3. Kingdom Plantae - part 06 - Plant life cycle and alternation of generations

 

3. Kingdom Plantae - part 06 - Plant life cycle and alternation of generations


Plant life cycle and alternation of generations:

  • Life cycle of a plant includes two phases or distinct generations namely - 
  1. sporophyte (diploid : 2n) and 
  2. gametophyte (haploid : n).
  3. Some special diploid cells of sporophyte divide by meiosis to produce haploid cells. 
  • These haploid cells divide mitotically to give rise to gametophyte. 
  • The gametophyte produces male and female gametes which fuse during fertilization to produce diploid zygote. 
  • It divides by mitosis to form diploid sporophyte.
  • The sporophytic and gametophytic generations generally occur alternately in the life cycle of a plant. This phenomenon is called alternation of generations.
  • Distinct alternation of these two generations is observed in Bryophytes and Pteridophytes. 
  • In Gymnosperms and Angiosperms, gametophyte is much reduced and exists within sporophyte. 
  • In algae, based upon the nature of dominant phase in life cycle, it is called haplontic, diplontic or haplodiplontic life cycle.
  • In Bryophytes haploid gametophyte is dominant. 
  • It is photosynthetic, independent thalloid or erect phase. 
  • Sporophyte is short lived, multicellular and depends totally or partially on gametophyte for nutrition and anchorage. 
  • Whereas in Pteridophytes, sporophyte is dominant, independent and vascular plant body. 
  • Haploid multicellular gametophyte is generally autotrophic and short lived. 
  • It alternates with Sporophyte.

Diplontic :

  • Here mitotic divisions occurs only in diploid cells. 
  • Gametes formed through meiosis are haploid in nature. 
  • The diploid zygote divide mitotically. 
  • In this process production of multicellular diploid organism or in the production of many diploid single cells takes place. 
  • E.g. Animals.

Haplontic :

  • Here mitosis occurs in haploid cells. 
  • It results in the formation of single haploid cells or a multicellular haploid organism.
  • These forms produce the gametes through mitosis. 
  • Zygote is formed After fertilization. 
  • This cell is the only diploid cell in the entire life cycle of the organism. Thus the same zygotic cell later undergoes meiosis.
  • E.g. Some Algae and Fungi.

Haplo-diplontic :

  • Here mitosis occur in both diploid and haploid cells. 
  • These organisms undergo through a phase in which they are multicellular and haploid (the gametophyte), and a phase in which they are multicellular and diploid (the sporophyte). 
  • E.g. Land plants and in many algae.

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