Plants include 375,000 species, with 297,326 identified, and are photosynthetic with chloroplasts. They consist of green algae and terrestrial plants, with vascular plants being the most dominant and bryophytes being less advanced. Seeds allowed plants to colonize drier areas, and flowering plants evolved to involve mobile animals in pollination.
There are an estimated 375,000 plant species, with more being discovered each year. This includes seed plants, bryophytes, ferns, and fern relatives called fern allies. About 297,326 plant species have been identified, of which 258,650 are flowering and 15,000 are bryophytes (liverworts, bagpipes, and mosses). All plants are photosynthetic and use carbon dioxide, water and light energy to produce oxygen and sugars. Within each cell there are numerous organelles called chloroplasts that perform this task. Chloroplasts are thought to be ancient cyanobacteria that were taken up by the larger eukaryotic cells that make up plants.
Plants consist of two main groups; green algae and terrestrial plants. Other types of algae, such as red algae and brown algae, were initially identified as plants, but later classified outside the kingdom Plantae. The latter organisms are part of the same unclassified 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.
Most plants, of course, are land plants, known as embryophytes. The most common are vascular plants, meaning they have internal tubes to transport water and nutrients. The minority are plant species called bryophytes, including liverworts, bagpipes, and mosses. Without vascular tubes, plants can only grow a couple of centimetres. Bryophytes first evolved during the Cambrian Era, about 500 million years ago, and have remained abundant ever since.
Vascular plants are a more advanced plant species. These evolved during the Silurian era, about 420 million years ago, and have continued to dominate the earth, both in terms of diversity and biomass. Vascular plants are the keystone that sustains the entire terrestrial ecosystem, in the same way that cyanobacteria sustain ecosystems 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 angiosperms, or flowering plants. These developed only 125 million years ago, during the Cretaceous. The aim of flower evolution was to involve mobile animals, especially insects, in the process of exchanging pollen and therefore genetic material.
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