Plants are all around us, attesting to their reproductive and evolutionary success. When the first terrestrial plants washed ashore from the primitive sea, the plant kingdom began the long process of adapting to life on land and differentiating in such a way as to survive in myriad challenging environments. Nowhere is this more evident than the plants' life cycles, where they harness the power of water, wind and helpful animals to ensure the continuation of their kind.
Like animals, plants reproduce sexually, exchanging genetic material with other members of the same species to increase genetic diversity. Unlike animals, plants are rooted into place, unable to seek out mates or defend their young, and their life cycles derive from adaptations that allow them to exchange genetic material with distant individuals. Plants also utilize vegetative means of reproduction, such as developing new bulbs.
To a large extent, plants are classified based on their life cycle, and as plants evolved, the life cycle underwent significant changes that made those species better adapted to a particular set of conditions. Early plants lived in water, and the first terrestrial plants were highly dependent on water for survival and reproduction. As plants evolved, their dependence on water lessened, and their adaptations better suited them for life on land. Botanists generally classify plants into four groupings based on life cycle: mosses, ferns, gymnosperms and flowering plants.
Mosses and ferns are spore-producing plants. Mosses produce sperm and eggs that are carried to each other by water, uniting to form a structure that develops into spores. The fuzzy dots that grow on the underside of fern leaves serve a similar function, and these plants release millions of spores into the air that develop into structures able to produce sperm and eggs, starting the cycle over again.
Ferns were the first plants to develop seeds, although the seed-bearing ferns are all extinct now. Nonetheless, the development of a tough coating around the female reproductive cells and the development of pollen to carry sperm cells on the wind rather than requiring water meant a huge evolutionary step forward for plants. When pollen reaches the female structure of a seed-bearing plant, the pollen extends a thin tube into the rudimentary seed, releasing sperm to fertilize the egg located within. The tough seed coat protects the embryo, allowing seed plants to produce fewer seeds with greater rates of success than spore-producing plants.
Flowering plants represent the pinnacle of evolutionary success in the plant kingdom. Flowers improve the plant's life cycle even further, reducing reliance on both water and wind in many cases. Flowers produce colors, scents and food that attract pollinators such as hummingbirds and bees; these pollinators collect pollen as they feed, which sticks on the female structures of further flowers that they visit. Flowers also develop into fruit, which entices animals to eat it and disperse the seeds more widely than the plant could manage without help.