Reproduction (aka Propagation)

Plant reproduction or propagation is the process of increasing the number of plants of a particular species or cultivar. There are two primary forms of plant propagation: sexual and asexual. In nature, propagation of plants most often involves sexual reproduction, or the production of viable seeds. When exposed to proper environmental conditions, these seeds germinate and grow into mature, reproductive plants. [source]

The simplest type of asexual reproduction consists in the division of the parent plant into two or more parts, each of which becomes independent. It is a characteristic property of most plants that a portion of the body, (particularly if it includes a bud) when removed and placed under favorable conditions, will replace the missing parts and develop into a new individual. This readiness for multiplication is made use of extensively in the various arts of plant propagation, by which new individuals are produced through cuttings and like processes. In a somewhat similar manner, a bud or twig from one plant may be united so intimately to another by budding or grafting that it thrives and grows as an integral part of the plant to which it has been transferred. One particular advantage of such a procedure is that it ensures a high degree of uniformity among the offspring, since each one of them is in fact a portion of the original individual. All true Concord grape vines or Baldwin apple trees, for example, are simply detached parts of the original vine or tree in which the variety originated.

In many plants, reproduction of this sort is a constant accompaniment of the slow spread and dispersal of the vegetative parts which is continually taking place. Thus a grass plant may become established in one spot and gradually extend its area until it forms a considerable patch of turf. Portions of this, through accident or decay, become separated from one another, and a colony of plants takes the place of a single individual. In a somewhat similar fashion the beech tree multiples itself, buds arising on the roots of the parent tree until it is surrounded by a grove of beeches. This method of reproduction, however, is not very common among the seed plants.

In many species, structures have arisen which are particularly adapted for aiding vegetative dispersal of the plant body and which thus partake somewhat of the nature of reproductive structures. To this group belong the runner or stolon of plants like the strawberry, the rapidly spreading rootstocks of the quack grass, and the long, arching stems of the blackberry, in which the tips touch the ground and there take root. Other vegetative parts are sometimes modified still further as reproductive organs. Perhaps the best known example of such is the tuber of the potato, which is merely a short and very much thickened underground stem, from the buds of which new potato plants arise the next season. The bulb, as in the onion and hyacinth, and corm, as in the crocus, are also short, stout stems with their lower leaves modified as scales. They carry the plant over from one season to the next and their buds ultimately give rise to a group of new plants.

Seed plants are characterized by their possession of a rather complex reproductive apparatus known as the flower. This consists typically of those structures intimately concerned with the development of the sexual cells, together with others which contribute indirectly to the success of the process of reproduction. Stamens and pistils are the essential organs of the flower. Each stamen bears an anther or pollen sac and within this sac are produced a great number of minute, single-celled pollen grains, from the contents of each of which two male gametes ultimately develop. The anther is commonly supported by a stalk or filament. The pistil consists of a closed chamber, the ovary, at the top of which is the stigma, a structure adapted to catch and hold the pollen grains. The stigma is often support by a stalk or style. In the ovary are borne the ovules or potential seeds, within each of which is a female gamete or egg. The fertilization of an egg by a male gamete starts the series of processes which result in the development of the ovule into a seed.

Chromosome Reduction

Certain noteworthy differences occur between the cell division which preceeds the formation of the gametes and those which we have studied in the ordinary vegetative tissues of the plant; and a knowledge of these differences is essential to a thorough understanding of the laws of inheritance, which we shall later discuss. In this division the chromosomes, as they make their appearance out of the nuclear network, are grouped in pairs; and when division takes place, the members of each pair separate, one going to one of the newly formed nuclei and the other to the other. The splitting of chromosomes which occurs in ordinary mitosis, and by which the chromosome number is maintained, does not occur here, and the resulting daughter nuclei (and the gametes derived from them) therefore contain only half the chromosome number found in the ordinary body cells of the plant. Such a division is accordingly known as a reduction division. When the gametes later unite in fertilization, each contributes its quota of chromosomes, and in the fertilized egg the original chromosome number is thus restored and then persists throughout all the cells of the plant which develops therefrom.

Perianth

The pistil occupies the center of the flower, with the stamens in a circle around it; and outside of these, in turn, is the perianth, composed typically of two circles of parts. The inner one of these is the corolla or a circle of petals. The petals are flat, somewhat leaf-like structures, usually conspicuous in color and rather delicate in texture, whose chief function is to attract to the flower those insects which are important in effecting pollination. Finally, outside the corolla is the calyx or circle of sepals. These are usually green or greenish structures which protect the delicate inner organs of the flower while in the bud. All floral parts are attached to the receptacle or enlarged tip of the flower stalk.

Variations in Floral Parts

These four groups of organs exhibit such great differences in the number, shape, size, color, texture and relative position of their parts as to give the flower a far greater range of structural diversity than the other organs of the plant, and we therefore depend upon the flower very largely for those characters which distinguish genera and families of plants from one another. In number, the circles may differ considerably. Among some of the more primitive orders there are several separate pistils, but this part of the flower is more often single, at least as far as its ovary is concerned, although from the number of stigmas or of chambers in the ovary we have reason to believe that in many cases there has been a fusion of several pistils, and that many-chambered ovary is thus a compound structure. The stamens are often the most numerous of the floral parts and are generally free from one another, although in some families they may be partially fused together. The petals are usually fewer than the stamens and rarely exceed ten in number. In certain orderst they are united to form a continous or gamopetalous (as opposed to a polypetalous) corolla. The sepals are generally of the same number as the petals and, like them, may sometimes be fused together into a gamosepalous (as opposed to polysepalous) calyx. Not only are the members of one circle sometimes joined together, but two entire circles may even be united. The corolla is sometimes attached to the calyx and is thus episepalous. Similarly, the stamens may be epipetalous (attached to the corolla) and the calyx epigynous (attached to the ovary) and so on. In shape, floral parts vary enormously. In higher plant groups, too, some of the sepals, petals or stamens are different from the rest, with the result that an unsymmetrical or irregular flower is produced, as opposed to the more primitive regular type. In color, flowers range through practically the entire spectrum, except that green is comparatively rare in the corolla. In size there is also great diversity, although flowers more than a decimeter in diameter are rare. In texture, flowers are generally soft except for the calyx, the firmness of their parts being produced turgidity of the cells rather than by skeletal issues. In certain cases, however, notably in the grasses and allied families, the perianth segments have become hard, dry, and chaffy. Any one, or more than one, of the floral circles may sometimes be absent. If both calyx and corolla are missing the flower is said the be naked. If it is either the stamens or the pistil which is absent, the flower is unisexual and is called either “male” or “female” according to the structures which it possesses. If both male and female flowers are distinct from one another but are on the same plant (as in corn, birch and many others) the plants are said to be monoecious; if the two sexes are on separate plants (as in the willow) the species is termed dioecious. In studying the evolution of floral parts, evidence has been obtained that the sepals, petals, and probably the stamens and units of the pistil are morphologically leaves; and that the earliest floral type was perfectly regular, with its various parts rather numerous and with no fusion whatever between circles or between members of the same circle.

Inflorescence

The arrangement of flowers on the plant is known as the inflorescence. The flowers may be solitary, arising from the ground, or singly in the axils of the leaves; or the leaves may be reduced to small bracts, the internodes shortened, and the flowers thus grouped into definite clusters. Such clusters are of various types as to shape and arrangement, the commonest among them being the raceme, spike, head, umbel, corymb, panicle, and cyme.

Pollination

The first step in the accomplishment of reproduction is the transfer of pollen from the anthers to the stigma, a process known as pollination. At about the time the flower unfolds, the anthers open and liberate the pollen grains. In rare cases the stigma lies so close to the anthers that the pollen transferred thereto directly, and this may sometimes happen even before the flower opens. In the great majority of cases, however, this transfer is brought about by some external agency, and pollen from the flowers of one plant is thus frequently carried to the flowers of another. The two most important agencies in effecting pollination are the wind and insects. Wind-pollinated or anemophilous flowers are prominently exposed on the plant but are generally small, inconspicuous and unisexual, possessing a poorly developed perianth, abundant dry and light pollen, and feathery stigmas. Insect-pollinated or entomophilous flowers, on the other hand, are conspicuous or possess marked odor, and are characterized by a well-developed corolla, pollen grains which tend to adhere in masses, stigmas which are sticky, and in many cases by the presence of nectaries secreting a sugary liquid. The insect is guided to the flower by its color or odor and visits it either to secure nectar, the source of honey, or pollen, the source of “bee bread”. Pollen readily adheres to the hairy bodies of these insects, and as it is thus carried about from flower to flower it often comes in contact with a stigma, to the sticky surface of which it is transferred. Insects belonging to the order Hymenoptera (the bees and their allies) are more important than any others in effecting pollination. In many cases we have evidence that offspring which arise from a cross, or union of sexual cells contributed by two different parents, are superior in vigor to those in which both gametes came from the same plant. Perhaps in response to this fact, there are an enormous number of devices among floweing plants which tend to insure cross-pollination, or the transfer of pollen from one flower to another, and to prevent self-pollination, or the transfer of pollen from anther to stigma of the same flower. Anthers and stigmas, for example, may ripen at different times, with the result that the anthers liberate their pollen either before the stigma of the same flower is ripe for pollination or after it has become no longer receptive. In many cases, also, pollen from another plant is better able to effect fertilization than the plants own pollen; and in extreme instances the plant may be actually self-sterile, its own pollen having no effect whatever upon its stigma. More striking than these methods, however, are the multitude of structural devices in entomophilous flowers whereby self-pollination through insect agency is rendered difficult or impossible and cross-pollination made easy. This is sometimes accomplished by floral dismorphism, in which there are two types of flowers, so constructed that the points where the anther and stigma touch the insects body are exactly reversed, with the result that the pollen of one is apt to reach the stigma of the other. More common are the various and often intricate devices in which hairs, springs, traps, and other agencies are employed. These reach their highest development in such families of irregular-flowered plants as the legumes and orchids, and have long excited the curosity and admiration of naturalists.

Fertilization

Pollination, however, is only a step toward the union of male and female gametes which we know as fertilization. Although the pollen grain is a single cell, it is not the male gamete. At about the time of pollination, the nucleus of the grain divides into two, one of which, the tube-nucleus, remains free in the cytoplasm. The other nucleus surrounds itself with a mass of cytoplasm of its own, sometimes with a separate wall, and is known as the generative cell. Shortly after the pollen has reached the stigma the thick wall of the pollen grain bursts at one point and out of the grain proceeds a thin-walled pollen-tube. Near the tip of this moves the tube-nucleus, followed by the generative cell. This tube bored through the tissues of the style and carries the contents of the pollen grain down into the ovary and to the mouth of an ovule. Meanwhile the generative cell divides into two male cells, which are the true male gametes. By this time the ovule has become prepared for fertilization. It possesses one or two coats or integuments, later developing into seed-coats, which cover it except at one point, the mouth or micropyle. Here the pollen-tube usually enters. Inside the integuments is a thin nutritive layer, the nucellus. Within this, in turn, and ordinarily occupying the whole central portion of the ovule, is the embryo-sac. This is a small, sap-filled cavity with three cells at each end and a naked nucleus, the endosperm nucleus, near its center. The three cells at the end of the sac farthest from the micropyle play no part in fertilization or seed development. Of the three at the micropylar end, however, one is usually distinguishable by its greater size and is the female gamete or egg. With the nucleus of this egg cell fuses on e of the male gametes which has come down the pollen tube. This union produces the fertilized egg, and from this single cell develops the entire embryo of the seed and thus the young plant which grows therefrom. This fertilized egg, in which are combined the protoplasm of the two parents, is the sole direct link between parents and offspring; and only across this exceedingly narrow bridge are characteristics transmitted by inheritance from one generation to the next. The fertilization of the egg by a male cell is not the only cell union which takes place at this time, for the other male cell fuses with the endosperm nucleus, and from the cell thus formed arises the endosperm or food-storage tissue of the seed. Fertilization effected by gametes from the same plant is known as self-fertilization; that by gametes from different plants as cross-fertilization.

Seed Development

After fertilization has been effected, the petals and stamens drop off and the ovule gradually develops into the seed. Various changes accompany this process. The whole structure grows markedly in size and the integuments increase in thickness, become hard and woody, and close over the micropyle. In many seeds a considerable mass of endosperm or food-storage tissue is developed, but in other this is much less abundant. Within the endosperm is the embryo or young plant, which has developed from the fertilized egg. In dicotyledonous plants the embryo is differentiated into three main portions; the hypocotyl or primitive stem and root, its tip directed toward the micropyle; the two seedleaves or cotyledons, attached to the upper end of the hypocotyl, and the plumule or bud, inserted between the cotyledons. The cotyledons may be very thick and serve entirely for food storage, as in the pea; they may be thin and leaf-life, serving as foliage leaves from the beginning, as in the morning glory, or they may combine both functions, as in the squash. In monocotyledonous plants endosperm is always well developed and the comparatively small embryo consists of a flat disc, the scutellum (which probably represents a single cotyledon) to the face of which are attached an upward-pointing, sheathed plumule or bud and a downward-pointing minature root or radicle. The scutellum serves to absorb food from the endosperm and to transmit it to the growing portions of the embryo. The ripe seed is thus a structure in which the partially developed young plant, well protected and provided with an abundant supply of food for future growth, is able to pass through a more or less extended period of dormancy.

The Fruit

The ripened ovary, together with its contents the seeds, and with any other structures intimately associated with these, is known as the fruit. The ripened wall of the ovary is called the pericarp. Fruits are various and many different types are recognized and named, but we shall mention here only the most common and important of them. Some are dry at maturity and split open. Such are the capsule (as in the lily), which arises from a compound ovary, and the pod (as in the bean), which arises from a simple or single-chambered one. Others are dry but do not split open. Such are achene (as in the buttercup), the commonest type of single-seeded fruit; the nut (as in the hickory), in which the pericarp becomes hard and woody, and the grain (as in the corn), the characteristic fruit of the grass family, in which seed coats and pericarp are firmly fused. These single-seeded fruits are often mistaken for seeds. Many fruits become fleshy, at least in part. In the berry (as in the blueberry), the entire fruit is so except the seeds, which have thick coats. In the stone fruit or drupe (as in the cherry), the outer part of the pericarp is fleshy but the inner portion, enclosing the seed, is a hard and woody “stone”. In the pome, represented by such fruits as the apple and pear, it is the receptacle grown around and enclosing the fruit, which becomes fleshy, the pericarp being represented here only by the tough membranes of the core.

Seed Dispersal

It is obvious that to leave successful offspring a plant must not only develop seeds but must provide for their dispersal; and in bringing this about, almost as great a variety of adaptive devices are employed as there are to insure cross-pollination. Dependence is placed upon various agencies, but chiefly the wind and animals. Many seeds or even entire fruits are light and provided with wings or tufts of long hairs, so that they present a large surface area for the wind to catch, and are often wafted many miles. In the case of the various “tumble weeds” the entire plant breaks off at the base of the stem and is rolled along over the ground by the wind. Other fruits and seeds develop hooks, spines, or sticky secretions which enable them to adhere to the fur of animals or the feet of birds and thus to be carried for long distances. In fleshy fruits, the fleshy portion or pulp is usually bright in color and is rendered attractive to animals by its taste. Birds are particularly important to the dissemination of the seeds of such fruits. In a few cases like the witch hazel, the fruit splits open with such force that the seeds are projected through the air for a considerable distance. The seeds and fruits on the water have been known to travel thus for hundreds of miles.

Seed Germination

The seed remains dormant until a favorable environment appears, when the embryo begins to grow and the seed is said to germinate. The conditions necessary for germination are a plentiful supply of water and oxygen and a moderately high temperature. When these are fulfilled, metabolism begins vigorously in the embryo and in the cells of the endosperm. Water is absorbed in large quantities and the embryo swells, bursts the seed coats, sends its root into the ground and its stem into the air, and becomes a seedling. The food stored in endosperm or cotyledons is digested and used for the development of new organs. It is generally sufficient in amount to provide for the growth of the seedling to a point where the latter can begin to manufacture its own food. Indeed, as soon as the young stem and leaves get above the ground they turn gree and commence photosynthetic activity, soon supplying an abundance of food which ensures the rapid development of the plant from the seedling stage to maturity, at which point the cycle of reproduction is complete.