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Orchids are easily distinguished, as they share some very evident apomorphies. Among these: bilaterally symmetric (zygomorphic) and resupinate, a petal (labellum) is always highly modified, stamens and carpels are fused, and the seeds are extremely small.

Leaves
Like most monocots, orchids generally have simple leaves with parallel veins, although some Vanilloideae have a reticulate venation. They may be ovate, lanceolate, or orbiculate and very variable in size. Their characteristics are often diagnostic. They are normally alternate on the stem, often plicate, and have no stipules. Orchids leaves often have siliceous bodies called stegmata in the vascular bundle sheaths (not present in the Orchidoideae) and are fibrous.

The structure of the leaves corresponds to the specific habitat of the plant. Species that typically bask in sunlight, or grow on sites which can be occasionally very dry, have thick, leathery leaves and the laminas are covered by a waxy cuticle to retain their necessary water supply. Shade species, on the other hand, have long, thin leaves.

The leaves of most orchids are perennial, that is they live for several years, while others, especially those with plicate leaves, shed them annually and develop new leaves together with new pseudobulbs, as in Catasetum.

The leaves of some orchids are considered ornamental. The leaves of the Macodes sanderiana, a semiterrestrial or lithophyte, show a sparkling silver and gold veining on a light green background. The cordate leaves of Psychopsiella limminghei are light brownish green with maroon-puce markings, created by flower pigments. The attractive mottle of the leaves of Lady's Slippers from temperate zones (Paphiopedilum) is caused by uneven distribution of chlorophyll. Also Phalaenopsis schilleriana is a lovely pastel pink orchid with leaves spotted dark green and light green. The Jewel Orchid (Ludisia discolor) is grown more for its colorful leaves than its fairly inconspicuous white flowers.

Some orchids, as Polyrrhiza lindenii (Ghost Orchid), Aphyllorchis and Taeniophyllum depend on their green roots for photosynthesis and lack normally developed leaves, as of course do all of the heterotrophic species.