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Distribution
Orchidaceae are cosmopolitan, occurring in almost every habitat apart from deserts and glaciers. The great majority are to be found in the tropics, mostly Asia, South America and Central America. They are found above the Arctic Circle, in southern Patagonia and even on Macquarie Island, close to Antarctica.
The following list gives a rough overview of their distribution:
tropical America: 250 to 270 genera
tropical Asia: 260 to 300 genera
tropical Africa: 230 to 270 genera
Oceania: 50 to 70 genera
Europe and temperate Asia: 40 to 60 genera
North America: 20 to 25 genera
[edit] Taxonomy
Main article: Taxonomy of the Orchid family
This family is totally recognised, and the APG II system of 2003 places it in the order Asparagales.
The taxonomy of this family is in constant flux, as new studies continue to identify more classificatory elements.
Five subfamilies are now recognised. The cladogram has been made according to the APG system:
Apostasioideae: 2 genera and 16 species, south-western Asia
Cypripedioideae: 5 genera and 130 species, from the temperate regions of the world, as well as tropical America and tropical Asia
Monandrae
Vanilloideae: 15 genera and 180 species, humid tropical and subtropical regions, eastern North America
Epidendroideae: more than 500 genera and more or less 20,000 species, cosmopolitan
Orchidoideae: 208 genera and 3,630 species, cosmopolitan
[edit] Ecology
A majority of orchids are perennial epiphytes, which grow anchored to trees or shrubs in the tropics and subtropics. Other species are lithophytes, growing on rocks or very rocky soil, or are terrestrial. Nearly all temperate orchids are terrestrial.
Some orchids, like Neottia and Corallorhiza, lack chlorophyll and are unable to photosynthesize. Instead, these species obtain energy and nutrients by parasitising soil fungi through the formation of orchid mycorrhizas. The fungi involved include those that form ectomycorrhizas with trees and other woody plants, parasites such as Armillaria, and saprotrophs.[5] These orchids are known as myco-heterotrophs, but were formerly (incorrectly) described as saprophytes due to the belief that they gained their nutrition by breaking down organic matter. While only a few species are achlorophyllous holoparasites, all orchids are myco-heterotrophic during germination and seedling growth and even photosynthetic adult plants may continue to obtain carbon from their mycorrhizal fungi.
[edit] Description
Orchids are easily distinguished, as they share some very evident apomorphies. Among these: bilaterally symmetric (zygomorphic), many resupinate, one petal (labellum) is always highly modified, stamens and carpels are fused, and the seeds are extremely small.
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