Orchidaceae

Growth Form and Habit

Mostly perennial herbs, exhibiting enormous diversity in habit:

• Terrestrial: Growing in soil, often with underground rhizomes, corms, or fleshy/tuberous roots.
• Epiphytic: Growing on other plants (typically trees), using them for support but not parasitizing. Very common in tropics.
• Lithophytic: Growing on rocks.
• Some are climbing vines (e.g., Vanilla), rarely aquatic or subterranean.

Many epiphytic and some terrestrial orchids develop thickened stem sections called pseudobulbs for water and nutrient storage.

Roots

Terrestrial species have typical fibrous, fleshy, or tuberous roots. Epiphytic species possess specialized aerial roots adapted for attachment and absorption. These roots are often covered by a velamen, a layer of dead, spongy epidermal cells that rapidly absorbs atmospheric moisture and nutrients and provides protection.

Leaves

Leaves are usually simple, alternate (often distichous or spiral), sometimes basal or absent (in mycoheterotrophic species like Corallorhiza). They are typically entire, often fleshy or leathery (especially in epiphytes), sometimes thin and pleated (plicate). Leaf bases characteristically sheath the stem or pseudobulb. Venation is parallel.

Inflorescence

Inflorescences are typically indeterminate spikes, racemes, or panicles, arising terminally or laterally from the stem, pseudobulb, or rhizome. Sometimes flowers are solitary. Flowers are often subtended by bracts.

Flowers

The orchid flower is highly specialized, usually bisexual, strongly zygomorphic (bilaterally symmetric), and typically undergoes resupination (twisting 180° during development so the median petal becomes lowermost). Derived from a 3-merous monocot plan:

• Perianth: Composed of 6 tepals in two whorls (3+3), usually petaloid.
  • Outer Whorl (Sepals): 3 segments, often similar to each other, though the dorsal sepal may differ.
  • Inner Whorl (Petals): 3 segments; the two lateral petals are typically similar to the sepals or more elaborate. The median petal is highly modified into a distinctive labellum (lip). The labellum usually differs significantly in size, shape, color, texture, and ornamentation (e.g., bearing spurs, pouches, crests, calli, hairs); it serves as the primary attractant and landing platform for pollinators.
• Androecium & Gynoecium (Column / Gynostemium): Stamens and style/stigma are fused into a single, complex structure called the column (gynostemium), located centrally, opposite the labellum. This fusion is a defining feature of the family. Typically, only 1 fertile stamen is present (median stamen of outer whorl), or rarely 2 fertile stamens (Cypripedioideae - slipper orchids). Other stamens may be present as sterile staminodes forming parts of the column.
• Pollen: Pollen grains are uniquely aggregated into cohesive, waxy or mealy masses called pollinia. Typically 2, 4, 6, or 8 pollinia per flower. Pollinia are often attached via stalks (caudicles or stipes) to a sticky base (viscidium), forming a single dispersal unit called a pollinarium, which adheres precisely to the pollinator.
• Stigma: Stigmatic surfaces (usually 3 ancestrally) are located on the column, often fused and appearing as a single sticky depression below the anther(s). Part of the stigma is often modified into a non-receptive beak-like structure called the rostellum, which aids in positioning the viscidium onto the pollinator.
• Ovary: Ovary is characteristically inferior (located below the perianth), composed of 3 fused carpels, usually forming a single chamber (unilocular) with three parietal placentas bearing numerous (thousands to millions) tiny ovules. The ovary itself often functions as the pedicel and twists during development (resupination).

Fruits and Seeds

The fruit is overwhelmingly a dehiscent capsule, typically opening via 3 or 6 longitudinal slits while remaining closed at the base and apex, allowing the seeds to sift out like pepper. Rarely (Vanilla), the fruit is a fleshy, indehiscent pod or berry. Seeds are extremely numerous (thousands to millions per capsule) and minute ('dust seeds'). They lack endosperm and have an undeveloped embryo, making them dependent on acquiring nutrients from a compatible mycorrhizal fungus for germination and early growth.

Chemical Characteristics

Chemical diversity is high, including various alkaloids, flavonoids, and phenolic compounds. Vanilla flavoring (vanillin) is derived from the cured fruits of Vanilla planifolia. Many epiphytic orchids utilize CAM photosynthesis for water conservation. Generally lack iridoids.