|
The orchid family (Orchidaceae) is the second largest family of flowering plants with approximately 20,000 described species and many thousands of cultivars (cultivated varieties). The family includes terrestrial and epiphytic species, mostly native to tropical regions of the world. Some species are mycotrophic, living off of mycorrhizal soil fungi which is attached to the roots of neighboring trees and shrubs. Orchids have three sepals and three petals. The perianth segments are showy and colorful because orchids are typically pollinated by insects. One of the petals (called the lip or labellum) is very different from the other two, and plays an important role in pollination. It often serves as a landing platform for insects. The number of stamens (anthers) is reduced to one or two, and the androecium is adnate to the stigma and style, forming a compound structure called the column. The column is also referred to as the gynandrium. A beaklike structure called the rostellum separates the anther from the functional stigma on the column of single-anthered orchids. The sticky (viscid) stigma is situated below the rostellum. A cap-like structure called the anther cap sits on the anther portion. The anther cap is readily detached and is a nice adaptation to prevent self pollination.
When the anther cap is dislodged by a pollinator, the anther is exposed to the insect's body. Two pollen masses (called pollinia) are attached to a sticky (viscid) padlike structure called the viscidium (also called viscidulum in some references). The viscidium readily adheres to the bodies of insects. When insects visit anther orchid blossom, the pollinia are transferred to the sticky stigmatic surface just below the rostellum. Following pollination, the epigynous ovary develops into a many-seeded capsule. The seeds are microscopic, and some orchids produce more than a million seeds per capsule.
Certain epiphytic orchids of the tropical rain forest produce the world's smallest seeds weighing only one 35 millionths of an ounce (1/35,000,000) or 0.81 micrograms. They are dispersed into the air like minute dust particles or single-celled spores, eventually coming to rest in the upper canopy of rain forest trees. The seeds of some species are no larger than fungal spores and occur in a loose cellular sheath. Since the seeds have no endosperm and underdeveloped embryos, there are practically no food reserves. In order to germinate under natural conditions, they must establish a symbiotic relationship with a compatible mycorrhizal fungus. During early stages of development, the fungus supplies critical nutrients to the orchid seedling. Later the orchid may become fully independent, or it may retain its mycorrizal relationship throughout its life. The coral-root orchid seed (Corallorhiza) grows into a nonphotosynthetic mycotrophic wildflower. It absorbs carbohydrates and minerals from its fungal host, which in turn absorbs these vital nutrients from the roots of nearby forest trees. Orchid seeds are also grown under aseptic conditions in nutrient agar, similar to bacterial and fungal cultures.
|
The ripened ovary (capsule) of an unknown orchid cut lengthwise (longitudinally) into two halves. The capsule is composed of three carpels and contains more than one million minute seeds. The withered flower (perianth) is attached to the top of the ovary, a condition known as epigynous (above gynoecium). Epigynous also refers to flowers with an inferior ovary. The term inferior does not refer to a lesser rank or degree. In fact, flowers with inferior ovaries are considered more advanced on the evolutionary scale, culminating in the orchids. The U.S. penny is shown as a size relationship. It has a diameter of 19 mm and a circumference of 59.7 mm. [One inch = 25.4 millimeters.]
|
|
Microscopic view of the seed of a coral-root orchid (Corallorhiza maculata). The individual seed is only about 0.2 mm in diameter. In fact, there are unusual bacterial cells that are larger than this orchid seed. The resolving power for an unaided human eye with 20-20 vision is about 0.1 mm. With its cellular sheath (seed coat) removed, this seed is barely visible to the naked eye. Certain epiphytic orchids of the tropical rain forest produce the world's smallest seeds weighing only about one 35 millionths of an ounce (1/35,000,000) or 0.81 micrograms. Some seeds are only about 1/300th of an inch long (85 micrometers). One seed capsule from a single flower may contain up to four million seeds. They are dispersed into the air like minute dust particles or single-celled spores, eventually coming to rest in the upper canopy of rain forest trees. The seeds of some species are no larger than fungal spores and occur in a loose cellular sheath. Since the seeds have no endosperm and a minute, undifferentiated embryo, there are practically no food reserves. In order to germinate under natural conditions, they must establish a symbiotic relationship with a compatible mycorrhizal soil fungus. During early stages of development, the fungus supplies critical nutrients to the orchid seedling. Later the orchid may become fully independent, or it may retain its mycorrhizal relationship throughout its life. The above coral-root orchid seed (Corallorhiza) grows into a nonphotosynthetic mycotrophic wildflower that is completely dependent on its mycorrhizal fungus. Throughout its life, the orchid absorbs carbohydrates and minerals from its fungal partner, which in turn absorbs these vital nutrients from the roots of nearby forest trees. In a laboratory, orchid seeds can be grown in nutrient agar, like a sterile (axenic) culture of bacteria or fungal spores.
|
Several remarkable examples of orchid pollination involve the seduction of male wasps and bees by orchid flowers. Flowers of the Australian orchid Cryptostylis leptochila resemble a female ichneumon wasp. The male ichneumon wasp attempts to copulate with the flowers, thus insuring cross pollination as he mounts the blossoms. Other species of Cryptostylis orchids also entice male wasps to visit their blossoms for pseudosexual encounters. Orchids of the genus Ophrys are well-known for their flowers that mimic bees and wasps. The Mediterranean orchid Ophrys fusca seduces a male bee before the female bees have emerged from their pupal cases. The following image shows an uncanny "face" on column of this vanda orchid. Nectar guide lines are visible of the lower "lip" petal (labellum). With some imagination, the flower superficially resembles the head and front legs of an insect. Whether hymenopterans perceive this flower as an insect is pure speculation by this author.
|
Go To The Diversity Of Flowering Plants