Plant Life Cycles and Alternation of Generations

Life cycle of plants: It is recurring morphological cytological phases in an organism that occur in each individual organisms from it’s birth to death.

According to it can be divided into three –

I. Haplobiontic - In this cases there is single somatic phase is present. Again it can be divided into two- 

1. Haplontic -  It has single somatic phase which is haploid. Diploid condition is present only in zygote or zygospore wherein meiosis occurs to produce haploid condition again. Example of haplontic is mainly algae (Spirogyra, Chlamydomonus, Ulothrix and Chara. Here alteration of generation is absent.

2. Diplontic –In this case single diploid somatic phase is observed. Haploid conditions occurs in gametes which is formed through meiosis and it is fused to restore diploid state. Here alteration of generation is absent. Examples of diplontic are – Cladophora, Bryopsis, Fucus, Sargassum etc.

II. Diplobiontic – Organisms which show diplobiontic life cycle have two somatic phases in their life history.

1. Diplohaplontic: Both haploid gametophyte and diploid sporophytic somatic phases occur in the life history. It has two subtypes. They are-

• Isomorphic or Monomorphic Diplohaplontic-  In this case gametophytic and sporophytic phases are morphologically similar though cytologically they are haploid and diploid respectively. There is an isomorphic or homologous alteration of generation. Example of this type are –Ulva, Dictyota.
• Heteromorphic or Dimorphic Diplohaplontic- Haploid and diploid phases are morphologically dissimilar. One of them is dominant. A heterologous or heteromorphic alteration of generation occurs. In some cases gametophytic body is dominant (Bryophyte) and in others sporophytic body is dominant ( pteridophytes).

2. Haplohaplontic – Only haploid cytological stage is observed which contain two somatic phases. They are dimorphic haplobiontic or diphasic haplohaplontic. Example of this is Porphyra.

III. Triplonbiontic – Two types of  somatic phases areoccurred. They are-

1. Haplohaplohaplontic – Three haploid phases are occurred in entire life history.

2. Diplodiplohaplontic – Triphasic or trimorphic life history with two diploid and one haploid stages. Example is Polysiphonia.

Plants alteration of generation mean occurrence of conversion of gametophytic stage and sporophytic stage on each other and their dependency in plant life cycle. In some plants sporophyte (2n) is dominant whereas in some plants gametophytic (n) is dominant.

Bryophyte: Bryophyte has haploid gametophyte dominant body. It produced eggs (n) in the archegonium and sperm (n) in antheridium. Sperms need water to swim towards the eggs for fertilization and zygote is produced (2n) which give rise to the sporophytic body (2n). Parasitic sporophyte (depending on female gametophyte) undergoes meiosis division to give rise gametophytes.

Pteridophytes: Sporophytic body is dominant in fern. Diploid cells undergoes meiosis to produce haploid spores (n)which burst out to give rise to haploid organisms and produces gametophyte.After maturation haploid organisms create haploid egg and sperm which fuse together to produce sporophyte.

Other plants have diploid dominant body and haploid gametes give birth to the diploid new plant.

Question and answer on Plant Life Cycles and Alternation of Generations:

What is hypnozygote?

Most of the green algae have a haplontic life cycle. The zygote nucleus is being the only diploid nucleus in this type of life cycle. The zygote becomes thick walled and germination occurs after a period of obligate dormancy. This thick walled zygote is called hypnozygote.

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