Characteristic Morphological Features

The main characteristics that differentiate the orchids as a group are found in the flower. At the bottom of an unspecialized non-orchid flower is the stalk which supports it, known as the pedicel. Directly above, and at the bottom of the flower itself, is a whorl of green, leaflike organs called sepals. Above and inside the sepals is a second whorl of coloured petals. Together the sepals and petals are called the perianth, which make up the nonreproductive parts of the flower. The perianth keeps the flower safe from harm or attracts pollinators or both. Inside (also arranged in whorls) are the sexual parts of the flower. First are the stamens producing pollen in up to several whorls; each stamen consists of an anther on a long slender filament. In the center of the flower is the pistil, which consists of an enlarged inferior ovary topped by a stalklike style with a stigma at its top part. The sepals and petals are generally similar, often highly coloured, and in sets of three. One petal is made as a landing platform for the pollinator and is known as the lip (or labellum).

The sexual portions of the orchid flower are fairly different from other generalized flowers, and they tend to portray the family. The filaments, anthers, style, and stigma are cut in number and are usually merged into a single structure known as the column. Most of the orchids keep only a single anther at the apex of the column.

In the orchid the ovary consists of three carpels merged so that the only outward facts of their existence is the three ridges on the outside of the seed containers. The fully grown seed pod opens down the middle between the lines of juncture. The ovules are organized along the ridges inside the ovary and do not develop till some time after the flower has been pollinated, thereby contributing to the long delay between pollination and the opening of a mature pod.

The sepals and petals are generally fairly distinct and hence keep their separate identification. The petal opposite the fertile stamen is known as the lip, or labellum. Frequently two, or even all three, of the sepals are attached, and the lip, petals, or the sepals may be attached to the column for some distance. One of the characteristic differences between the orchid family and other sophisticated monocots is that the fertile stamen or stamens which are on one side of the flower and the lip are facing each other. This makes the flower bilaterally symmetrical.

The lip is oriented upward in the bud, but, when it later develops, the pedicel or ovary twists so the lip is generally oriented downward by the time the orchid flower opens, a process named resupination.

There are many types of nectaries in the orchids, including extrafloral types which produce nectar on the outside of the buds or inflorescence (flower cluster) while the flower is growing. Shallow cuplike nectaries at the base of the lip are ordinary. Some nectaries are in long spurs that grow either from the connected sepals or from the base of the lip. Members of the Epidendrum complex have long tubular nectaries embedded in the base of the flower next to the side of the ovary. Nectaries on the side lobes of the lips are known, and general nectar production along the middle groove of the lip is common. The nectaries of the Orchidales are existing on the sepals or petals, if they are existing at all.

In most orchids the anther is a caplike structure at the top of the column. The anther of some of the more simple structured orchids is superficially almost the same to that of a lily or amaryllis. In Habenaria and its allies the anther protudes beyond the top of the column but is thoroughly fixed.

The pollen grains are generally bound together by threads of a clear, sticky substance (viscin) in masses known as pollinia. Two basic kinds of pollinia exist: one has soft, mealy packets bound together to a viscin core by viscin threads and is known as sectile; the other kind ranges from soft, mealy pollinia, through more compact masses, to hard, waxlike pollinia; the latter generally have some mealy pollen with viscin strands that connect the pollinia to each other or to a viscidium. This portion of the pollinium is known as the caudicle.

Most of the orchids, a different larger part of one of the three stigma lobes makes the rostellum, a thin flat piece of tissue that protudes down in front of the anther being between the stigma and the anther. When the visiting insect backs out of the flower, it touches lightly the rostellum, which is layered with sticky stigmatic liquid. The pollinia are then picked up from the anther and sticked firmly to the body of the insect. Some simple structured species have no rostellum, and the pollinia simply stick to stigmatic liquid that is first spreaded on the back of the insect. A further specialization happens in more modern orchids in which the caudicles of the pollinia are already fixed to the rostellum and a different larger part of it comes off as a sticky pad known as a viscidium. In the most modern genera a strap of nonsticky tissue from the column attaches the pollinia to the viscidium. This strap of tissue is known as the stipe and should not be confused with the caudicles, which are gotten from the anther. Orchids that have a stipe also have caudicles that connect the pollinia to the top of the stipe. The pollinia, stipe, and viscidium are known as the pollinarium.

Orchid seeds are extremely small and have an undifferentiated embryo which lacks endosperm inside them. A single seed container produces a large amount of small seeds, which are ideally suitable for spreading over a wide area by wind. Orchid seeds need the presence of a fungus (mycorrhiza) in order to sprout and grow in nature. The fungus apparently goes through the seed and gives help to the growth of the seedling by producing or providing some of the necessary nutrients for growth. It has not yet been firmly established if the fungus is necessary for the continued growth of the fully grown plant, but it appears probably that the presence of the fungus aids in the uptake of nutrients and prevents the removing of nutrients from the root material of epiphytic species by water passing through. In some cases the presence of a specific species of fungus is essential, while in other cases a number of fungi may have the ability to get involved in the process. It is viable to grow and germinate some orchids without the fungus help in artificial cultures that provide the required nutrients regularly.

The most of the tropical orchid species are epiphytes; almost all the orchids in the temperate zones, nevertheless, are terrestrial. The predominant and perhaps simple structured growth form in a wide range of monocots is sympodial growth, a creeping habit consisting of an axis that seems to be continuous but is actually made up of a stream of elements. Each of these elements comes not from a terminal bud but as a fork of a dichotomy, the other fork being weaker in growth or suppressed entirely. The usual form of a sympodium is a horizontal rootlike stalk structure known as a rhizome that ends each “branch.” Most simple structured orchids which have a fairly ordinary monocot appearance with a short rhizome stem and erect, nonthickened annual stalks having scattered, spirally arranged leaves and a terminal inflorescence (flower cluster).

Another growth form found in the orchid order is the monopodial habit, in which the stalk has unlimited apical growth and the roots are not limited to its basal portion.

A great large number of orchids, particularly the epiphytes, show variously thickened stems, or “pseudobulbs.” While these structures are pretty diverse in form, they belong to a limited number of morphological types and seem to show some evolutionary trends. One of these apparent trends is from pseudobulbs (or corms, bulblike stalk structures) of several or many internodes (nodes are stalk areas at which leaves fix, internodes are the stalk areas between such nodes) to pseudobulbs of a single internode. Thickened stalk bases may be found in either terrestrial or epiphytic groups, but pseudobulbs of a single internode are limited to mainly epiphytic groups.