There are about 375,000 species of plants, with more being discovered each year. This includes seed plants, bryophytes, ferns and relatives of ferns called fern allies. Some 297,326 species of plants have been identified, of which 258,650 are flowering and 15,000 bryophytes (liverworts, hornworts, and mosses). All plants are photosynthetic, utilizing carbon dioxide, water and light energy to produce oxygen and sugars. Within each cell are numerous organelles called chloroplasts which perform this task. Chloroplasts are thought to be ancient cyanobacteria that were absorbed by the larger eukaryotic cells which constitute plants.
Plants consist of two main groups; green algae and land plants. Other types of algae, such as red algae and brown algae, were initially identified as plants, but later classified outside the kingdom Plantae. These latter organisms are part of the same unranked group as plants, Archaeplastida, but are usually not referred to as plants except in a colloquial sense. An example of a familiar green algae is seaweed.
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Most plants, of course, are the land plants, known as embryophytes. The most common are the vascular plants, meaning they have internal tubes to carry water and nutrients. The minority are species of plants called bryophytes, including liverworts, hornworts, and mosses. Without vascular tubes, plants can only grow a couple inches tall. Bryophytes first evolved during the Cambrian era, about 500 million years ago, and have remained abundant ever since.
The vascular plants are a more advanced species of plant. These evolved during the Silurian era, about 420 million years ago, and went on to dominate the land, both in terms of diversity and biomass. Vascular plants are the keystone that holds up the entire terrestrial ecosystem, in the same way that cyanobacteria hold up the ecosystem in the oceans.
The first serious innovation after the initial evolution of vascular plants was that of the seed. Using seeds, plants could colonize drier areas than before, and survive seasonal drought. By comparison, plants that reproduce via spores, such as ferns, require a moist environment.
The most recent plants to evolve were the angiosperms, or flowering plants. These developed only 125 million years ago, during the Cretaceous. The point of evolving flowers was to get mobile animals, especially insects, involved in the process of exchanging pollen and therefore genetic material.