Fern Structures and Reproduction
Ferns are seedless, vascular plants. They contain two types of vascular tissue that are needed to move substances throughout the plant. Evolutionarily, this addition of vascular tissue to plants is what allowed ferns to grow up and out rather than just spreading along the ground. The more primitive mosses rely on osmosis and diffusion for material movement and need to stay in close contact with the ground. With the addition of vascular tissue, water, nutrients and food could now be transported throughout a taller plant. The first type of vascular tissue, xylem, is responsible for moving water and nutrients throughout the plant. As the xylem cells reach maturity they die, losing their cellular contents. The external cell walls remain intact. These cell walls are stacked end to end forming long tubes from the roots, through the stems, up to the leaves. As water vapor exits the leaves through the stomata, a process known as transpiration, a vacuum is created, pulling more water from the roots up the xylem tube. This vacuum/suction moves water throughout the entire plant. The stiff cell walls of the xylem also provide support for the fern plant as it grows taller. The other vascular tissue, phloem, is responsible for moving glucose throughout the plant. Phloem tissue is comprised of sieve elements; live cells which have lost their nuclei, and companion cells; which seem to “take care of” the sieve cells. To move food throughout the fern, glucose is pumped into the sieve elements (this process requires cellular energy). Water also moves into the phloem tissue via osmosis, creating a pressure that “pushes” the glucose throughout the plant.
Ferns are only capable of primary growth i.e. growing upward. They do not increase in diameter, a type of growth known as secondary growth. This primary growth occurs at the tips of the plant’s shoots and roots within areas called apical meristems. The apical meristems contain meristematic tissue which gives rise to all other types of plant tissue.
Ferns also contain true roots, stems and leaves. The fern leaves are considered to be megaphylls, meaning they have several vascular strands within them. Fern leaves are also known as fronds. When leaves first emerge they are often tightly coiled and called “fiddleheads” since they resemble the very top part of a fiddle instrument. The fronds arise from an underground stem known as a rhizome. Underground roots are attached to the rhizome and serve as an anchor for the plant along with absorbing water and nutrients from the ground.
Modern ferns are descended from some of the oldest plants on Earth. They are believed to have arisen between 420-360 million years ago. Their phylogeny is as follows:
Domain-Eukarya (their cells contain nuclei)
Kingdom-Plantae (they contain chlorophyll for photosynthesis and cell walls)
Division-Pteridophyta (also called Polypodiophyta when used as a part of Tracheophyta or vascular plants)
Order-Athyriales (one of the largest)
Further classification is based upon the fern’s sporangia, whether or not they are protected by a covering called an indusium, the shape of the fronds, and how the fronds unfold.
vascular plants feature an alternation of generations within their life
the sporophyte generation and the gametophyte generation. In ferns, the
multicellular sporophyte is what is commonly recognized as a fern
plant. On the
underside of the fronds are sporangia. Within the sporangia are spore
cells called sporogenous cells. These cells undergo meiosis to form
spores. The spores on most ferns are the same size and perform the same
function. Therefore ferns are known as homosporous plants. The
usually in clusters known as sori, found on the underside of the fern
ferns have a covering over the sporangia known as an indusium. When the
are mature, they are released from the sporangia. If a spore lands on a
suitable site, it will germinate and grow via mitosis into a mature
plant. A gametophyte is the plant that produces gametes. The fern
is a small (approximately 5 mm), bisexual, heart-shaped plant called a
prothallus. The prothallus is haploid, since it grew from a spore which
been formed by meiosis. It does not have any vascular tissue and uses
rhizoids to anchor it to the ground. On the underside of the prothallus
organs form. The female structure, called an archegonium, contains a
egg. The male structure, the antheridium, contains many flagellated
sperm are released from the antheridium and swim through a thin film of
to a nearby archegonium to fertilize the egg. Since the antheridium and
archegonium are on the same prothallus the fern has several strategies
prevent self-fertilization. These strategies will be discussed later in
paper. Once fertilization of the egg has occurred, a diploid zygote has
created. As the zygote grows into an embryo it remains attached to the
prothallus. The embryonic plant depends upon the prothallus for water
nutrients. As the embryo grows and develops into a mature diploid plant
prothallus dies. This mature plant is called the sporophyte generation
produces spores. The sporophyte plant is the one most commonly
recognized as a
fern. The sporophyte then produces new spores as described above.
Another method of reproduction ferns use is clonal spreading. Underground rhizomes grow and sprout new sporophyte plants. Huge clonal colonies of ferns have been found that are made up of thousands of individual clonal plants called ramets (Klekowski, 2003). This extensove clonal spread is especially adaptive for the sporophyte phase. Their long life span (several years) and extensive competition for space with other plants allows ferns to quickly become the dominant understory plant in newly disturbed areas of the forest. However, aproximately 10% of fern species do have gametophytes that reproduce vegetatively using an asexual bud-like structure known as a gemmae (Haig, 2006).
With archegonia and antheridia on the same gametophyte, one would assume a very high level of inbreeding. If intragametophytic selfing occurred, where sperm fertilize an egg on the same gametophyte plant, the resulting sporophyte would become 100% homozygous in only one generation, losing all genetic variability necessary for future evolution to occur. Any lethal recessive gene would be expressed and the species would very quickly die out. Intergametophytic selfing, where sperm from one gametophyte fertilized the egg on another gametophyte both having come from the same sporophyte, would also quickly result in homozygosity. In fact, this does not happen. Homosporous pteridophyte species are not all highly inbred or evolutionarily stagnant (Haufler, 2002). There are several mechanisms ferns use to prevent self-fertilization and the resulting homozygosity. Intergametophytic crossing, sperm fertilizing an egg on a different gametophyte where both plants arose from separate sporophytes, occurs. This is similar to pollen from one lily landing on the stigma of another separate lily plant several meters away. Some fern gametophytes produce a pheromone called antheridiogen. This pheromone causes neighboring immature gametophytes to produce only antheridia. This allows many more sperm to be produced for possible cross-fertilization of the egg. Archegonia and antheridia mature at different rates to prevent intragametophytic selfing. One of the most interesting strategies used by ferns to prevent evolutionary stagnancy is polyploidy. Over 95% of ferns have been found to be polyploid (Haufler, 2002). This results from a process known as allopolyploidy. Allopolyploidy occurs when two haploid sets of chromosomes have come from two different species of ferns. The result is that during meiosis, the homologous chromosomes cannot match up because they are somewhat dissimilar. When intergametophytic selfing occurs, the chromosomes are similar enough that a sporophyte zygote can form and grow. However, when the sporophyte tries to produce spores via meiosis, the chromosomes are not similar enough to pair up as needed. They are homoeologous, not homologous. This inability to undergo meiosis and produce spores renders the plant infertile. However, sometimes the chromosomes replicate creating polyploid individuals. The homologous chromosomes can now pair up and separate during meiosis, creating spores. These now diploid spores are released, germinate and grow into healthy gametophytes. The diploid gametes self-fertilize creating healthy sporophyte plants. Although this self-fertilization may sound like the intragametophytic selfing described above, it is not. The polyploidy that was created when the chromosomes doubled before pairing provides genetic variation within the plant to prevent homozygosity. The presence of homoeologous chromosomes also helps prevent lethal recessive gene expression. With extra dominant copies of genes available, fewer lethal genes are expressed. Studies have also shown that occasionally homoeologous chromosomes will pair up with a homologous chromosome during meiosis, creating new gene combinations.
Although today’s common ferns have descended from some of the oldest plants on Earth, their reproductive cycle and genetic variability are complex.
Glen, Rene. “Pteridophytes: the mysterious plants
no seeds.” Lantern 42. 1 (1993): 48-51.
Haig, David and Wilczek, Amity. "Sexual conflict and alternation of haploid and diploid generations." Philosophical Transactions of the Royal Society B: Biological Sciences 361. 1466 (2006): 335-343.
Haufler, Christopher H. “Homospory 2002: An
Progress in Pteridophyte Genetics and Evolutionary Biology.” Bioscience
52. 12 (2002): 1081-1094.
Klekowski, Edward. "Plant clonality, mutation, diplontic selection and mutational meltdown." Biological Journal of the Linnean Society 79. 1 (2003): 61.
Krogh. Biology: A Guide to the
Solomon, Berg, Martin. Biology Belmont: Brooks/Cole-Thomson Learning, 2005.