REPRODUCTION AND ITS TYPES - Key Concepts 
   » 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

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. 
 

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:
         1. Pre fertilization
         2. Fertilization
         3. Post – fertilization 
¤ Pre fertilization events:
        1. Gametogenesis
        2. Gamete transfer
Gametogenesis: 
» It is the process of formation of two types of gametes.
» If both the gametes are similar they are called homogametes or isogametes.
         e.g.: Cladophora

     » If the gametes are dissimilar and distinguished into male and female they are referred as
    heterogametes e.g.: 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. e.g.: 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.
         e.g.: Papaya and date palm.
     » A haploid parent produces gametes by mitotic division.
     » A diploid parent produces gametes by meiosis (reduction division).
Gamete transfer:
     » 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.

¤ Fertilization:
     » 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.