Orchids are classified in the family Orchidaceae which contains the largest group of plants on Earth. There are more than 25,000 known species of orchids and more are being discovered every day. The orchids are recognized as the most highly evolved plants with one of the widest distributions. They come in many sizes from miniatures that need to be seen with a magnifying glass to giants that grow vines up to 100 feet or weigh tons. They are found on all continents with the exception of Antarctica and Arctic regions. As a result of their wide distribution, orchids have some of the most diverse species imaginable.
The growth patterns of orchids can be broken down into two main groups. Sympodial growth is characterized by new growth from the base of old growths in a semi-linear manner. Examples of sympodial plants are Cattleyas, Dendrobiums and Oncidiums. Monopodial growth has one primary stem from which all leaves and flowers grow. Examples of monopodial plants are Vandas, Phragmipediums and Phalaenopsis.
Orchids are also classified by how they grow. Epiphytes grow on other vegetation; Lithophytes grow on rocks; Terrestrial grow in or on the ground; Saprophytes grow under leaves or even underground. The majority of tropical orchids are epiphytes or lithophytes; temperate orchids are usually terrestrial.
The orchid flower is the main predicator of whether a plant is an orchid or not. The flower must have three petals and three sepals, with the third petal or lip modified and differentiated from the other two. In most cases the flower is zygomorphic or the same on both sides of the flower as in a mirror image and perfectly symmetrical. The main test is the fusion of the male and female parts in a structure called a column with a complete separation so that it is almost impossible for the flower to self-pollinate naturally. The pollen is held in sacks called pollinia and different species have different numbers of pollinia.
Orchid seeds are also different from other types of plants. The pod or capsule can contain millions of seeds. These seeds are tiny and contain no food storage capacity as do most other plant seeds. Orchids depend upon specific types of fungus in order to germinate and develop. You can understand why millions of seeds are needed in order to keep a species going if the seed will only grow in the right place, where there are the correct fungi and under proper conditions.
Under laboratory conditions, the vast majority of seeds can be germinated and this is one of the main reasons that the costs of orchids has gone down enormously since the 1920s when the modern method of germinating and growing seed was discovered by Knudsen and put into commercial production. It is estimated that up to 90% germination is accomplished by this method.
The last major development that reduced the cost of orchids was the development of mericloning from undifferentiated cells of a highly acclaimed orchid. This produces clones of the mother plant and allows growers to know exactly what the flower and plant will look like when they get a seedling or mature plant.