Learn About Orchids and Orchid Care

Learn About Orchids and Orchid Care

Tuesday, March 15, 2005

Plant thallus and roots

All orchids are perennial herbs.
  • Some orchids are terrestrial, growing rooted in the soil. Terrestrial orchids may be rhizomatous, forming corms or tubers. These act as storage organs for food and water. The root caps of terrestrials are smooth and white. Terrestrials are mostly found in colder climates.
  • A great many orchids are epiphytes, growing out of soil on tree branches. They occur in warmer regions. Epiphytic orchids have modified aerial roots and, in the older parts of the root, an epidermis modified into a spongy, water-absorbing velamen, which can have a silvery-gray, white or brown appearance. The cells of the root epidermis grow at a right angle to the axis of the root. This allows them to get a firm grasp on their support. These roots can sometimes be a few meters long, in order to take up as much moisture as possible. The aerial roots of epiphytes that lack leaves have an additional function. They contain chlorophyll and take up carbon dioxide.
  • A few species, especially in Australia and Tasmania, are lithophytes., i.e. they grow on rocks.

The base of the stem of sympodial epiphytes, or in some species essentially the entire stem, may be thickened to form what is called a pseudobulb. These contain nutrients and water for drier periods. Pseudobulbs have a smooth surface with lengthwise grooves. They typically stay alive for five or six years. They look on the inside more like a corm than the embryonal stage of leaf sheaths. They have different sizes and shapes. They can be conical or oblong. In the Black Orchids (Bulbophyllum), the pseudobulbs are no longer than 2 mm. The largest orchid in the world, the Giant Orchid (Grammatophyllum speciosum), has pseudobulbs with lengths of 2-3 m. When the orchid has aged and the pseudobulb has shed its leaves, the pseudobulb becomes dormant and is called a backbulb. The next year's pseudobulb then takes over, exploiting the last reserves of the backbulb. Eventually, the backbulb also dies off, having given life to newer growths. At the end of the pseudobulb typically appear one or two leaves, though there may be up to a dozen or more. Some Dendrobiums have long, canelike pseudobulbs with short, rounded leaves over the whole length.

Some sympodial terrestrials, such as Orchis and Ophrys, have two subterranean tubers (more like tuberous roots) between the roots. One is used as a food reserve for wintery periods, and provides for the development of the other pseudobulb, from which visible growth develops.

In warm and humid climates, many terrestrial orchids do not need pseudobulbs.

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