equal parts, the wood (x) lying toward the inner, flat side of the leaf, the bast (T) toward the outer, convex side. This type of bundle, called “collateral,” is the common form found in the stems and leaves of seed plants. The cells of the wood or xylem are rather larger than those of the bast or phloem, and have thicker walls than any of the phloem cells, except the outermost ones which are thick-walled fibres like those under the epidermis. ”

“ The central cell, which contains dense, glistening protoplasm, is destitute of chlorophyll, but the outer cells have a few small chloroplasts. The former divides repeatedly, until a mass of about thirty-two sperm cells is formed, each giving rise to a large spirally-coiled spermatozoid. When ripe, the mass of sperm cells crowds so upon the outer cells as to render them almost invisible, and as they ripen they separate by a partial dissolving of the division walls. ”

“ To study the earlier stages of germination it is best to choose specimens in which the masses of spores have not been moistened. By thoroughly wetting these, and keeping moist, the process of germination may be readily followed. Many usually begin to grow within twenty-four hours or less. Each cell of the spore sends out a tube (Fig. 47, C) , through an opening in the outer wall, and this tube rapidly elongates, the spore contents passing into it, until a short filament (basidium) is formed, which then divides into several short cells. ”

“ If a mass of actively growing filaments is examined, some of the cells will probably be found in process of fission. The process is very simple, and may be easily followed (Fig. 13, C) . A ridge of cellulose is formed around the cell wall, projecting inward, and pushing in the protoplasm as it grows. The process is continued until the ring closes in the middle, cutting the protoplasmic body completely in two, and forms a firm membrane across the middle of the cell. ”

“ In other cells, either of the same filament or else of the small male plants already mentioned, small motile cells, called spermatozoids, are formed. These are much smaller than the egg cell, and resemble the zoöspores in form, but are much smaller, and without chlorophyll. When ripe they are discharged from the cells in which they were formed, and enter the oögonium. By careful observation the student may possibly be able to follow the spermatozoid into the oögonium, where it enters the egg cell at the clear spot on its surface. ”

“ Owing to the absence of a definite membrane, a distinction between fission and internal cell division can scarcely be made here. Often the cells escape from the gelatinous envelope, and swim actively by means of two cilia at the colorless end (Fig. 12, B) . In this stage they closely resemble the individuals of a Volvox colony, or other green Flagellata, to which there is little doubt that they are related.
There are a number of curious forms common in fresh water that are probably related to Protococcus, but differ in having the cells united in colonies of definite form. ”

“ All living organisms are dependent for existence upon inorganic matter, and sooner or later return these elements to the sources whence they came. Thus, a plant extracts from the earth and air certain inorganic compounds which are converted by the activity of the plant into a part of its own substance, becoming thus incorporated into a living organism. After the plant dies, however, it undergoes decomposition, and the elements are returned again to the earth and atmosphere from which they were taken. ”

“ With a higher power, each cell (Fig. 11, A) is seen to have a distinct cell wall, within which is colorless protoplasm. Careful examination shows that the chlorophyll is confined to several roundish bodies that are not usually in immediate contact with the wall of the cell. These green masses are called chlorophyll bodies (chloroplasts) . Toward the centre of the cell, especially if it has first been treated with iodine, the nucleus may be found. The size of the cells, as well as the number of chloroplasts, varies a good deal. ”

“ The spore fruit now enlarges rapidly, and the outer cells become first yellow and then dark brown, the walls becoming thicker and harder as they change color. Sometimes special filaments or appendages grow out from their outer surfaces, and these are also dark-colored. Shortly before the fruit is ripe, the upper cell of the archicarp, which has increased many times in size, shows a division of its contents into eight parts, each of which develops a wall and becomes an oval spore. ”

“ At first sight it would appear that most flowers are especially adapted for self-fertilization; but in fact, although stamens and pistils are in the same flower, there are usually effective preventives for avoiding self-fertilization. In a few cases investigated, it has been found that the pollen from the flower will not germinate upon its own stigma, and in others it seems to act injuriously. One of the commonest means of avoiding self-fertilization is the maturing of stamens and pistils at different times. ”

“ This body is called the nucleus of the cell, and the small one within it, the nucleolus.
The membrane surrounding the cell is known as the cell wall, and in young plant cells is always composed of cellulose.
The granular substance lining the cell wall (Fig. 1, pr.) is called “protoplasm,” and with the nucleus constitutes the living part of the cell. If sufficiently magnified, the granules within the protoplasm will be seen to be in active streaming motion. This movement, which is very evident here, is not often so conspicuous, but still may often be detected without difficulty. ”

“ The act of fertilization may be easily observed by laying fresh antheridia into a drop of water containing recently discharged egg cells. To obtain these, all that is necessary is to allow freshly gathered plants to remain in the air until they are somewhat dry, when the ripe sexual cells will be discharged from the openings of the conceptacles, exuding as little drops, those with antheridia being orange-yellow; the masses of oögonia, olive. Within a few minutes after putting the oögonia into water, the egg cells may be seen to escape into the water, when some of the antheridia may be added. ”

“ The segments divide by vertical walls in various directions so that the young plant rapidly assumes the form of a flat plate of cells attached to the ground by root hairs developed from the lower surfaces of the cells, and sometimes from the marginal ones. As the division walls are all vertical, the plant is nowhere more than one cell thick. ”

“ The plant is composed of much-branched, slender filaments, closely resembling Cladophora in structure, but with smaller cells (Fig. 29, B) . The non-sexual reproduction is by means of special spores, which from being formed in groups of four, are known as tetraspores. In the species under consideration the mother cell of the tetraspores arises as a small bud near the upper end of one of the ordinary cells (Fig. 29, C i) . This bud rapidly increases in size, assuming an oval form, and becoming cut off from the cell of the stem (Fig. 29, C ii) . ”

“ If the ovule is from a flower that has been open for some time, we shall find in the centre of the embryo sac a large nucleus (k) (or possibly two which afterward unite into one) , and at each end three cells. Those at the base (g) probably represent the prothallium, and those at the upper end a very rudimentary archegonium, here generally called the “egg apparatus.” ”