Ferns and Their Allies
Denny PoliFERNo

Ostrich_Fern.jpg
Ostrich Fern (2)


Diagnostic Characteristics of the Group
Ferns are land plants that have vascular systems, true roots and leaves, and reproduce using spores. A fern plant generally consists of one or more fronds attached to a rhizome(6 VM). A frond is simply the leaf of the fern(6 VM). The leaves are made up of two parts: the petiole (stipe), and the blade. The pattern of the stipes are diverse and can often help identify a certain class. (11) (JAC) A rhizome is a specialized, root-like stem to which the true roots are usually attached(6 VM). The vascular system, a system for transporting water and minerals, contains vascular tissue, which is used in transporting materials between roots and all other parts of the fern. The two types of vascular tissue are called xylem (transports water and nutrients) and phloem (transports food). (1) Ferns are
Habitats
Since the beginning of their existence, ferns have grown in swampy, moist areas, such as in swamps, on the shores of bodies of water and streams, in the tropics, and in temperate forests that receive adequate amounts of rain. Very few ferns are able to grow in arid climates, but those that do are called xerophytic ferns (8, DJ). Most live in the Southwestern area of the United States and are dormant during the summers (8, DJ). These ferns are able to exist in dry areas due to their waxy epidermis (8, DJ). A specific example is the Notholaena jonesii which lives in Utah, Nevada and California (8, DJ). Ferns are highly dependent on water for their reproduction, which is why they need to be in areas with constant moisture. (1) Although some types of ferns are adapted to dry habitats, most species prefer moist and shady environments. Ferns are particularly abundant in rainforests and woodlands. Ferns also tend to thrive in areas of higher temperatures. In fact, the majority of fern species live in areas where the average temperature is between 30 and 40 degrees Celsius (3, NK).
Ferns live in a wide variety of habitats across the globe. They can thrive on the high elevations of mountains, in deserts, in open fields, or in water. Ferns are notable in that they can often survive in places that are inhospitable to flowering plants. Some ferns have become so prolific in their habitats that they have been classified as weeds. (7) (KN)

A fern in a desert habitat (KN) (9)
A fern in a desert habitat (KN) (9)


Major Types
There are two major types of ferns: lycophytes from the phylum Lycophyta and ferns, whisk ferns, and horsetails from the phylum Pterophyta. (1)

Lycophytes are commonly called club mosses, although they are not mosses. They have true leaves with vascular tissue, differentiating them from mosses. The spores of lycophytes are clustered on the tips of branches on sporophylls (specialized reproductive leaves) and give lycophytes a club-shaped structure on top, called strobili. The strobili give the club mosses the look of a moss. (1)
Lycophytes are the oldest existing group of vascular plants. They are very small organisms now, but in the past, they would grow to be over 35 meters in height. The lycophytes evolved the microphyll leaf, a leaf with only one vein.(13) (RJS)

The whisk ferns, also called psilophytes, have a peculiar Y-like (dichotomous) branching, similar to the branching of ancient vascular plants. In addition, they lack true leaves and roots. Both of these characteristics caused plant biologists to believe that whisk ferns were remnants of a very early lineage of vascular plants. However, DNA comparison and analysis of certain structures show that whisk ferns are closely related to modern ferns, and the branching and lack of true leaves and roots evolved after the whisk ferns branched off from ferns. (1)

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The fiddlehead growth on a Sadleria cyatheoides fern in Hawaii. (12)(SS)

As you can see, these whisk ferns are y-branched, leafless and lack roots. Instead of roots, these plants have subterranean rhizomes (rootlike stems) which hold the plant in place and absorb nutrients from the soil through filaments called rhizoids. This is considered one of the most primative vascular plants in of our time.(AR) (12).

Horsetails, or sphenophytes, used to be their own phylum, but molecular evidence suggests that they are closely related to ferns and are now classified with them. Often found in marshy places or alongside streams and sandy roadsides, horsetails have vertical green stems and horizontal stems (rhizomes) that produce roots. The stems are jointed and have rings of little leaves emerging from them; however, it is the stem, not the leaves, that does the most photosynthesis (using energy from the sun and water to make food). Horsetail stems produce cones, which are made of clusters of sporophylls, at their tips, and the sporophylls have sporangia, little capsules in which spores grow. (1)

Ferns are a very broad range of plants, containing over 12,000 species. Most ferns grow in areas with lots of moisture, like the tropics and temperate forests, but some are known to grow in arid places. Like horsetails, ferns have horizontal rhizomes, from which grow large leaves with vascular systems. Fern leaves are also called fronds and are compounded (divided) into many leaflets. As the coiled tip of the fern, called the fiddlehead, unrolls, the frond grows. On the underside of the fern’s sporophylls, clusters of sporangia, called sori, grow, and they may be arranged in patterns, such as parallel lines or dots. Most sporangia have little spring-like mechanisms that launch the spores into the air, where the wind can blow them far away from their mother plant. Most terrestrial ferns are homosporuous because they need to use the little water where they grow to help transport their sperm to the archegonium. If the terrestrial ferns were heteroporous, successful fertilization would be rare. However, some ferns (aquatic ferns) are still heteroporous (gametophyte produces only one type of gamates but sporophyte produces two types of spores) ; they have a lot of water to use to successfully transport the sperm, so they remain heterosporous. (1) (SV)

Basic Anatomy
Ferns have full vascular systems made up of both xylem and phloem that travel throughout the entire fern. In addition, ferns have true roots, which most likely developed from the lowest part of the stems in ancient vascular plants. Ferns have stems with leaves that protrude from these stems. Terrestrial plants before ferns did not have leaves, and ferns developed leaves only because they had vascular tissue; it is hypothesized that microphylls, small leaves, started out as small stem outgrowths into which vascular tissue grew. Megaphylls, large leaves, formed when many stem outgrowths were clustered together; over time, a tissue webbing joined the clustered outgrowths and the vascular tissue branched off into the outgrowths. The leaves in lycophytes are called microphylls and the leaves in pteridophytes are called megaphylls. Besides their comparative sizes, the main difference between microphylls and megaphylls is that microphylls only have a single, unbranched vein of vascular tissue while megaphylls have very branched veins of vascular tissue. Since a branched vascular system can supply a leaf with more water and nutrients and export more sugars to the rest of the plant, megaphylls perform more photosynthesis than microphylls. (1)
I10-22a-fernanatomy.jpg
(This image shows the anatomy of Ferns. YS)

Transport of Materials
Ferns were the first plants to evolve with a vascular system. In early vascular plants, there were no true roots, only stems. Using fossils of the early vascular plants and comparing them to the ferns of today, it is clear that the stems of the early vascular plants and the roots of present-day ferns are almost identical, indicating that the roots of present-day ferns evolved from the part of the stem closest to the ground in early vascular plants. (1)

Some ferns have megaphylls, which have high productivity rates for photosynthesis. Due to the highly branched vascular tissue, water, nutrients, and food can be transported in and out of the megaphylls in relatively high amounts. Compared to other modern vascular plants, lycophytes have small leaves called microphylls that contain a single, unbranched vein. Because there are no branched veins in the microphylls, a lesser amount of water and minerals can be supplied to the leaves and a lesser amount of sugar can be exported from the leaves to the rest of the fern. Therefore, there is less photosynthetic activity happening in the lycophyte leaves than in the leaves of other vascular plants. (1)

Reproduction


fern_lifecycle.jpg
This is a diagram of the fern's alternation of generation reproduction cycle (explained below).


(4 SES)


Ferns follow an alternation of generation cycle; that is, part of the plant’s life is spent as a sporophyte (a plant that produces spores, reproductive cells that can divide without binding to other cells (OZ) ) and part of the plant’s life is spent as a gametophyte (a plant that produces gametes, or sex cells). However, a fern spends most of its life as a sporophyte, which is the green, leafy plant we see when we think of a typical fern.
The following description of a fern’s reproductive cycle will start with a mature sporophyte. (1)

The sporophyte is a diploid, multicellular organism, meaning every cell contains the normal amount of chromosomes (23 pairs, or 46 chromosomes). On the underside of the special reproductive fern leaves, called sporophylls, there are sporangia (singular: sporangium. Clusters of sporangia, whether arranged in clumps or in lines, are called sori (singular: sorus). The spores mature inside the sporangia until they are ready to be released. The spores are haploid, meaning they only have half of the normal amount of chromosomes (23 chromosomes). (1)

When a spore lands on a promising surface, it begins to grow a small haploid gametophyte that is able to sustain itself through photosynthesis. If the fern lives on land, the spore will grow into a gametophyte with both male sex organs (antheridia) and female sex organs (archegonia); if the fern lives in an aquatic environment, the spore will grow into a gametophyte with either archegonia or antheridia. The archegonia will produce eggs that stay inside them, while the antheridia will produce sperm that will leave and swim to the eggs if water is present. In ferns that grow in an aquatic environment, needing water is not an issue; the terrestrial ferns are more dependent on water, so they grow in damp areas to ensure they will have a film of water around them when it is time to fertilize. When the sperm and egg are matured, the sperm will swim to the egg, causing fertilization. Since both the egg and the sperm are haploid, fertilization causes the sporophyte (fertilized egg) to be diploid (23 chromosomes + 23 chromosomes = 46 chromosomes). As the sporophyte grows, it develops into the leafy ferns, and the gametophyte dies off. (1)

Ferns spend most of their time in the sporophyte stage. A fern gametophyte is very, very small; so small, you would have to crawl on your hands and knees and look at the very tip of the stem going into the ground to find it. In addition, the gametophyte only lasts long enough for the egg to be fertilized and the sporophyte to become self-sustaining, a relatively short time. Refer to the figure below. (1)

external image I10-68-fern.jpg(7) (BMB)

Environmental Adaptations
In order to move away from aquatic environments, ferns had to undergo a number of adaptations that would enable water to reach each cell of the plant (ER). One of them was developing its leaves into megaphyll. A megaphyll is a leaf with a complicated system of veins to transport nutrients with ease.(SJB)(5). The a major adaptation (ER) was to develop a vascular system that can transport water and nutrients (using xylem) and sugars (using phloem) around the plant efficiently(ER). In smaller plants, there are very few cells; water, nutrients, and food can be passed from cell to cell. The xylem and the phloem function as different types of cells in the fern. The phloem cells transport sugars throughout the plant and are alive. The xylem cells, on the other hand, transport water and nutrients throughout the plant and must be dead to function properly. In vascular plants there are “vascular bundles” in the stem of the plant and “vascular bundles” in the leaves of the plants. In the stem, the xylem cells are found inside the bundles, whereas in the leaves the xylem cells are “on top” of the bundles. In a vascular transport system, the roots absorb soil, minerals and water, and the leaves absorb air, light, and carbon dioxide. (Jesse Carmen) (4) Due to the vascular system, ferns were capable of growing to much larger proportions and were able to move farther away from water. However, ferns are not able to move completely way from water, for they still are dependent on water to reproduce. (1) The spores of ferns have to travel on the wind and land in an area that is suitable for ferns to grow. In order to prevent damage, spores are protected by sporopollenin, a substance coating the outside of the spores that is resistant to most environmental damage. (1)

Review Questions
1) Describe the characteristics ferns evolved that allow them to live on land. [HZ]
2) Explain the function of the leaves of ferns and the basic structure of both microphylls and megaphylls. (HS)
3.) What type of environment do ferns need in order to thrive? (RK)

4) Why specifically do ferns have to live in moist environments? (BL)
5.) Discuss the basic types of ferns, and explain what features about them lead scientists to believe they are ancestors of the first vascular plants. (GR)
6.) What key characteristics differentiate ferns from other vascular plants? (Nangia)


Sources:
(1)
Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2002.
(2) "Ostrich Fern." Http:www.plantcare.com/. <http://www.plantcare.com/oldsite/httpdocs/images/namedimages/ostrich_fern.jpg>.
(3) "Ferns." Www.ferns.com. 4 Dec. 2008.(4) "Primitive Plants: Mosses, Ferns, and their Allies." University of Maryland. 2 November 2002. 7 December 2008. <http://biology.clc.uc.edu/courses/bio106/mosses.htm>.
(4) "Plant Reproduction, Growth and Development." 7 Dec. 2008. <http://www.bio.miami.edu/dana/dox/altgen.html>
(5) http://science.jrank.org/pages/2685/Ferns-Evolution.html
(6) "Ferns-General Charactersitics." 7 Dec. 2008 <http://science.jrank.org/pages/2679/Ferns-General-characteristics.html>
(7) "Anatomy of Plants." 7 Dec. 2008 <http://universe-review.ca/r10-34-anatomy2.htm>.
(8) Pearson, Lorentz C. The Diversity and Evolution of Plants. New York: C R C P LLC, 1995. 482.
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(9) Flickr. Flickr. <http://farm1.static.flickr.com/27/55809597_f7149a794b.jpg?v=0>.
(10) http://universe-review.ca/I10-22a-fernanatomy.jpg
(11) Pryer, Kathleen M. and Alan R. Smith. 1997. leptosporangiate ferns. Version 01 January 1997 (under construction). http://tolweb.org/leptosporangiate_ferns/21666/1997.01.01
in// The Tree of Life Web Project, http://tolweb.org/(12) "A whisk fern." 14 Dec 2008. <http://www.washjeff.edu/greenhouse/Pnudum/>.
(12) "Fern fiddlehead (Sadleria cyatheoides), Kilauea, Hawaii." Flickr. Yahoo! 17 Dec. 2008 <http://www.flickr.com/photos/38037974@n00/487632904/>.
(13) brs, "Introduction to the Lycophyta." 18 Dec 2008 <http://www.ucmp.berkeley.edu/plants/lycophyta/lycophyta.html>.

OZ, Sarah Vlach, NK, Jesse Carmen, Kevin Nayer, Sarah Schwarzschild, Ethan Richman, Sam Blatchford, Vonai Moyo, Brittany Marcus-Blank, Daisy Joo, Hanna Zhu, Josh Czik, Arielle Reiter, SS, Becca Levenson, Meru Nangia