Standard 11 | note 12 | Plant Kingdom | Plant life cycles and alternation of generations | Short Note For NEET BIOLOGY

 

Chapter 3: Plant kingdom

Plant Kingdom | Plant life cycles and alternation of generations | Short Note For NEET BIOLOGY| standard 11

Note 12

Plant life cycles and alternation of
generations

both haploid and diploid cells can divide by mitosis - in plants 

So different plant bodies can form - haploid and diploid. 

The haploid plant body - 

• produces gametes- through mitosis. 

• This plant body is a gametophyte. 

• After fertilisation of gametes - diploid zygote produced

The diploid plant body - 

• the zygote divides by mitosis - produce a diploid plant body. 

• This plant body is a sporophyte.

• Cells of this body divide- by meiosis

• Produce spores - haploid spores

• Haploid spores divide - by mitosis

• This forms haploid plant body 

So, during the life cycle of any sexually reproducing plant, there is an alternation of generations occur.

Definition of alteration of generations:- Two plant bodies occur one after another -- gamete producing haploid gametophyte and spore producing diploid sporophyte.

But, different plants have different patterns:

1. Haplontic life cycle

• In some plant groups, Sporophytic generation is represented only by the single-celled zygote. 

• There are no free-living sporophytes.

• Meiosis in the zygote results in the formation of haploid spores.

• The haploid spores divide mitotically and form the gametophyte. 

• The dominant and photosynthetic phase in such plants is the free-living gametophyte.

• Example: Many algae such as Volvox, Spirogyra and some species of Chlamydomonas.

2. Diplontic life cycle:

• In this type, the diploid sporophyte is the dominant, photosynthetic, independent phase of the plant. 

• The gametophytic phase is represented by the single to few-celled haploid gametophyte. 

• Example : an alga, Fucus sp., represents this pattern. 

• In addition, all seed bearing plants i.e., gymnosperms and angiosperms have diplontic  life cycle with some variations - the gametophytic phase is few to multi-celled.

3. Haplo-diplontic life cycle 

• this is an intermediate condition of haploid life cycle and diploid life cycle 

• both phases are multicellular.  

• Pattern 1: A dominant, independent, photosynthetic, thalloid or erect phase - haploid gametophyte

•  it alternates with the short - lived,  multicelluler sporophyte 

• Sporophyte is totally or partially dependent on the gametophyte - for its anchorage and nutrition

•  All bryophytes follow the above pattern.

• Pattern 2: The diploid sporophyte is represented by a dominant, independent, photosynthetic, vascular plant body. 

• It alternates with multicellular, saprophytic/autotrophic, independent but short-lived haploid gametophyte. Such a pattern is known as haplo-diplontic life cycle. All pteridophytes exhibit this pattern. 

Life cycle pattern in alga:  

• Interestingly, while most algal genera are haplontic, 

• some of them such as Ectocarpus, Polysiphonia, kelps are haplo-diplontic. 

• Fucus, an alga is diplontic.

Note down these in your note-book for quick read during your examination. You can freely ask us any doubt regarding concepts, we will surely answer you back.

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Manish Mevada
Urvi Bhanushali