The Orchid Plant Family and Orchid GerminationThe Orchid Plant Family and Orchid Germination
The orchid plant family has a very wide range of orchid flowers, and the flowers are highly specialized 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 general, 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 color 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 colors 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 color 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 an 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.
In most species, small seeds, which are like dust, are dispersed by the wind and require nutrients provided by a mycorrhizal fungus to germinate. Some members of Vanilloideae and Cypripedioideae have fleshy fruits that ferment in situ, releasing fragrant compounds (e.g., vanillin) to attract birds and mammals, which act as propellants.
These seeds are composed of an embryo of a few cells (between 100 and 200), covered by a hard testa. The number of seeds can vary from 13,000 to 4,000,000 per capsule. The weight range of an orchid seed varies from 0.3 to 14 mg and measure from 0.25 to 1.2 mm long and is from 0.009 to 0.27 mm wide.
These seeds have no endosperm, and consist of a small embryo suspended within a membrane, typically transparent, although sometimes pigmented. Forms can be very variable, such as elliptic, filiform, fusiform, round, globular or prominently winged. All these features appear to maximize fertility and the effectiveness of wind dispersal of seeds of orchids.
The germination of these seeds occurs through a process that is different from the majority of angiosperms, because embryos of orchids are, anatomically and structurally, extremely small and simple. The embryos of orchids germinate and grow to produce a mass of cells called protocorms.
These protocorms, with its rhizoids (root-like structures), may or may not immediately begin to photosynthesize. Nevertheless, for the protocorms to survive, develop and turn into a shoot, they must first establish a symbiotic relationship with a fungus.
The role of the fungus is to provide the protocorms sugars (especially those who have no chlorophyll). The fungus gets the sugar sections of the substrate (soil or another solid object to serve as a host organism to the plant) of the orchid, that is, the bark of a tree or the soil. The protocorms, in turn, provides the fungus with certain vitamins and a habitat to live.
The fungus lives in the area of the protocorms and substrate. Over time, the young scion will start producing its own nutrients, and symbiosis will no longer be necessary.