Orchids are easily distinguished from other plants, as they share some very evident apomorphies. Among these are: bilateral symmetry (zygomorphism), many resupinate flowers, a nearly always highly modified petal (labellum), fused stamens and carpals, and extremely small seeds.
Stem and roots
All orchids are perennial herbs, lack any permanent woody structure, and can grow according to two patterns:
• Monopodial: The stem grows from a single bud, leaves are added from the apex each year and the stem grows longer accordingly. The stem of orchids with a monopodial growth can reach several metres in length, as in Vanda and Vanilla.
• Sympodial: The plant produces a series of adjacent shoots which grow to a certain size, bloom and then stop growing, to be then replaced. Sympodial orchids grow laterally rather than vertically, following the surface of their support. The growth continues by development of new leads, with their own leaves and roots, sprouting from or next to those of the previous year, as in Cattleya. While a new lead is developing, the rhizome may start its growth again from a so-called 'eye', an undeveloped bud, thereby branching.

Phalaenopsis hieroglyphica
Ophrys tenthredinifera
Paphiopedilum concolor

Paphiopedilum rothschildianum

Anacamptis lactea showing the two tubers
Terrestrial orchids may be rhizomatous or form corms or tubers. The root caps of terrestrials are smooth and white.
Some sympodial terrestrials, such as Orchids and Ophrys, have two subterranean tuberous roots. One is used as a food reserve for wintry periods, and provides for the development of the other one, from which visible growth develops.
In warm and humid climates, many terrestrial orchids do not need pseudo bulbs.
Epiphytic orchids have modified aerial roots that can sometimes be a few meters long. In the older parts of the roots, a modified spongy epidermis, called velamen, has the function to absorb humidity. It is made of dead cells and can have a silvery-grey, white or brown appearance. In some orchids, the velamen includes spongy and fibrous bodies near the passage cells, called toilsome.
The cells of the root epidermis grow at a right angle to the axis of the root to allow them to get a firm grasp on their support. Nutrients mainly come from animal droppings and other organic detritus on their supporting surfaces.
The pseudo bulb of Prosthechea fragrans
The base of the stem of sympodial epiphytes, or in some species essentially the entire stem, may be thickened to form a pseudobulb that contains nutrients and water for drier periods.
The pseudobulb has a smooth surface with lengthwise grooves, and can have different shapes, often conical or oblong. Its size is very variable; in some small species of Bulbophyllum, it is no longer than two millimeters, while in the largest orchid in the world, Grammatophyllum speciosum (giant orchid), it can reach three meters. Some Dendrobium species have long, canelike pseudobulbs with short, rounded leaves over the whole length; some other orchids have hidden or extremely small pseudobulbs, completely included inside the leaves.
With ageing, the pseudo bulb sheds its leaves and becomes dormant. At this stage it is often called a back bulb. A pseudo bulb then takes over, exploiting the last reserves accumulated in the back bulb, which eventually dies off, too. A pseudobulb typically leves for about five years.
A close-up of a Phalaenopsis orchid leaf, the parallel veins and cuticle are visible.
Like most monocots, orchids generally have simple leaves with parallel veins, although some Vanilloideae have a reticulate venation.

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