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WHAT IS AN ORCHID?

This is not as silly a question as it might seem, nor is the answer easy. As new evidence is brought to light, taxonomists constantly reassess the limits of the orchid family: some, for instance, place the "most primitive" orchids, Apostasta and Neuwiedia, in their own family, Apostasiaceae.

In describing a plant family so vast and so diverse, it is almost safe to say that there are no rules, only exceptions. Biologists generally agree, however, that one feature above all others defines the orchid and differentiates it from virtually all other flowering plants - the fusion of the male portion of the flower (stamens) and female portion (pistils). Orchids have three stamens and three pistils, but unlike those of a lily, for example, which are separate from each other, those of the orchid me fused into a structure termed the column or gynostemium, which is located to one side of the flower.

In slipper orchids (Cypripedium Paphiopedilum, Phragmipedium, Selenipedium), only two of the stamens are fertile and produce pollen in aggregated. sticky masses (the sterile stamen is termed the staminode). Most other orchids have only one fertile stamen with pollen massed into "packets" termed massulae or more cohesive units known as pollinia (sing. pollinium). There are thousands, even hundreds of thousands of pollen grains in one pollinium and two, four, six or eight pollinia in a given flower depending on the species. Orchid genera have traditionally been defined (and still are, in part) by the number of pollinia present.

The ovary of the orchid flower is inferior, that is, it appears to be situated below the points of attachment of the other flower parts. It, too, is noteworthy because each at maturity can contain hundreds of thousands if not millions of ovules. After pollinia are deposited on the stigmatic surface of the flower (at the top of the pistil), pollen tubes grow through the styles down to the ovules and toward the embryo sac and egg cell in each.

One sperm cell fertilizes the egg to form the embryo. A second sperm may or may not unite with two other nuclei (polar nuclei) in the embryo sac. In most flowering plants, this fusion product will develop into a tissue called endosperm that nourishes e embryo. in orchids, however, the development of endosperm is arrested at a very early stage (if it forms at all) so that at maturity seeds will comprise an embryo, a seed coat, and little else. Millions of seeds can fit in the fruit or matured ovary of the flower termed the capsule, commonly called (incorrectly) a "pod."

Without endosperm, then, how is the orchid embryo nourished until it is established and can sustain itself by photosynthesis? As mentioned earlier, on contact with the appropriate fungal strain in nature, the orchid embryo derives carbohydrates and mineral nutrients from a mycorrhizal fungus. This fundamental relationship remained unknown until the turn of the twentieth century, and its discovery had a ripple effect, allowing advances first in scientific laboratories and culminating in the tidal wave of hybridization that continues today.

Another feature that characterizes most orchids is the specialization of the perianth - the sepals and petals. Every orchid flower is bilaterally symmetrical and has an outer whorl of three sepals, one dorsal sepal and two lateral sepals. In some genera such as Paphiopedilum the two laterals are fused into a synsepal. just inside the whorl 4 sepals is a whorl of three petals. One of these is often different in size, shape, and/ or color specialized for attracting and perhaps manipulating pollinators in extraordinarily complex and specific ways. This is the labellum, also known as the lip. The Labellum is usually lowermost in the orchid flower with the column just above it. Strangely enough. in the bud the labellum is uppermost, but as the flower develops, the flower stalk or pedicel mists by 180 degrees in response to gravity so that by the time the flower opens, the labellum is lowermost. Flowers which undergo this twist are known as resupinate flowers. Some orchid species, however, have flowers that either do not twist at the bud stage (e.g., Cycnoches) or else rotate a full 360 degrees (e.g. Malaxis). The result is a flower with the labellum uppermost and column lowermost - a non-resupinate flower.

The labellum is perhaps the most intriguing, even spellbinding element of the orchid flower. As you will see further the variations seem never-ending. The labellum can be relatively large (Cattleya labiata) small (Disa uniflora) or minute (Platystele stenostachya); gaudy (Schomburgkia tibicinis) or dull (Neottia nidus-avis); narrow and linear (Platanthera chlorantha) or shaped like a bucket (Coryanthes speciosa The margin may be smooth or heavily fringed (Rhyncholaelia digbyana Dendrobium brymerianum) The labellum may be extended at the base into a tube called a spur (Angraecum, Comparettia which usually but not always contains nectar. In many genera such as Stanhopea the labellum -is morphologically and functionally very complex and comprises three distinct parts: the basal hypochile which is often the fragrance source, the middle mesochile, and the apical epichile which is opposite the apex of the column.

The labellum may have other accessory structures as well, generally functioning somehow in the attraction of pollinators or the pollination process itself Fragrance glands called osmophores may be generalized over the labellum or localized as hairs or small areas of tissue. Osmophores may also occur at the tips of sepals and/or petals (e.g. Restrepia Caladenia). The upper surface of the labellum is frequently furnished with fleshy tissue called the callus, which may have ridges "warts," bumps or papillae. The shiny, blue callus on the labellum of Calochilus campestris demonstrates this very well. as do the papillate calli of many Oncidium species. Orchid flowers are generally on stalks termed pedicels and arranged on unbranched inflorescences called racemes.

In a few species the flowers are not stalked and are borne directly on the main axis of an inflorescence type called a spike. Occasionally, as in some oncidiums, the inflorescences are branched; these are called panicles. Some flowers are solitary on stalks called scapes; examples abound in Lycaste, Paphiopedilum, Pterostylis and Maxillaria. Lockhartia flowers appear in cymes, inflorescences in which the first flower to open is terminal, but subsequent flowers arise laterally from buds below. Other inflorescence types present in Orchidaceae are the corymb (see Glossary) as in Coryimborkis, the capitulum as in Rhizanthella and the umbel as in some bulbophyllums (section Cirrhopetalum). The vegetative diversity of orchids is one of nature's marvels. Entire flowering-size plants may be as small as a few millimeters or as large as several meters and weighing a ton. They may spend their lives in the Andean treetops at 3,000 meters or most of their lives completely underground in Australia (Rhyzanthella).

There are two fundamental growth habits of orchids. Sympodial orchids such as cattleyas have a horizontal stem called a rhizome that successively produces new shoots with determinate growth from axillary buds. Monopodial orchids such as vandas lack rhizomes but have upright or pendent stems with indeterminate growth; new leaves develop continuously from the same meristem or growing point. Cross-cutting these two types of growth habits are two basic ecological habits of orchids: terrestrial and epiphytic. Terrestrial orchids are those that spend most of their lives rooted in the soil. Epiphytes are those that spend most of their lives growing on other plants. There is a third type. Actually a subset of the epiphytes, that lives on thin layers of detritus on rocks: these orchids are called lithophytes. Epiphytic orchids, spending their lives on tree branches and separated from the steady supply of water in the soil, have effectively solved the dual problems of uptake and storage of water by the specializations listed below. Both terrestrial and epiphytic orchids have successfully exploited vastly different ecological microsites around the world because of certain vegetative specializations. Perhaps at the heart of this is the vast array of orchid root systems. Roots of epiphytic and many terrestrial orchids have a specialized epidermis of one or many layers of dead cells called the velamen. Although the velamen probably has several functions debated for well over 100 years (Pridgeon, 1987), it seems clear that its primary role is to capture and hold water long enough for it to be absorbed by the cells interior to it. In and situations it probably also serves as barrier to excessive, damaging water loss by transpiration, and protects underlying cells from high levels of ultraviolet radiation. Many terrestrial orchids possess fleshy roots or underground storage organs (called tuberoids) with both stem and root tissues. In fact, it was the paired tuberoids of the genus Orchis that prompted the botanist of ancient Greece, Theophrastus, to name the genus, after a fancied resemblance of the tuberoids to testicles (Gr. orchids testis). A new tuberoid is formed during each growing season, while the previous year's tuberoid is still present. Orchid stems may also serve as storage organs. Terrestrial orchid genera such as Bletia have underground storage stems termed corms. Sympodial epiphytes often have variously thickened stems commonly called pseudobulbs. Monopodial orchids d not have pseudobulbs but do have fleshy leaves and roots.

The leaves of epiphytes are generally persistent from year to year, but leaves of terrestrials are often shed at the end of the growing season. Leaves may be pleated or plicate (folded many times longitudinally, as in Bletia and Phaius) or conduplicate (folded only once, in the middle, as in Cattleya). Plicate leaves are always thin, but conduplicate leaves may be quite thick, fleshy, and leathery (coriaceous). Some leaves are terete that is, pencil-like and round in crosssection, as in Brassavola, Luisia, and some species of Dendrobium and Oncidium Epiphytes will generally have fleshy leaves with some water-storage tissue, although some such as species of Taeniophyllum, Microcoelia and Chiloschista, are essentially leafless.

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  The Illustrated Encyclopedia of Orchids by Alec Pridgeon

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