The kingdom Plantae contains four major groups of land plants: bryophytes, pteridophytes, gymnosperms, and angiosperms.
Plant life is diverse, adaptive, and environmentally successful. This means they exist in many of Earth's ecosystems. All plants are autotrophs, producing their own food through photosynthesis, which makes food using light, and thus plants provide food to an array of consumers and decomposers. The kingdom Plantae consists of four major plant groups on land: bryophytes (mosses), pteridophytes (ferns), gymnosperms (cone-bearing plants), and angiosperms (flowering plants). Plants may be categorized as vascular or nonvascular. A vascular plant has tissues for transporting water or sap. Nonvascular plants do not. Plants may also be classified as seed-bearing or spore-producing. Gymnosperms and angiosperms produce seeds, while bryophytes and pteridophytes produce spores.
Plants have varied, unique structures, yet all plants are eukaryotic organisms, meaning they have a cell nucleus and membrane-bound organelles. Plants exist in all types of ecosystems. Angiosperms have more than 260,000 living species. Gymnosperms include the oldest (bristlecone pine), the tallest (redwoods), and the thickest (Montezuma bald cypress) living species. At the opposite end of the plant spectrum are green algae, some of which are single-celled. Phytoplankton produce more than half the world's oxygen, far more than the Amazon rainforest or the large areas of grasses on the plains. Although these phytoplankton technically fall under the kingdom Plantae, they are not part of the four major groups already discussed, which include only land plants.
Although the life cycles of plant groups may seem different, they all have some characteristics in common. All plants undergo a process called alternation of generations, a cycle that consists of a sporophyte generation – diploid (which has a double set of genetic material) and a gametophyte generation – haploid (which has only a single set of genetic material).