REPRODUCTION IN PLANTS | DEFINITION | IMPORTANCE & TYPES
What is a Reproduction in Plants? Reproduction is the mechanism that produces new generations and maintains a species. Every species of organism can reproduce new individuals of that species. In organisms, methods of reproduction are varied and some are quite complex. In one respect, reproduction differs from all other functions of animals. it is not necessary for the well being of the individual. Although reproduction has nothing to do with the survival of the individual. it is very important to the survival of a species or a population.
TYPES OF REPRODUCTION IN PLANTS
Reproduction in plants is of two types;
- Asexual reproduction
- Sexual reproduction
Asexual reproduction in plants requires only a single parental organism which gives rise to offspring by mitotic cell division, during which the complete adult number of chromosomes is exactly replicated and passed on so that the offspring are genetically identical to the parent. Methods of asexual reproduction are fission, sporulation, budding, vegetative propagation, artificial propagation (layering, grafting), parthenogenesis, apomixes etc:
Fission: Asexual reproduction in unicellular organisms in which a single cell divides to produce two (or more) daughter cells that are similar to parents. e.g. bacteria, Paramecium, Amoeba,
Sporulation; It is asexual reproduction with the help of spores e.g., fungi etc.
Budding: In this case, a new individual is formed as an outgrowth (bud) of the parent e.g., yeasts, Hydra, sponges, urochordates etc.
Vegetative propagation: Any method of nonsexual reproduction that involves the detachment, of fairly large usually differentiated multicellular bodies from the parent plant. Organs of vegetative propagation are bulbs, corms, rhizomes tubers, stolons, gemmae, etc.
Artificial propagation: It is also vegetative reproduction but done by man.
Layering: In layering, a new plant develops from a stem that is still attached to the parent plant. In nature, the stem simply arches over and spreads out on the ground. The parts of the stem that are in contact with the soil develop the root sterns, and leaves of a new plant, Examples are blackberries black raspberries and several other species.
cuttings: For many plant species, a leaf, section of stem, or piece of root cut from a plant and lightly covered in soil, peat moss, or another growth medium develops a new, independent plant-by generating the missing parts. Conifers, including pine and spruce; a variety of shrubs, roses and honeysuckle are examples.
Grafting: In grafting, a freshly cut section of stem with buds, called a scion, is joined to another plant called the stock. The upper stem of the stock is severed and the scion is joined to the lower stem. The scion is securely attached to the stock, and the tissues of the two plants grow into each other, forming a single plant. The scion produces the stems, leaves, and flowers on the new plant and the stock provides the root system.
Parthenogenesis: Growth of an organism from an unfertilized gamete, or sex cell. Common in insects. Less commonly occurs in lower plants.
Apomixes: A seed develops directly from tissues of the ovule rather than from a fertilized egg e.g. dandelions and blackberries. In asexual reproduction, although an increase in a number of genetically alike .individuals from a parent is very rapid this is not an adaptive method and may at some stage jeopardize (put at risk) the survival of a species.
Tissue culturing and cloning
Man has favoured this type of reproduction for his own needs, commonly in plants but now tissue culturing in plants and cloning in animals is being adopted for producing organisms of valuable characteristics, without a change in their genetic makeup. Cloning has been practiced successfully but its disadvantages like rapid aging and low resistance to environmental stress and diseases are still the limitations in commercial ventures. Also, it is still not being accepted socially and morally in general.
Sexual reproduction in plants usually involves two parents. A fertilized egg is produced through the union of meiotically produced specialized sex cells (egg and sperm) from each parent. Meiosis or reduction division give rise to gametes (gametogenesis) in which not only the chromosome number is halved (haploid) but the reshuffling of genes lead to new recombination of genes. This not only maintains the chromosome number in a species but also produce genetic variations, an important factor in the survival and adaptation of a species or a population. In plants, if there is an alternation of generation between a diploid sporophyte and a haploid gametophyte meiosis occurs during spore formation (Sporogenesis).
Diplohaplontic life cycle
Plants have a diplohaplontic life cycle with alternating diploid sporophyte and haploid gametophyte generations. If the two generations are vegetatively similar such alternation of generation is referred to as isomorphic, and if they are dissimilar it is called heteromorphic.
Reasons for success of seed plants
Seed plants are predominantly present all around us due to their better sexual reproduction mechanism:
a) modification of flower and inflorescence for pollination.
b) involving gamete transfer by pollen tubes.
c) food storage for developing an embryo.
d) protection by seed coats.
e) dispersal with the help of fruit formation (Angiosperms).
f) Seeds are capable of enduring unfavourable conditions in dormant form (Seed dormancy) and as soon as, conditions become favourable for establishing the seed in it terminates.