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Orchid Family (Orchidaceae) - Biology And Ecology Of Orchids

species fungus usually mycorrhiza

Almost all orchids are perennial (or long-lived) and grow from a rhizome or corm, from which the leaves and flowering shoot emerge. Many orchids are evergreen, meaning their foliage and shoots remain green and functional for more than a year. Many others are herbaceous perennials, meaning their above-ground tissues die back periodically, generally at the end of the growing season.

Orchids exploit two broad types of habitats—terrestrial and epiphytic. Terrestrial orchids have their perennating tissues in the surface substrate of the ground, such as the soil or organic floor of a forest, or the surface sediment or peat of a wet meadow or bog. In contrast, epiphytic orchids use trees as a platform upon which to grow within the canopy. Epiphytic orchids do not obtain their moisture or nutrients from their host—they only use the tree as a physical substrate upon which to grow, either perched on a branch or as a climbing, vine-like plant.

Orchid leaves are usually arranged alternately on their stem, or are basal around the flowering stalk. In a few species the leaves are reduced to small scales, and photosynthesis is mostly carried out by the green flowering stem. Orchid leaves are simple, have non-toothed margins, are usually strap-shaped or linear, have a longitudinally parallel venation and sheath to their base (that is, they lack a petiole).

The flowers of orchids are strongly zygomorphic, and are perfect, containing both female and male reproductive structures. The flowers may be born singly or as a group (or inflorescence). There are three sepals, which may be green or colorful and petal-like, and there are three petals. The two lateral petals are known as wings and are mirror images of each other, while the central petal is highly modified as a so-called lip (or labellum). The lip generally serves as a landing platform for pollinating insects. The lip commonly has a nectar-bearing sac or spur, which is usually brightly colored and pleasantly odorous and is intended to attract insect pollinators. There are one or two stamens, which are largely united with the stigma and style to develop a composite structure, known as the column. The anthers produce one to eight large pollen masses, known as pollinia. The pistil is composed of three united carpels and contains numerous ovules, usually numbering in the thousands. During its development, the flower of many orchids twists 180 degrees on its supporting stalk, so the mature flower is actually presented upside-down. The seeds of orchids are very numerous within the ripe capsule. The seeds are tiny and dust-like, contain almost no energy reserves, and are dispersed by the wind.

Pollination is usually by an insect, which is attracted by a combination of the bright color of the flower, a fragrant aroma, and nectar. Some species of orchids have tightly co-evolved with one or a few species of pollinating insects, so now the two species are highly dependent on each other. In the case of the bee orchid (Ophrys apifera), the floral structure and coloration mimic the shape and color of the female of their pollinating species of wasp or bee. In addition, the orchid produces chemicals that closely mimic the sex pheromones of their pollinator and further help to attract the male wasp or bee. The orchid is pollinated when the male is tricked and attempts to copulate with the orchid flowers.

Almost all species of orchids develop a mutualistic symbiosis (that is, a mutually beneficial relationship) with a fungus, an alliance known as a mycorrhiza. This relationship is beneficial to the fungus, which gains access to some of the energy fixed by the orchid during photosynthesis. The mycorrhiza is extremely important to the orchid, because it greatly enhances the rate at which nutrients, especially phosphate, can be extracted from the environment. The mycorrhiza may also allow the orchid to utilize the energy and nutrition of organic matter in its growth substrate, which can be absorbed by the mycorrhizal fungus and partly passed along to their host plant. Orchid seeds are very tiny and have few energy reserves, and their seedlings rarely survive for long if they do not develop a mycorrhiza soon after germination.

In a few specialized cases, saprophytic species of orchids rely entirely on their mycorrhizal fungus to provide them with organic nutrition, which is obtained by tapping into the decomposer food web of the forest floor. In these cases, the saprophytic orchid can be considered to be parasitic on its mycorrhizal fungus.


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