Plants are living organisms of the kingdom Plantae, which includes multicellular eukaryotes that produce their own food through photosynthesis. Plants are the primary producers in most terrestrial ecosystems, playing a vital role in the global carbon and water cycles. They provide us with oxygen to breathe, food to eat, and many other useful products.
There are an estimated 390,900 dicot species and 419,300 monocot species alive today. The total number of plant species has been estimated at c. 1 million (van der Pijl 1979). angiosperms (flowering plants) are by far the most diverse group of land plants, with c. 270,000 species (Stevens 2006). Gymnosperms (conifers and cycads) come in a distant second with c. 1000-1200 species (Stevens 2006). pteridophytes (ferns and lycophytes) are third with about 11000-12000 species (Stevens 2006), although this number is likely to be revised upwards as more data becomes available from tropical regions where fern diversity is particularly high (Pryer et al. 2004). bryophytes (mosses and liverworts) come next with about 16500-17000 described species worldwide (Goffinet & Buck 2000). Finally there is a small but significant group of parasitic plants known as holoparasites which have lost the ability to photosynthesize and rely on their host plant for all their needs; these include broomrape family Orobanchaceae (~1400 spp.), mistletoes Viscum album L., Phoradendron leucarpum Nutt., Santalales Rafflesiaceae (~1000 spp.) among others (Weber 2003).
The vast majority of plant diversity occurs in tropical regions, especially in wet forest habitats where canopy cover can exceed 100 m2/ha resulting in very little light reaching the ground layer vegetation; this is known as closed forest or rainforest. The richest areas for plant diversity tend to be located around the margins of continents where there is greater disturbance from geological activity such as volcanoes and earthquakes, or from climatic factors such as El Niño Southern Oscillation events which bring warm moist air from the Pacific Ocean over South America causing extensive flooding followed by drought conditions further inland. Other major hotspots for plant diversity include: southern China/Tibet due to its geography straddling both subtropical and temperate zones; New Caledonia in the southwestern Pacific due to its isolation since Gondwanan times leading to high levels of endemism; Madagascar off the east coast of Africa again due to its isolation allowing plants time to evolve independently from those on mainland Africa; parts of southern Africa including Cape Floristic Region which has Mediterranean-type climate not found elsewhere on the continent; western Australia including Shark Bay area because it has arid conditions similar to those foundin deserts further inland but without being too hot or too cold year-round so allowing a greater rangeof life forms including many unique carnivorous plants such