Life span is the period from birth to the natural death of an organism. The life span of organisms varies from a few days to a few thousand years. Reproduction is the process that ensures continuity of living organisms generation after generation.
Reproduction is defined as the biological process in which an organism gives rise to young ones similar to itself.
Reproduction is of two types in plants
1. Asexual reproduction
2. Sexual reproduction
6.2 ASEXUAL REPRODUCTION
Asexual reproduction is a method in which a single individual parent is capable of producing offspring.
The offsprings produced during asexual reproduction are identical to one another and also exact copies of their parent.
Different kinds of asexual reproduction in lower plants:
1. Binary fission: Ex:- Protists and Monerans (Euglena and Bacteria)
2. Budding: Yeasts
3. Zoospores(motile): Chlamydomonas
4. Aplanospores (non- motile): Rhizopus (Bread mold)
5. Conidia : Pencillium
6. Fragmentation: some forms of colonial algae, moulds and mushrooms.
7. Gemmae: liverworts
Asexual reproduction in flowering plants takes place by vegetative propagules like runners, stolons, suckers, offsets, rhizomes, corms, tubers, bulbs, bulbils and reproductive leaves.
Plants produced vegetatively or asexually are called clones.
Water hyacinth – “Terror of Bengal” is an example for profuse vegetative propagation.
Simple organisms like algae and fungi shift to sexual mode of reproduction just before the onset of adverse conditions.
6.3. SEXUAL REPRODUCTION
Sexual reproduction involves formation of the male and female gametes either by the same individual or by different individuals of the opposite sex.
The gametes unite to form a zygote which develops into a new organism.
The offsprings produced by sexual reproduction are not identical to the parents or among themselves.
In annuals and biennials vegetative, reproductive and senescent phases are clear.
In perennials these phases are not distinct.
Hormones and environmental factors regulate reproductive process.
Events in sexual reproduction:
3.Post – fertilization
Pre fertilization events:
2. Gamete transfer
It is the process of formation of two types of gametes.
If both the gametes are similar they are called homogametes or isogametes. Ex: Cladophora
If the gametes are dissimilar and distinguished into male and female they are referred as heterogametes Ex: Funaris, Pteris, Cycas.
If both male and female reproductive structures are present in the same plant it is said to be bisexual. Homothallic in fungi and monoecious in plants. Ex: cucurbits and coconuts.
If male and female reproductive structures are present on different plants it is said to be unisexual. Heterothallic in fungi and dioecious in plants. Ex: Papaya and date palm.
A haploid parent produces gametes by mitotic division.
A diploid parent produces gametes by meiosis (reduction division).
Water is the medium of gamete transfer in algae, bryophytes and pteridophytes.
In few fungi and algae both male and female gametes are motile.
In many plants male gametes are motile and female gametes are stationary.
In seed plants (angiosperms and gymnosperms) pollen grains produce male gametes and ovule produces egg (female gamete).
Hence the pollen grains are transferred to the stigma of a flower by pollination.
Fusion of gametes to form a diploid zygote is called syngamy or fertilization.
Fertilization is external in majority of algae (in water).
Fertilization is internal in bryophytes, pteridophytes, gymnosperms and angiosperms.
In seed plants the male gametes are carried by pollen tubes to the female gamete.
Post fertilization events:
Every sexually reproducing organism begins life as a single celled zygote.
In fungi and algae the zygote develops a thick wall and undergoes a period of rest. It later divides by meiosis to form haploid spores that grow into haploid individuals.
Zygote undergoes cell division (mitosis) and cell differentiation to form an embryo.
The process of development of embryo from the zygote is called embryogenesis.
As the zygote develops into embryo, the ovules develop into seeds, the ovary develops into fruit.
Seeds germinate under favorable conditions to produce new plants.