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Plant Life cycles and Alternation of Generations

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Plant Life cycles and Alternation of Generations - Lesson Summary

The alternation of sporophytic generation with a gametophytic generation in a sexually reproducing plant is called alternation of generations. In the lifecycle of a fern, a mature sporophyte bears sporangia which produce haploid spores through meiosis. These spores undergo mitosis and germinate into a prothallus, a haploid gametophyte. The gametophyte bears the antheridium and the archegonium that respectively produce the antherozoids and the egg.

The fusion of these haploid gametes result in the formation of a diploid zygote which undergoes mitosis to produce a diploid, young sporophyte. The young sporophyte soon matures into an adult sporophyte, completing the lifecycle. However, in different plant groups, either the sporophytic generation or the gametophytic generation persists for a longer period, called the dominant phase.
 
In chlamydomonas, spirogyra and volvox, the sporophytic generation is represented by a single-celled, diploid zygote. This zygote undergoes meiosis to form haploid spores, which divide mitotically to produce a free-living, haploid gametophyte. Since mitosis occurs only in the haploid phase, the life cycle is called the haplontic life cycle, wherein the gametophyte is the dominant phase.
 
In gymnosperms and angiosperms, the single or few-celled, short-lived, haploid gametophyte alternates with the free-living, photosynthetic, long-lived, diploid sporophyte. So, the life cycle is called diplontic life cycle.
 
In bryophytes, free living, photosynthetic, long-lived, haploid gametophyte alternates with short-lived, diploid sporophyte. So the life cycle is called haplo-diplontic life cycle.

In pteridophytes, the photosynthetic, long-lived, diploid sporophyte alternates with the independent, multi-cellular, saprophytic or autotrophic, short-lived, haploid gametophyte. So the life cycle is called haplo-diplontic life cycle.

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