Before we talk about Reproduction in Plants, it is important to understand its purpose.
Every living organism has a fixed lifespan and dies one day. For the maintenance of the population of a particular organism, new organisms of that species must be produced.
Thus reproduction is the process of production of new individuals from their parents. Hence, it is an essential process to ensure the survival of any species on the earth.
Reproduction In Plants
Plants can reproduce in many ways to produce their offspring. But broadly it can be classified into two types –
- Asexual reproduction
- Sexual reproduction
It is the formation of new offspring without the formation of gametes and seeds. Asexual reproduction can be of many types. Since there is no formation of gametes, each new offspring is genetically identical to its parent, so they are clones.
In this process, a small bud-like projection starts forming on the original cell. This bud grows in size while being attached to the original cell, getting its nutrition, cytoplasmic and nuclear membrane from the cell. Finally, the membrane of the bud closes pinching of the bud along with the cytoplasm within it thus creating a separate cell. This new cell can grow, mature and produce further cells by budding. Multiple buds can form on one cell at different places. A chain of buds may also be formed with each bud forming on one cell and the next bud forming on the former cell.
When the bud detaches it leaves behind scar tissue. Since reproduction is asexual, the newly created organism is a clone. In multi-cellular organisms, bud develops as an outgrowth due to repeated cell division at one specific site. These buds develop and when fully mature, detach.
This occurs usually in algae. A strip of algae breaks up into two pieces. Each piece can now regenerate and grow into two new individuals. This is a very rapid method of division. That is why algae are able to spread rapidly.
The splitting may also occur due to man-made or natural damage by the environment or predators.
The parent cell first duplicates its genetic material. In plants the genetic material is DNA. Then the cell undergoes cytokinesis (a division of the cytoplasm) and divides into two with each new cell receiving one copy of each DNA. This is called binary fission.
Sometimes the single cell may divide into more than two new offspring. This is called multiple fission.
Spores may help in both sexual and asexual reproduction. They are a popular reproductive unit in plants, fungi, bacteria, and protists. Spores are prevalent among algal groups. Spores are haploid and are produced by meiosis in the sporophyte. The spore produces the gametophyte, which produces gametes. When two of these gametes fuse, a zygote is formed. This zygote develops into a new sporophyte. A wide variety of spores are possible.
- Zoospores- flagellated spores. They are motile and found in algae.
- Meiospores -haploid spores formed due to meiosis. They give rise to a gametophyte. They are of two types -microspores and megaspores. Microspores give rise to male gametophyte and megaspores give rise to the female gametophyte.
- Aplanospores- spores that are immobile but may have flagella.
- Autospores-immobile spores without flagella.
- Carpospores and tetraspores are characteristic spores produced by red algae.
- Rhizomes- prevalent in banana and ginger they are stem-like structures that grow across the ground from which new roots and shoots grow.
- Sucker- new shoots that grow from buds present at the base of plant stems or roots (adventitious buds).
- Runner/stolons- modified stems of the original plant which run horizontally just under the soil surface. Buds grow on these stems which grow into new plants.
- Plantlets- tiny plants form from the meristem of the leaves (like bryophyllum) of the parent plant which later falls off to form new plants.
- Eyes of tubers- in potato these new buds on the stem grow into new plants
This is a mechanism present in plants that mimic sexual reproduction. Seeds are formed in this case. In some species, the diploid egg cell is formed without meiosis. It develops into the embryo. Sometimes few cells of the nucellus around the embryo sac start dividing. They protrude into the embryo sac and develop into the embryos.
Sexual reproduction is facilitated by the production of gametes. The haploid spores divide by mitosis to form the gametophyte. The male gametophyte is formed by microspores, and the female gametophyte is formed by megaspores. The gametophytes form their respective gametes which are haploid. These gametes fuse to form the diploid zygote. This zygote divides and redivides to form the embryo which grows into the new plant. Sexual reproduction in plants is most developed in angiosperms.
Flowers are the location of sexual reproduction in angiosperms. They are bright and fragment to attract insects to facilitate pollination. The male whorl (androecium) is the anther and filament. The anther contains the microsporangium and is bilobed. The pollen grains are formed inside the anther when cells of microsporangium undergo meiosis. The female whorl(gynoecium) consists of the stigma, style, and ovary. The pollen grains from anther land on the stigma, travel through the style and enter the ovary. Each pollen grain releases 2 male gametes.
The ovary has the megasporangium which produces embryo sac by meiosis. This is surrounded by a protective nucellus layer. The haploid embryo sac undergoes repeated mitosis to form a 7 celled 8 nucleated structure( 2 synergies, 1 egg cell, 2 polar nuclei, and 3 antipodal cells). A male gamete fuses with the egg (female ova) to form a zygote and the resultant embryo; the male gamete fuses with the two polar nuclei to form a triploid endosperm which provides nutrition. The embryo grows into a mature plant. The ovary develops into a fruit, and the ovules develop into seeds.