Ferns are non-flowering plants with large leaves called megaphylls that reproduce by spore formation. Spores are reproductive cells that are capable of maturing into an adult plant without fusing with other cells. Ferns are in the plant division, Pteridophyta, and to date there are 10,400 known species of true ferns. Fern allies are plants that have shared characteristics with ferns, but have distinct structural differences. For example, fern allies have smaller, undivided leaves. In addition, although fern allies reproduce by spores also, the sporangia are located in the axils or on the apex of sporophylls, as opposed to the underside of the leaf. These plants consist of fork-ferns, tassel ferns, whisk ferns, horsetails, club mosses, and quillworts. There are 1,600 species of fern allies. The greatest abundance of ferns was late in the Carboniferous period (369-280 million years ago), this era is known as the "Age of Ferns". Many of the ferns found in fossil records are ancestors of modern, primitive fern families.
Ferns are more commonly adaptive to wet regions. Roughly, 70% of ferns live in tropical climates, while the remaining 30% are found in temperate climates. Ferns have great diversity in terms of size. Tree ferns can reach heights of 24 meters, while some aquatic ferns may have leaves only 2 centimeters long. They also have a wide range of variability with regards to shape, texture, color, sheen and practicality. Ferns can be used for decorations, ornaments, fertilizer, medicine, food or conservation.
Ferns are considered vascular cryptogams. Vascular means that within the plant water and nutrients are transported through specialized tissues. The word cryptogam is derived from the Greek words kryptos and gamia which translates to "hidden marriage". This is because ferns reproduce by spores, which form in bundles called sori, on the underside of the fertile leaves. The spores and the reproductive process are thus hidden from view. Since reproduction of ferns is executed by spores instead of seeds, they are considered lower vascular plants.
Ferns differ from other vascular plants so much that the features of ferns are given different names. The stem of a fern is referred to as the rhizome. A fern can be thought of as an erect plant that is laying on its side. The rhizome develops horizontally beneath the surface of the soil. Some rhizomes elevate closer to the surface level of the ground at the tip. In rare cases, the rhizome may protrude from the soil to form a small trunk and is referred to as erect or caudex. The rhizome has a growing tip that produces new fronds. Rhizomes can be comprised of solid, hard wood or of fleshy, soft skin. Within the rhizome is vascular tissue that transports water, minerals, and food. Ferns can have either a short, medium, or long creeping rhizome. The shorter rhizomes have less surface area for fronds, resulting in a cluster of leaves.
The megaphylls (large leaves) or the microphylls (small leaves) of a fern are called fronds. Megaphylls are large leaves, with an intricate vascular system, that create a leaf gap in the stem when developing. The leaf gap is the separation or break in the vascular tissue. The gap is formed as the large fronds branch out from the smaller rhizome. Microphylls are small leaves with a single vein that does not have a well developed, patterned, vascular system. The fronds are the largest structures of the fern that can be viewed above ground. The fronds consist of two parts, the stipe and the lamina or leaf blade. The stipe is the stalk that connects the leaf blade to the rhizome. The blade is the leaf portion that expands outward from the rachis or the mid-vein of the frond. Dissection of fronds starts from the edge of the blade and goes inward to the midrib or mid-vein of the leaf. A frond with no dissection is called a simple frond and a pinnatid frond is dissected all the way to its mid-vein. A pinnate frond is divided all the way to its rachis and the leaf segments are called pinnae. The fronds often have elaborate or intricate dissection and are one of the most distinguishable characteristics of ferns.
Another characteristic that is distinct to ferns is how the frond develops. The leaves roll out from the base to the end of the frond. The base of the frond grows faster than the tip, giving the frond a "fiddle-head" shape. This fiddle-head shape is termed crozier. However, some ferns, such as the grape fern, do not have croziers, making them difficult to distinguish.
The fronds and the rhizomes of ferns are usually coated with hairs or scales. These tiny structures serve as a method of defense from a variety of fern predators including; nematodes, ants, caterpillars, beetles, crickets, gnats, moths, slugs, snails, cockroaches, fungi and many more. The hairs can be unicellular, multicellular, or glandular. Some longer hairs can secrete poisonous fluids and emit toxic or fragrant odors. Some glandular hairs can produce wax and colorful substances. Wax can give fronds bright color and glossy sheen.
The scales of ferns have great variability of size, shape and color. They are usually of single-celled thickness and multicellular width. Scales are mostly shaped linear, lanceolate, or ovate. Although, scales are usually monocolored, some are bicolored or multicolored. A clathrate scale can transmit light, causing it to appear a wide range of colors. The clathrate scale can be thought of having a "stain glass window effect". Although color is often not used as a taxonomic characteristic of ferns it gives great diversity and beauty to the entire species.
The diversity of ferns also extends to their many practical uses. Ferns are used for medicinal, economic, decorative and environmental purposes in many countries including the United States, Europe, New Zealand, Japan, Africa and the Philippines. In the United States, the Asplenium ruta-mauraria, Dryopteris crisata and Pellaea atropurpurea species are used to expel parasitic worms from the stomach and intestinal tract. Species such as Adiantum phillipense and Asplenium adiantum-nuigrum are used to treat diarrhea and bowel disorders. The rhizomes of many fern species are exploited and eaten as a rich, source of starch, because the rhizome is where ferns store their excess amounts of carbohydrates.
Several parts of the fern are edible, however it is the young, newly developing crozier, which is regarded as a delicacy. The crozier is eaten raw or cooked. When prepared raw the crozier is often added to salads or eaten plain. When cooked, preparation consists of removal of hairs and scales and boiling in salty water for 30 to 60 minutes. The croziers can also be steamed until they are soft or tender enough to eat. Many species of ferns contain carcinogenic substances, in particular several species of Pteridium (Bracken fern). In Japan, where large quantities of Bracken are eaten, the cases of stomach cancer are the highest in the world. Ferns are also consumed in the form of beverages. In Europe, the fronds of the species Drypteris fragrans are dried and made into tea. The Bracken fern can also be used instead of hops when fermenting beer. In Asia, rice farmers use ferns of the genus Azolla as green manure, to fertilize their crop. The Azolla fern contains blue-green alga, which can fix nitrogen from the atmosphere. The Azolla and the alga have a symbiotic relationship, in which the fern can use the fixed nitrogen to promote its own growth, and in return provides the alga with excess carbohydrates.
Ferns are also use for decorative and ornamental purposes, because they keep well in water. In the United States, fronds of the species Polystichum acrostichoides are sold during the Christmas season, because they maintain their green color even during the winter months. This species is aptly called the Christmas Fern. When added with other flowers, ferns can create unique floral arrangements. The fronds of various ferns are dried and spray-painted. Gold or silver paint adds beautiful, decorative color to floral patterns. The inner trunk of tree ferns can be cut and shaped into ornamental artwork such as vases, cups, or bowls.
In nature, ferns serve a far greater purpose to the environment. In
particular those species that are weedy or that can thrive in disturbed
soil. Ferns help aid in erosion control and soil stabilization. The main
structures of ferns, which perform these tasks are the rhizomes and the
root systems. The rhizomes, because they are thin, long and grow horizontally
beneath the surface of the ground, help to stabilize the soil. The root
systems of ferns are well branched and add moisture to soil, helping to