Plants are classified according to how they reproduce. Angiosperms and gymnosperms both reproduce through bearing seeds, though in different forms. The vast array of ferns reproduce through spores, instead of seeds. All of these types of plants can be found the world over, minus the subzero arctic tundras.

Ferns come in a wide variety of sizes. Some are 1 inch tall and others grow to be 65 feet tall. There are two stages of the fern's life cycle known as alternation of generations, called asexual and sexual. In the asexual stage the fern is known by botanists as a sporophyte. During this stage, the leaves produce sporangia, the dark spots found on the underside of the fronds. Once the plant matures, thousands of spores are released from the sporangia to be carried on the breeze to fertile soil, to begin the sexual stage of the fern, as a plant known by botanists as a gametophyte. This plant is seldom seen as it only lives for a few weeks. It is in this stage that both male and female reproductive parts are developed in the plant.

Gymnosperms are seed-bearing plants. The seeds are produced through conelike structures instead of inside a fruit or fleshy covering. The name "gymnosperm" derives from the Greek for "naked seed." It is believed that gymnosperms evolved from the primeval ferns that produced seeds. The wide majority of gymnosperms are conifers, such as pine trees, fir, cedar and juniper. Other types are called cycads, and the one that has a single surviving species is the gingko. Many types of trees and shrubs are classified as gymnosperms.

Angiosperms are the flowering plants, which produce their seeds through flowers and fruits. The name comes from the Greek for "vessel" and "seed." It is believed that angiosperms are the most advanced of the plant kingdom. Most of the plants throughout the world fall into this classification, including all edible food consumed by humans and animals.

All the plants have varying forms of the alternation of generations. Many scientists believe that the progression from spores to seeds represents the evolution of the gametophyte staying with the parent plant instead of growing on its own. From this progression came the appearance of the seeds in gymnosperms and angiosperms. Within the seeds are an embryo sporophyte that grows into a mature sporophyte.