The formation by a plant of offspring that are either exact copies or reasonable likenesses. When the process is accomplished by a single individual without fusion of cells, it is referred to as asexual; when fusion of cells is involved, whether from an individual or from different donors, the process is sexual.

Using the technique of tissue culture, higher green plants can be regenerated from a single cell and can usually flower and set seed normally when removed and placed in soil. This experiment shows that each cell of the plant body carries all the information required for formation of the entire organism. The culture of isolated cells or bits of tissue thus constitutes a means of vegetative propagation of the plant and can provide unlimited copies identical to the organism from which the cells were derived.

All other vegetative reproductive devices of higher plants are elaborations of this basic ability and tendency of plant cells to produce tissue masses that can organize into growing points (meristems) to yield the typical patterns of differentiated plant organs. For example, a stem severed at ground level may produce adventitious roots. Similarly, the lateral buds formed along stems can, if excised, give rise to entire plants. The “eyes” of the potato tuber, a specialized fleshy stem, are simply buds used in vegetative propagation of the crop. In many plants, cuttings made from fleshy roots can similarly form organized buds and reconstitute the plant by vegetative propagation. Thus, each of the vegetative organs of the plant (leaf, stem, and root) can give rise to new plants by asexual reproduction. Plant propagation

While in asexual reproduction, the genetic makeup of the progeny rarely differs greatly from that of the parent, the fusion of cells in sexual reproduction can give rise to new genetic combinations, resulting in new types of plants. The life cycle of higher green plants consists of two distinct generations, based on the chromosomal complement of their cells. The sporophyte generation is independent and dominant in the flowering plants and ferns, but small, nongreen, and dependent in the mosses, and contains the 2n number of chromosomes. The diploidy results in each case from the fusion of sperm and egg to form the zygote, which then develops into an embryo and finally into the mature sporophyte. The sporophyte generation ends with the formation of 1n spores by reduction division, or meiosis, in a spore mother cell. The spore then develops into the gametophyte generation, which in turn produces the sex cells, or gametes. The gametophyte generation ends when gametes fuse to form the zygote, restoring the 2n situation typical of sporophytes.

In flowering plants, the gametophyte or 1n generation is reduced to just a few cells (generally three for the male and eight for the female). The male gametophyte is formed after meiosis occurs in the microspore mother cells of the anther, yielding a tetrad of 1n microspores. Each of these microspores then divides mitotically at least twice. The first division produces the tube nucleus and the generative nucleus. The generative nucleus then divides again to produce two sperms. These nuclei are generally not separated by cell walls, but at this stage the outer wall of the spore becomes thickened and distinctively patterned—a stage typical of the mature male gametophyte, the pollen grain. Flower Pollen Pollination

Each pollen grain has a weak pore in its wall, through which the pollen tube emerges at the time of germination. Pollen germinates preferentially in the viscous secretion on the surface of the stigma, and its progress down the style to the ovary is guided through specific cell-to-cell recognition processes. Throughout its growth, which occurs through the deposition of new cell wall material at the advancing tip, the pollen tube is controlled by the tube nucleus, usually found at or near the tip. When the pollen tube, responding to chemical signals, enters the micropyle of the ovule, its growth ceases and the tip bursts, discharging the two sperms into the embryo sac, the female gametophyte of the ovary.

The female gametophyte generation, like the male, arises through meiotic division of a 2n megaspore mother cell. This division forms four 1n megaspores, of which three usually disintegrate, the fourth developing into an eight-nucleate embryo sac by means of three successive mitotic divisions. The eight nuclei arrange themselves into two groups of four, one at each pole of the embryo sac. Then one nucleus from each pole moves to the center of the embryo sac. One of the three nuclei at the micropylar end of the embryo sac is the female gamete, the egg, which fuses with one of the sperm nuclei to form the zygote, the first cell of the sporophyte generation, which produces the embryo. The second sperm fuses with the two polar nuclei at the center of the embryo sac to form a 3n cell that gives rise to the endosperm of the seed, the tissue in which food is stored. The entire ovule ripens into the seed, with the integuments forming the protective seed coat. The entire ovary ripens into a fruit, whose color, odor, and taste are attractive to animals, leading to dispersal of the seeds. The life cycle is completed when the seed germinates and grows into a mature sporophyte with flowers, in which meiotic divisions will once again produce 1n microspores and megaspores.

Nonflowering higher plants such as the ferns and mosses also show a distinct alternation of generations. The familiar fern plant of the field is the sporophyte generation. Meiosis occurs in sporangia located in special places on the leaves, generally the undersides or margins. A spore mother cell produces a tetrad of 1n spores, each of which can germinate to produce a free-living, green gametophyte called a prothallus. On the prothallus are produced male and female sex organs called antheridia and archegonia, which give rise to sperms and eggs, respectively. Sperms, motile because of their whiplike flagella, swim to the archegonium, where they fertilize the egg to produce the zygote that gives rise to the sporophyte generation again.

In mosses, by contrast, the dominant green generation is the gametophyte. Antheridia or archegonia are borne at the tips of these gametophytes, where they produce sperms and eggs, respectively. When suitably wetted, sperms leave the antheridium, swim to a nearby archegonium, and fertilize the egg to produce a 2n zygote that gives rise to a nongreen, simple, dependent sporophyte. The moss sporophyte consists mainly of a sporangium at the end of a long stalk, at the base of which is a mass of tissue called the foot, which absorbs nutrients from the green, photosynthetic gametophyte. Meiosis occurs in the sporangium when a spore mother cell gives rise to four reduced spores. Each spore can germinate, giving rise to a filamentous structure from which leafy gametophytic branches arise, completing the life cycle.

Various members of the algae that reproduce sexually also display alternation of generations, producing sperms and eggs in antheridia and oogonia. Sporophyte and gametophyte generations may each be free-living and independent, or one may be partially or totally dependent on the other. Fruit Plant physiology Population dispersal Seed