Orchid stems are also specialized into what are called pseudobulbs. These are basically expanded stems that allow the plant to store excess water and food. The roots of epiphytic (tree-dwelling) orchids have a special outer layer of dead cells called velamen. Because orchids metabolize slowly in order to not outpace their resources, the roots also absorb slowly, meaning that they need help catching water as it passes by. The velamen cells catch and fill with water as it passes by so that the true root (the core of the root structure) has ample time to access as much water as it can. Velamen is silver-white in color, but becomes transparent when wet, so that when the root is wet it turns green as you are able to see through to the inner structures of the root that contain chlorophyll.
The most common type of orchid is probably the moth orchid, or the Phalaenopsis orchids. These are the plants that you can buy from a standard grocery store. Or if you happen to live close by an Asian supermarket, you will find truck-loads of the white or purplish-pink variety. Some fashion/interior designers even dubbed this purplish-pink color as “orchid.” (This is ultra confusing for an orchid grower, as orchids come in a million different shades and colors, but well, these are the same people who invented colors like sour lemon and spiced mustard. It’s just all marketing!)
Phalaenopsis, the Moth Orchid, is one of the most commonly available and easiest to grow orchid genera. It is an especially good choice for beginners to orchid growing. They have large, showy flowers that come in a wide variety of colors. Most species have several flowers per stem, but some have more, and others have as few as one or two. There are a great many hybrid varieties on the market.
Epiphytic orchids, those that grow upon a support, have modified aerial roots that can sometimes be a few meters long. In the older parts of the roots, a modified spongy epidermis, called a velamen, has the function of absorbing 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 tilosomes.
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Cymbidium carry tall stems with many flowers that have a more typical orchid shape. They are easily identified by their grassy leaves, and they readily form large clumps. Most common are the cool-flowering types, which need to experience a cooler winter (nights in the 40s for some time in the fall) in order to initiate flowering. Once the flowers open, they can last for up to two months or more, especially in a cool environment. These are very showy and rewarding plants, and a plant in a six-inch pot can carry three stems with 15 or more flowers each, depending on the type.