|Reproduction in Plants|
In unicellular organisms three steps result in cellular reproduction. Among prokaryotic organisms (made of cells that have no nuclei), the single loop of DNA (deoxyribonucleic acid, the molecule that carries genetic information) replicates; then one copy is carried to each daughter cell as the original cell elongates and then “pinches” in two, a process called fission.
In eukaryotic organisms (whose cells have nuclei), the DNA is located within a nucleus in discrete chromosomes that must be precisely divided between the two daughter cells.
Nuclear division to produce two identical daughter cells (asexual reproduction) is called mitosis. A chromosome during cell division consists of two halves, sister chromatids, each of which is identical to the other.
During mitosis, every chromosome in the nucleus splits in half so that one chromatid will migrate to the first daughter cell, and the second chromatid migrates to the other.When cell division is complete, the result is two genetically identical daughter cells.
In sexual reproduction, a nucleus must divide by meiosis. In sexually reproducing organisms, at least some cells will have pairs of every type of chromosome.
Such cells are diploid, or 2n, where n is the number of different types of chromosomes. During meiosis, the pairs separate so that each daughter cell has only one of each type of chromosomes and is haploid, or 2n.
If two different haploid cells fuse, the resulting cell will have pairs of every type of chromosome but with one of each pair contributed by each of the two parents. The offspring will thus be different from either parent.
Sexual Reproduction: Alternation of Generations
Sexual reproduction provides an opportunity for an organism to have different kinds of cells at different stages of its life cycle. The most familiar example is what is known from animals, including humans.
The body cells of the adult are diploid, but in the reproductive organs, meiosis occurs to form haploid cells, either eggs or sperm. If these haploid reproductive cells (gametes) fuse, a new diploid cell is formed, the zygote.
The zygote divide smitotically to form an embryo and eventually a new adult consisting of diploid body cells similar to those of both parents. This is a gametic life cycle, in which the gametes are the only haploid cells, and they are formed directly by meiosis. Fertilization occurs immediately after meiosis.
Some algae, particularly diatoms and some of the green algae, also have a gametic life cycle. In many plant species the gametes are large, immotile eggs and small, motile sperm, just as in animals. This condition is oogamy.
However, many other plants are not oogamous. In some cases the two gametes appear to be identical (isogamous), while in others there may be two distinctive sizes of gametes, but their shape and motility are the same (anisogamous).
Some of the green algae have a life cycle exactly the opposite of the gametic cycle described above. In these plants, the diploid zygote divides bymeiosis to produce haploid daughter cells, which multiply to form either a population of haploid unicellular plants or a multicellular plant with a haploid body.
Eventually some of these haploid cells will differentiate into gametes, and two gametes will fuse to form a new zygote. In this type of zygotic life cycle, the zygote is the only diploid cell in the plant’s life cycle, and fertilization is delayed after meiosis occurs.
Even more interesting is the sporic life cycle, in which a plant will have both a haploid gametophyte and diploid sporophyte body at different stages of its life. This type of life cycle is often called alternation of generations.
These bodies may look the same (isomorphic), or they may look completely different (heteromorphic). In some cases two different species have been described and later discovered to be simply the haploid and diploid forms of the same species.
Most plants, including most algae and fungi, have some form of a sporic life cycle. In a sporic life cycle, the diploid adult sporophyte plant forms reproductive organs in which meiosis occurs to form spores.
These spores germinate and undergo mitosis to form a multicellular haploid gametophyte body. The gametophyte forms reproductive organs in which gametes are produced. Following fertilization, the resulting zygote undergoes mitosis to form the new sporophyte.
Many red algae and some of the green and brown algae are isomorphic; that is, the gametophyte and sporophyte bodies look identical. Bryophytes, some green algae, some brown algae, and most fungi are heteromorphic, with the gametophyte being the dominant, more conspicuous body.
For instance, in mosses the green leafy plant is the gametophyte. When it is mature, the gametophyte will produce two types of gametangia, archegonia and antherida.
Archegonia are flask-shaped structures that produce an egg, and antheridia are sac like structures that produce multiple sperm. After a sperm cell fertilizes the egg, the zygote grows into a sporophyte plant, still attached to and growing out of the gametophyte.
The tip of the sporophyte swells to become a capsule,or sporangium, where meiosis occurs to form haploid spores. Each spore that falls onto a suitable place will germinate and grow into a new, leafy gametophyte plant.
Vascular plants and many brown algae are heteromorphic,with the sporophyte dominant. For instance, leafy ferns are the leaves of a sporophyte plant with an underground stem and roots.
Sporangia typically develop on the underside of the fern, and meiosis occurs inside the sporangia to produce haploid spores. If a spore falls onto a suitable habitat it will germinate to form a small, inconspicuous gametophyte plant.
The gametophyte typically produces archegonia and antheridia. Following fertilization, a new sporophyte plant grows out of the archegonium, but it is usually not seen until the first leaves enlarge enough to be visible above the soil surface.
In theory, any cell from a plant body should be capable of generating an entire new plant, because the nucleus of each cell has identical genetic information to every other cell in that body. In fact, since the mid-1950’s it has been possible to clone many plants—produce entire new plants from single cells of a parent plant.
The techniques of plant cell and tissue culture are used to propagate many commercially important species of ornamental plants. These techniques are also valuable tools in plant research. The basis for these tools is found in nature—the variety of methods of asexual reproduction found in plants.
In non vascular plants, particularly fungi and filamentous algae, fragmentation can be an effective way of increasing the number of individuals of a plant. If the plant body is physically broken into pieces, each piece may continue to grow and develop as an independent plant.
Most algae also form sporangia, asexual reproductive organs that produce motile zoospores. When the unicellular zoospores are released, they will swim for a period and, if they settle in a suitable environment, will germinate and grow to form a new plant.
In some cases specialized multicellular asexual propagules are formed. For instance, lichensmay formisidia or soridia, and liverworts may form gemmae. If the propagule breaks off and lands in a suitable environment, it will grow into a new plant.
Asexual reproduction may also occur in vascular plants. If a stem is laid horizontally on the ground, it may produce adventitious roots, which will allow that portion of the stem to grow as a separate plant.
Similarly, some plants have roots that form buds and produce new stems at some distance from the original stem. The most dramatic examples of this are aspen groves in which all the trees are clones of one another. These natural phenomena are the basis of horticultural plant propagation by means of stem, leaf, or root cuttings.
Other plants form specialized structures for asexual propagation. For instance, strawberries produce stolons, specialized stems that grow out from a plant, then root and form new plantlets.
Bulb-forming plants, such as gladiolus, frequently form new bulblets, and if a rhizome is split, such as on an iris, each half will continue to grow as a new plant. Some plants, such as kalenchoe, produce complete plantlets on the edges of leaves, which fall off and disperse to propagate the plant.