The life cycles of plants are observable all around you, from dandelion fluff drifting on the breeze, tiny ferns uncurling from the earth and pollen thick upon your windshield on a spring morning. The ancient kingdom of the plants has survived cataclysms that extinguished species and reshaped the continents because of reproductive success and life cycles that adapted to ever-changing conditions. The results are the diverse range of plant species you can see today.
The life cycle of all plants include an alternation of generations, referring to life stages during which different numbers of chromosomes are found in the cell. Cells are either diploid or haploid, meaning that they have the full number of chromosomes or half the number of chromosomes, respectively. Plants have two distinct generations, a diploid generation formed from a spore or seed, and a haploid generation that develops the sex cells.
There are two types of life cycles evident in plants today. In the first type, seen in mosses and ferns, plants produce spores that mature into adult plants. In the second type, which appeared later in evolutionary history, plants produce seeds from which the adult plant germinates. Most plants today are seed-bearing plants.
As plants have evolved, their life cycles and reproductive strategies have changed too, better adapting plants to life on land. Plants began as aquatic organisms, so the life cycles of the most primitive plants--the mosses and the ferns--require water to carry sperm to the egg, producing the spore. Gymnosperms, seed-bearing plants that do not produce flowers, produce pollen, which requires wind rather than water. The most recently evolved type of plant, the flowering plant, continues to use pollen but attracts specific pollinators with its flowers. These pollinators carry pollen, which contains the sperm cells, from plant to plant, removing much of the happenstance from plant reproduction.
Moss is the only type of plant in which the dominant structure is haploid, containing only half the number of chromosomes. Mosses sprout, producing eggs and sperm at the tips, which are carried to each other with the help of water. The fused egg and sperm develop into the diploid generation, which produces spores that, carried by the wind, are able to develop into new moss plants.
Fern life cycles are similar to mosses in that they produce spores, but the dominant generation is diploid, not haploid. Ferns produce spores on the undersides of their leaves that the wind deposits into new locations. The spore develops into a small structure called the prothallus--the haploid generation--that produces eggs and sperm that are carried by water to each other. When they fuse, they develop into a new fern plant.
Seed-bearing plants produce tiny pollen grains that are easily caught and carried by the wind, and each pollen grain contains sperm cells. Within each seed lies an egg, and when the pollen lands on the seed or female structure of a flower that leads to the seed, the pollen extends a tube into the seed, releasing sperm beside the egg. The resulting seed drops to the ground or is carried away by an animal and, when conditions are right, germinates into a new plant.