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The orchid plant family

The orchid plant family has a very wide range of orchid flowers, and the flowers are highly specialised in relation to their pollinators. There are well known structural changes that facilitate pollination by a particular species of insect, bird or bat.

Depending on genus and species, flowers can arise at the base of the leaf, the rhizome internodes or some of the pseudobulbs. They are hermaphrodite (rarely unisexual), usually zygomorphic (bilateral symmetry), usually resupinate (i.e., floral parts rotate 180 degrees during development), and often conspicuous and epigyne (i.e., the perianth parts are arranged above the ovary).

In the vast majority of genera, the flowers are formed by three external parts called sepals, two lateral and one dorsal, and three internal elements called petals; the lower lip, or labellum, which are modified into a larger one of more intense colour than others. Some authors interpret the orchid perianth as a perigone composed of six tepals arranged in two verticilos. The different parts of the perianth may be separated or fused at the base.

The sepals, or external tepals (similar to petals), are usually imbricated. Sometimes the two lateral sepals are fused into one element called synsepal. The petals, or inner tepals are always separated, and sometimes of different colours and stains.

The so-called “lip” petal is larger than the two lateral petals, and its shape is highly variable: it usually has three lobes, unusual shapes, and fleshy ridges or a basal spur, and many times has a different colour pattern than the lateral petals.

The androecium is usually formed by one or two stamens (sometimes three); if one comes from the yarn through the outer whorl it is usually with two ancestral ones and derived from vestigial side staminodes of ancestral internal whorl stamens.

In some subfamilies, as Apostasioideae and Cypripedioideae, there are two or three fertile stamens. When there are two, they are derived from the two lateral stamens of the ancestral inner whorl, and when there are three, they have originated from the two sides of the inner whorl of stamen and through the outer whorl.

The androecium is of fused style and stigma, which are highly modified, forming a structure known as the column or gynadrium. The nests of the anthers are arranged in the portion of the column called the rosetllum.

The pollen is granular, in tetrads or in two to eight bonded soft or hard masses called pollinia. These pollinia have an appendix shaped like a filament, called a caudicle, which binds with a sticky mess (the retinaculum or viscidium), on the rostellum, a stigma derived structure shaped like an elongated lobe which lies on the portion receptive stigma.

The set of pollinia, retinacula and caudicle is called the pollinator, which is the transport unit of the pollen during pollination. The longitudinal dehiscence anthers often are connective and modified into an “operculum” covering the anther until pollination.

The gynoecium comprises of three carpels fused together, with inferior ovary, which may present a loculus, and numerous ovules (even millions) of placentation, which is usually parietal, but occasionally axillary.

Orchids generally produce nectar, a substance used as a reward to pollinators. The nectaries are variable in position and type. For example, they can be at the spur of the lip, or at the tips of the sepals, or on the inner walls of the gynoecium. Species that do not produce nectar are self-pollinating or apomictic, i.e., they do not require pollinators to produce seeds.



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