growing in low light, and most are not as susceptible as other plants to aphids, mites, mealybugs, and similar pests. Outdoors, ferns are equally popular as ornamentals. Some eventually become subjects for an artist's brush or a natural history photographer's camera.
Apart from the pleasing aesthetic aspects of ferns, they function well as air filters. For example, during a single hour, one average-sized Boston fern can remove about 1,800 micrograms of formaldehyde (a common pollutant from carpets) from the air in a typical room measuring 3 meters by 3 meters (10 feet by 10 feet). According to the Environmental Protection Agency, if properly maintained, the fern can almost completely rid the air of pollutants on a daily basis.
Ferns also have other practical value. Commercial growers of the brilliantly colored anthuriums in Hawaii and elsewhere have found that native tree ferns provide the ideal amount of shade and other environmental conditions needed for bringing their flowers to perfection. Tree fern rhizome or root bark1 and rhizome bark of certain other species, such as the royal fern (which produces osmunda bark), are a favorite medium of orchid, bromeliad, and staghorn fern growers. Its texture is well suited to the growth of the orchids' aerial roots, and as the bark slowly decomposes, rain water trickling over its surface picks up nutrients that are particularly appropriate for these plants. The demand for fern bark for orchids has exceeded the supply for a number of years, and it has become very expensive on the market (Fig. 21.24).
1. Ferns do not have a cork cambium and do not, therefore, produce bark that is typical of woody seed plants. The cells of the outer layers of fern roots and rhizomes, however, do become impregnated with suberin and consequently become bark-like in appearance and function.
As the young "fiddleheads" of many species of ferns unroll, a dense covering of hairs is visible on the petiole and rachis. In the past, the silky hairs of some of the larger tropical tree ferns (Fig. 21.25) were stripped and used for upholstery, pillow, and mattress stuffing. During the late 1800s, over 1,900 metric tons (2,094 tons) of this material were shipped from Hawaii to the mainland, and if it were not for
Figure 21.25 A. A Brazilian tree fern. B. The growing tip of a tree fern, showing the protective rust-colored hairs.
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