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Orchids

coral-root
Spotted Coral Root (Corallorhiza Maculata.) Photo: Forest Service - Northern Region via Flickr. CC BY.

This episode of Nature Notes was previously aired on February 18, 2010, and was written by Cathryn Hoyt, then-Executive Director and current Director of Research at the Chihuahuan Desert Research Institute. 

Ever received an orchid for a special occasion? Or maybe you’ve walked through a hot, steamy, greenhouse, delighting in the color and shapes of the orchids on display. Without a doubt, orchids are special. However, we most often associate them with the warm, moist conditions of the tropics. But did you know that 14 species of orchids grow right here in the Northern Chihuahuan Desert of Texas?

Worldwide there are 25,000 species of orchids known to botanists. Most are epiphytic, growing high in the trees of the tropics. However, in Texas, our wild orchids are all terrestrial, with their roots firmly embedded in the soil.

36 of the 54 species of wild orchids in Texas are found in the eastern part of the state. There, abundant rainfall supports moist woodlands, swamps, bogs and meadows that provide ideal habitat for these water-loving plants. So if orchids are water-loving plants, how in the world can there be 14 species of orchids growing in the Chihuahuan Desert region?

To find our desert orchids, you need to be patient and diligent. They may be tucked into a canyon, where a seep provides just enough water to create damp soils, or you may discover a patch of orchids in the leaf litter of a mountain woodland, but only if there’s been enough rain that year.

Coral root orchids are myco-heterotrophs. They don’t produce their own food through photosynthesis as most plants do. Instead, they rely on a vast fungal network in the soil to provide the nutrients they need to grow and to flower.

The life cycle of a coral root orchid is a pretty risky business. It depends entirely on the establishment of a mycorrhizal relationship, with a particular species of fungus. Mycorrhiza is a Greek word meaning “fungus roots.” In a mychorrhizal association, a fungus growing underground colonizes the roots of a plant, such as an oak or a madrone tree. The fungus receives carbohydrates from the host plant, and the host plant receives mineral nutrients and water from the fungus.

Many orchids, and especially the coral root orchids, are considered cheaters in this system. They depend completely on a mycorrhizal association to grow, but they don’t appear to give anything in return. An orchid’s association with a mycorrhizal fungus begins at the very beginning of its life. Orchid seeds don’t have an endosperm or layer of cells that provide nutrients to the developing seedling. Instead, the thread-like hyphae of just the right species of fungus must penetrate the orchid embryo through a tiny pore in the seed.

The orchid responds by releasing chemical substances that control the fungus, and cause it to form dense coils of nutrient-rich material. This material is then digested by the orchid to provide the nutrients needed for seedling development. Some orchids will eventually develop green leaves and start producing their own nutrients, but not the coral root orchids. For their entire lifespan, they depend on the mycorrhizal fungus for their nutritional needs.