Orchids belong to the family of the Orchidaceae, which in turn is part of the order Asparagales.
It is the largest family of flowering plants with 880 genera and more than 20,000 species, or twice as many as there are bird species and four times as many as there are mammal species.
The family Orchidaceae is divided into five sub-families:
The name comes from the Greek órkhis, literally meaning “testicle”, because of the shape of the tuber. The term was introduced in 1845 by John Lindley in “School Botany”.
Habitat and distribution:
Orchids are found all around the world and in any habitat, except on glaciers.
Although the majority grows in the tropics, there are species north of the Arctic Circle, in southern Patagonia and even near Antarctica.
Orchids are perennial herbs. They are easy to distinguish from other plants by a few very characteristic features such as:
- bilateral symmetry
- apparently inverted or upside standing flowers
- one petal is almost always highly modified
- stamens and carpels are fused
- very small seeds
Stem and root:
Orchids can grow two ways:
- monopodial: the stem grows from a single node, new leaves are formed at the top and the stem grows longer
- sympodial: the plant produces a series of adjacent shoots which each grow, flourish and wither. The plant spreads laterally across its support.
Terrestrial orchids have a rhizome, corm or tuber.
Some sympodial terrestrial orchids have 2 tubers: a pseudo-tuber where water and food can be stored and used as stock for the other tuber from which the growth takes place.
Epiphytic orchids have aerial roots that can grow a few meters long. Some sympodial epiphytic orchids have a pseudo-bulb at the bottom of the plant to store water and food for bridging drier periods.
Orchids, like most monocots, have parallel veined leaves. They can be oval, lanceolate or rounded and very different in size.
They appear alternately on the stem are often pleated and have no stipules.
The structure of the leaves varies greatly and is adapted to the habitat: in a dry and sunny habitat they are usually wider, leathery and covered with a waxy film while in a shady place the leaves are usually long and narrow.
Most orchids are evergreen, the leaves survive multiple years. Others, mainly the species with pleated leaves form new leaves each year. And some species rely on green roots for photosynthesis.
The flowers of orchids are quite varied in shape and often specialized. They appear as a single flower or an unbranched inflorescence.
The flowers are zygomorphic, they exhibit a bilateral symmetry.
As with most monocots, orchid flowers have two whorls of sterile elements. The outer whorl has 3 sepals and the inner whorl has 3 petals. Often the sepals are the same as the petals and are then called tepals.
The upper center petal, called the labellum or lip, is always modified and quite large. The inferior ovary rotates 180 ° so that ithangs above the labellum and this serves as a runway for insects for fertilization.
In some species of orchids, the sepals fused. In Paphiopedilum they form a pouch and they are all fused in Masdevallia.
Orchids with an abnormal number of sepals and petals are called peloric.
The genus has Neuwiedia 3 stamens. The genera and Apostasia Cypripedioideae have 2 stamens. All other genera have 1 stamen and the two remaining stamens reduced to staminodes.
The filaments of the stamens are fused with the filament and form a cylindrical structure, called gynostemium or column.
Fruit and seed:
The ovary typically develops into a capsule that burst with 3 or 6 longitudinal slits, while at both ends closed. The ripening of a capsule may take 2 to 18 months.
The seeds are generally almost microscopic and very numerous, in some species more than one million per capsule. After maturing, they are blown away like dust or spores. They have no endosperm and have a symbiotic relationship with various fungi which provide the necessary nutrients to germinate.
The chance for seeds to germinate is very small. In culture the fungi are replaced by nutritious gel (agar agar) when sowing in vitro.
Orchids have flowers that are highly specialized pollination often limited to few insect species. This is offset by a bloom that lasts for months.
The pollinating insects are enticed in various ways: the visual appearance of the flower, the smell, the structure that forms a trap for insects from which the only way out is past the stamens.
Some species form offshoots or plantlets in one of the nodes on the stem by the concentration of growth hormone at that site. These extensions are also called keiki.
Most tropical orchids are epiphytes that grow on trees or shrubs. In temperate climates are found mostly terrestrial orchids. And a few species are lithofytic: they grow on rocks or rocky soil. Some orchids do not photosynthesize but live in symbiosis with fungi that provide the necessary power supply.
Although orchids are often grown for their beautiful flowers, they have sometimes other uses:
- vanilla is used as seasoning in the kitchen
- underground tubers are pulverized and ground for use in food or drink
- dried leaves are used for perfuming rum