Excerpt
Ferns bring to mind pleasant, cool glens and shaded forests. We recall such things when we use ferns in our gardens and homes. Their ferny look is well known and loved, and some of us are content to sit back and relax as we enjoy the soft green array of patterns and textures in ferns. But there is more to appreciate about ferns than their mere appearance.
Ferns are usually recognized by their finely divided leaves, a type of leaf so characteristic that it is called a "ferny leaf." There are, however, many plants with ferny leaves that aren't ferns, and many ferns that don't have typical ferny leaves. What then, makes a fern a fern?
Ferns are spore-bearing plants; they lack flowers, fruits, and seeds. The so-called asparagus fern (Asparagus setaceus) is not a fern despite its finely divided appearance because it bears flowers and seeds (it is actually a member of the lily family; its fruits are the orange berries often seen on the plant). Spores and seeds greatly differ. Spores are simple one-celled microscopic structures, whereas seeds are complex, many celled structures usually visible to the unaided eye. Although plants such as algae, liverworts, and mosses produce spores, they differ from ferns by lacking the large, thin, true leaves. Ferns further differ from these spore-bearers by their development of specialized tissue (xylem and phloem) to conduct food and water. These tissues also strengthen the stem and enable ferns to grow taller than other spore-bearing plants.
Unlike seed plants, ferns depend on water to complete their typical life cycle. They grow in places where, when the time comes to reproduce, enough water is available for the sperm to swim to the egg. Seed plants, such as pines and flowering plants, produce cones or flowers that use wind or insects to complete their life cycle. They do not need water in the external environment for fertilization, and therefore they can grow in drier conditions and dominate more of the landscape.
Nevertheless, some ferns have the surprising ability to thrive in extreme climates. Desert ferns, of which there are few, often grow in the shade of rocks and boulders, using every bit of available water. Their roots grow deep in the soil between the cool rocks, and their fronds are often covered with woolly hairs of scales to protect them from water loss. When water is insufficient for new growth, many desert ferns curl their leaves or shed their leaflets and suspend growth until the next rain.
Like desert ferns, alpine ferns are also adapted to climatic extremes. They tend to be small and have hard-textured fronds that can endure the cold, dry winds. They grow only during the short summers as weather permits.
Most ferns of temperate regions grow in the ground or on rocks, and only a few speices grow on trees. Approximately 200 species of ferns are native to the temperate areas of the United States. Temperate to subtropical areas noted for their abundance of ferns include parts of the Himalayas, Australia, New Zealand, and Japan. The tropics, however, harbor the largest number of fern species, particularly at middle elevations in the mountians. Here the mountain fog encourages an abundance of ferns.
New ferns are still being discovered in well-known parts of the world, and in the lesser known parts many more remain to be discovered. There are about 12,000 named species, and many of these have never been introduced into horticulture. Temperate places like Japan and the cool, mountain slopes of the tropics have many ferns suitable as new introductions for temperate gardens in the United States.