Orchids plant comprise about 7 percent of all flowering plants. Some taxonomists recognize as many as 35,000 species in 1,000 genera. Other taxonomists recognize only about 20,000 species, but even this number ranks the orchidaceae as the largest family of flowering plants.
Orchids are perennials, with only one species, Zeuxine strateumatica, known to be an annual. They are also typically herbaceous, but some forms may be viny or somewhat woody. Orchid rootstock may become thickened to form overwintering tubes or pseudobulbs. From early classical times through the Middle Ages, terrestrial orchids plant with testiculate tubers were associated with human form the Greek word for testicles, orchids.
Orchids are cosmopolitan, being found from the Arctic (Habenaria hyperborea) to the tropics and in almost every kind of habitat except desert. Most orchid plants species, however, are tropical, and in the tropics most orchids are epiphytes, living upon other plants epiphycally but not parasitically. Temperate zone orchids are mainly terrestrial, growing in soil. Some orchids are saprophytic, living on dead or ganic matter, such as leaf mold. A few grow completely underground. Cultivation of orchids is an important world wide greenhouse industry. Thousands of hybrids have been created.
Flowers
Most orchids have small flowers, some as tiny as 3 mm (1/2 in), as in Pleurothallis. Others have large, broad petaled flowers up to 23 cm (19 in) across, as in Cattleya, or long, threadlike petals that may span 60 cm (12 ft) from tip to tip, as in Brasilia, China, Australia, Indonesia, and many other countries. Orchid flowers may be bisexual or unsexual, and they demonstrate a wide range of complexity that involves modifications for attracting or admitting only specific pollinators, such as certain butterflies or bees. Only about 200 species of orchids are known to be self pollinating and none is known to be air or water pollinated.
The orchid flower's floral envelope consists of an outer whorl of three, usually equal sized, petalike sepals and an inner whorf of three petals, two of which resemble the sepals; the third petal; called the lip or labellum, may be highly modified. The lip is in the upper position in the bud, but in most orchids it is eventually positioned in the lower part of the flower, a process called resupination.
The seed bearing, or female, organ of the flower is the pistil. The top of the pistil, called the stigma, is three lobed in orchids and serves as the deposition site for pollen. In most orchids a part of one of the three lobes is modified into a structure called a rostellum, used in transferring pollen from the anthers to the pollinating insect or other agent.
Pollination
Polen grains, produced by anthers, are shed either singly or in attached groups. The groups range from four unit tetrads to large pollen masses called polinia. Pollination is affected by many kinds of insects an by hummingbirds and sunbirds. Because the pollen is commonly packeted and made unavailable, other means have been evolved to attract pollinatirs, the shape, color, and odor of the flowers and the presence of nectar and edible tissues. In some cases, to attract pollen eaters, the orchids form "pseudo-pollen" from grains of tissue. In certain instances the flower mimics the shape of the female of the pollinating insect, and males are sexually attracted to and try copulate with flower.
Seed and Seeding
The tiny seeds, perhaps as many as 4 million dustlike seeds in a single capsule of some species, are an excellent adaption for dispersal by wind, but because of their small size the seeds contain no nutritive endosperm and no stored water. The embryo is unformed, consisting of only a few unorganized cells. Development of the seed leaf and other features does not occur until germanation, and this commonly depends on the seed's association with certain mycorthizal ("root") fungi. The fungi penetrate the seed and provide the embryo with carbohydrates and other nutrients. Adult orchids ultimately digest some or the fungi threads.