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Fern

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Fern
488px-Oaxaca fern.jpg
Scientific Classification
Families

Order: Hydropteridales

Order: Marattiales

Order: Ophioglossales

Order: Polypodiales

[1]

Ferns are any of the species of plants belonging to the taxonomic Family Pteridophyta, which one of the most diverse taxa of plants. Ferns are vascular plants without seeds because they reproduce through spores, but have vascular tissue that transports water and dissolved material through the plant. They have many interesting characteristics. For example; ferns go through a distinct reproductive process called the alternation of generations in which it alternates between two different generations: a spore-producing sporophyte, and a gamete-producing gametophyte. Ferns come in many shapes and sizes from the tall tree fern, to the small boston fern that is commonly used as an indoor potted plant. A large majority of ferns are epiphytes. These are plants that grow on other plants, but are not parasitic. They can grow in several different environments including cool, damp forest floors, desert-like areas, cold places, and even aquatic .[2]

Contents

Anatomy

Tree Fern Fiddlehead

Ferns come in many diverse forms, including the giant tree fern which can reach heights of over eighteen meters, to ferns that grow as vines, and even ones that grow floating on water.

Frond

Each fern leaf is called a frond.[3] Circinate vernation is the term used to describe the protective coiling of a new frond or fiddle head.[4] Fronds are pinnately compound leaves, meaning each vein branches off of one large central vein and multiple leaflets are arranged down the midrib.[5] The midrib of a fern leaf is called petiole, and the leaflet, a blade.[6] On the underside of each leaf, sporangia are located in clusters called sori. These spore producing structures are used for reproduction.[7] The stems, or stipes, as well as the fronds of a fern, usually have a protective covering of scales and/or hair.[8]

Each frond grows from a specialized stem called a rhizome which grows sideways at the surface or underground.[6] The rhizome also serves the purpose of producing roots. Much like the fronds, the rhizome usually posses a protective covering of scales and/or hair. One of their main functions is to provide a vital link between the roots and frond.[8] Fern roots tend to be thin and wiry, though some may be fleshy and slender or have roots with a diameter up to thirteen millimeters.[9] The root hairs act as sponges to absorb water and retain nutrients.[10] Ferns usually are brown or green and posses no flowers because they reproduce through means of spores. They will often grow in clumps from a single rhizome and will look long and delicate.[3]

Tissue

Ferns are vascular plants without seeds, meaning they possess vascular tissue but do not use seeds as a means for reproduction.[3] The vascular tissue of a fern is used to transport nutrients and water through the plant. It is made up of two types of tissues: the xylem and the phloem. The xylem's main purpose is to transport water as well as dissolved minerals throughout the plant, though mainly from the roots up. It also provides support for the fern. The xylem is made up of two types of cells, the tracheids and the vessel cells. The tracheids are narrow, thick walled cells tapered at the ends. The vessel cells are stacked on top of each other forming long continuous tubes. The other important type of vascular tissue, phloem, is usually thinner and forms conduits for the transportation water as well as dissolved foods (usually the products of photosynthesis) from the leaves throughout the plant. The two main components of the phloem are the sieve tube cells that are the actual tubes and their adjacent companion cell responsible for cellular respiration, protein synthesis, and other metabolic processes of the sieve tube cell.[11]

The rest of the fern is composed of ground tissue which remains alive throughout the plant's entire life. Its main purposes are to provide support, storage (for water, sugar, and starch), and to perform the metabolic processes in the fern. Ferns are made up in large part of ground tissue. Filament-like cells of the outer cortex of a fern's roots absorb moisture directly from the atmosphere. The concertina-like extensible and collapsible cells of the leaf make incredible water storage tissue. Both of these tissues are examples of ground tissue.[10] Another very important type of tissue is the meristematic tissue. They are the stem cells of the fern, they are undifferentiated cells found usually in the growth regions of the fern.[11]

Reproduction

Fern life cycle illustrating the alternation of generations.

As stated earlier ferns are vascular plants without seeds; instead of reproduction by means of seeds, they use spores. Ferns use a distinct reproductive process called the alteration of generations in which it alternates between two different generations: a spore-producing sporophyte, and a gamete-producing gametophyte. The sporophyte generation uses spores to reproduce while the gametophyte uses sperm and egg to reproduce. On the underside of some leaves are small insect egg-like structures which are actually sori. The sori are a collection of spore producing sporangia. Physically they vary between species and their location as well as type are used by botanists to classify ferns. Ferns have two types of fronds; the fertile fronds produce spores and the sterile fronds do not. In some species the former appear the same as the sterile except for the absence of sori. However, in others, the fertile fronds may look strikingly different and may not even contain chlorophyll.[3]

When a fertile frond matures it releases spores that are carried on the wind. When the proper conditions present themselves the spore germinates into a one cell layer thick, heart shaped prothallus. On the underside of the prothallus rhizoids develop to absorb water and minerals. Several archegonia (the ovum producing female reproductive organ) develop near the notched end as well as several antheridia (sperm producing male reproductive organs) at the pointed end. When a fern reproduces, flagellated sperm is released from the antheridia and it than swims its way to the ovum at the bottom of the vase shaped archegonium. The prothallus is the gametophyte generation of a fern. The zygote at the bottom of the archegonium matures and sends its first leaf as well as root down beginning the sporophyte generation.[3]

The young sporophyte starts as a parasite on the gametophyte until it is large enough to make its own chlorophyll and begin carrying out photosynthesis. Soon after the sporophyte can do that the prothallus dies. In ferns the prominent generation is the sporophyte, the inconspicuous prothallus is hardly noticed and often overlooked or mistaken for a leaf on the ground. The first leaf of a new fern is a fan shaped blade, while the second is a coiled leaf called a fiddlehead. This fiddlehead has tissue in the coiled end that has the ability to produce new cells even after the base of the leaf reaches its full size. This characteristic allows the fern to have unusually large leaves. Young fiddleheads can be purchased to be eaten in salads though, to some animals, it may be poisonous.[3]

Ecology

The distribution of the western swordfern; common in the Pacific North West

Ferns are an incredibly diverse phylum; most are like the common forest floor fern, though a majority can grow on all kinds of surfaces. Some are parasitic while others grow on other plants, but are not parasitic. Others grow as vines or floating on water. Ferns are most prominent in shaded, cool areas of tropic regions. Though there are species that grow in desert like conditions or next to glaciers. There are fern species that are among the most delicate plants on Earth and parish at the slightest environmental change.[3] Ferns are the most common plants of the shaded, damp forests of the temperate as well as tropical zones. Some fern species grow just as easily on rocks (where they sprout up in fissures, crevices or off cliff faces) as in soil, though some are confined to a certain habitat.[12]

Some factors that affect where they grow are; moisture in the soil, moisture in the air, suitable nutrients in the soil, sufficient light for photosynthesis, suitable temperatures, protection from wind, protection from too much sunlight, protection from freezing, dependability and continuity of the previous requirements. Another factor that is less obvious is whether the conditions enable reproduction. For example, a fern may be fine living in a harsh environment, but if the gametophyte it produces does not survive it will result in ferns will no longer be in that area once the parent plant dies. Ferns are good nitch plants simply because they adapt well to particular environmental niche, most seldom exist outside that nitch.[13]

Ferns, for the most part are not the primary food source of many insects. This might be due to the role of secondary plant compounds, which work as a chemical defense. Though there are insects that do use ferns as a food source. These insects are usually in the orders Hemiptera, Coleoptera and Lepidoptera with large proportions of them being sap or specialist feeders restricted to ferns. The most affected ferns are Polypodiaceous.[14] Humans also eat the young fronds or fiddleheads in salads. Also, ferns can be poisonous to some animals.[3]

Gallery

References

  1. Pteridophyta U.S. Department of Agriculture
  2. Porch, Thomas. Biology, Greenville,South Carolina: BJU Press, 2005. p337
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Porch, p337-338
  4. The Ferns C.M. Sean Carrington, April 10, 1998. Cavehill.Uwi
  5. Porch, p349
  6. 6.0 6.1 Fern Fairfax County Public Schools. Islandcreekes Ecolog.
  7. Phylum Pterophyta: Ferns Wiley Publishing, Inc. Cliffsnotes.com.
  8. 8.0 8.1 Ferns and Fern Allies The Biological Sciences Greenhouse Facility The Ohio State University. Biosci.Ohio-State
  9. Fern Root Enclyclopædia Britannica, Inc. Britannica.com.
  10. 10.0 10.1 Niphobolus Adnascens Springer Science+Business Media. Springerlink.com.
  11. 11.0 11.1 Porch, p345-346.
  12. Fern: Ecology by Enclyclopædia Britannica, Inc. Accessed May 17, 2010.
  13. About Ferns American Fern Society. Accessed May 17, 2010.
  14. Insect-fern associations Department of Biology, Boston University.
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