Question

1. Draw the generalized alternation of generations life cycle all plants follow. When are plants gametophytes? Sporophytes? Know this for bryophytes (mosses), seedless vascular plants (ferns), gymnosperms and angiosperms for the exam.

Answer 1

All plants undergo a life cycle that takes them through both haploid and diploid generations. The fluctuation between these diploid and haploid stages that occurs in plants is called the alternation of generations. The way in which the alternation of generations occurs in plants depends on the type of plant.

In most plants meiosis and fertilization divide the life of the organism into two distinct phases or "generations". The gametophyte generation begins with a spore produced by meiosis. The spore is haploid, and all the cells derived from it (by mitosis) are also haploid.

Bryophytes are nonvascularized plants that are still dependent on a moist environment for survival. Like all plants, the bryophyte life cycle goes through both haploid (gametophyte) and diploid (sporophyte) stages. The gametophyte comprises the main plant (the green moss or liverwort), while the diploid sporophyte is much smaller and is attached to the gametophyte. The haploid stage, in which a multicellular haploid gametophyte develops from a spore and produces haploid gametes, is the dominant stage in the bryophyte life cycle. The mature gametophyte produces both male and female gametes, which join to form a diploid zygote. The zygote develops into the diploid sporophyte, which extends from the gametophyte and produces haploid spores through meiosis. Once the spores germinate, they produce new gametophyte plants and the cycle continues.

The life cycle of the fern has two different stages; sporophyte, which releases spores, and gametophyte, which releases gametes. Gametophyte plants are haploid, sporophyte plants diploid. This type of life cycle is called alternation of generations.