A plant is an organism in the kingdom Plantae. According to the five-kingdom classification system used by most biologists, plants have the following characteristics: they are multicellular during part of their life; they are eukaryotic, in that their cells have nuclei; they reproduce sexually; they have chloroplasts with chlorophyll-a, chlorophyll-b, and carotenoids as photosynthetic pigments; they have cell walls with cellulose, a complex carbohydrate; they have life cycles with an alternation of a sporophyte phase and a gametophyte phase; they develop organs which become specialized for photosynthesis, reproduction, or mineral uptake; and most live on land during their life cycle. Biologists have identified about 500,000 species of plants, although there are many undiscovered species yet to be classified.

There are nine phyla within the kingdom Plantae. Bryophyta is a phylum with three classes, the largest of which is the mosses. Lycopodophyta contains about 1,000 species, most of which are abundant...