Ferns and Dandelions

Joy Paul Environmental Science

This paper seeks to explore some of the similarities and differences between two plants; the common lady fern and the common dandelion.

Their phylogeny is as follows:

Domain: Eukarya (both)

Kingdom: Plantae (both)

Division: fern-Polypodiophyta; dandelion-Magnoliophyta

Class: fern-Filicopsida; dandelion-Magnoliopsida

Order: fern-Polypodiales; dandelion-Asterales

Family: fern-Dryopteridaceae; dandelion-Asteraceae

Genus: fern-Athyrium; dandelion-TaraxacumG.H.Weber ex Wiggers

Species: fern-filix-femina; dandelion-Taraxacum officinale G. H. Weber ex Wiggers

Both are multi-cellular photosynthetic organisms. Dandelions and lady ferns have vascular tissue for the transport of materials. They have leaves, (although differently shaped), stems and roots. The dandelion uses a tap root and the lady fern absorbs material via a fibrous root system. The lady fern’s stem is an underground rhizome. The dandelion has an above ground stem.

While both plants can reproduce sexually and asexually, their sexual reproductive life cycle using alternating generations between haploid and diploid organisms is quite different. As seen on the previous web page, the fern uses two very distinct, separate plants within its life cycle. The sporophyte generation is the one generally recognized as a fern plant. Clusters of spores called sori form on the underside of the leaf fronds. In ferns, the spores are all the same size. Therefore, ferns are known as homosporous plants. Thousands of spores are produced on each fern. If a released spore lands on a suitable site, it germinates growing into a gametophyte (haploid) plant. Fern gametophytes are generally bisexual, although they do have several mechanisms to prevent self-fertilization (see biology paper link). Water is necessary for fertilization to occur. Once the egg has been fertilized, a new sporophyte (diploid) plant grows from the zygote.

While the dandelion also uses alternating generations between haploid and diploid plants during its life cycle, the reproductive cycle is a bit more complex. The plant commonly recognized as a dandelion is the sporophyte (diploid) generation. Within the male reproductive structure called the anther, microspore mother cells undergo meiosis to produce haploid microspores. These microspores develop into pollen grains that contain sperm. Each pollen grain has an outer coating, a tube cell and a generative cell which forms two sperm. This microspore is the male gametophyte plant or generation. Within the female reproductive structure called an ovule, the female gametophyte plant develops. The ovule contains a stalk, a central core of cells and layers of surrounding tissue. Within the central core of cells, a megaspore mother cell undergoes meiosis to create four megaspores. Three die; one develops into the female gametophyte plant. One cell within the gametophyte develops into the egg. Another cell containing two haploid nuclei will develop into endosperm once fertilized by a sperm. Insect pollination is required for the sperm to reach the female reproductive structures (stigma, style and ovary). The fertilized egg develops into a seed. Dandelions produce an abundant number of seeds per plant. Each seed has a parachute like tuft to aid in wind dispersal. If a seed lands upon a suitable site will grow into a new sporophyte plant. To summarize the reproductive differences above: 1). The fern has two distinct plants within its life cycle; the dandelion’s gametophyte plants are enclosed within the sporophyte plant. 2). The fern gametophyte is bisexual; the dandelion has separate plants for each sex. 3). The fern is homosporous; the dandelion heterosporous (spores of two different sizes, microspores and megaspores). 4). The fern requires water for fertilization; the dandelion requires animal pollinators such as bees or other insects to transfer pollen to the female reproductive structures. 5). Since animal pollinators are needed for dandelion fertilization to occur, it has developed highly specialized organs for reproduction (anther, stigma). No such organs exist on the fern.

As stated above, fern gametophytes are bisexual. This could lead to a large amount of self-fertilization known as intragametophytic selfing. Intergametophytic selfing (sperm from one gametophyte fertilizing another gametophyte, both from the same sporophyte) also occurs in ferns. Both instances would result in high levels of homozygosity in ferns, leading to evolutionary stagnation (Haufler, 2002). In fact, this is not the case. A high incidence of polyploidy occurs in ferns and other homosporous plants. Polyploidy helps ensure the genetic variation necessary for evolution to occur. 95% of all pteridophytes may be called polyploid, as compared to only about 50% of flowering plants (Haufler, 2002). Polyploidy results from spores that have not undergone meiosis to reduce their chromosome numbers. These spores germinate and produce a diploid gametophyte. If the resultant gametes fertilize another unreduced gametophyte, a polyploid sporophyte plant results. Since ferns produce an extraordinary number of spores, the chance of some of these spores not reducing meiotically (and therefore producing polyploid organisms) is reasonable (Haufler, 2002). This results in the high level of polyploidy stated above. Contrast this situation with angiosperms. Both the microspore and the megaspore would have to forego meiosis. Then the polyploid microspore would have to depend on its specialized fertilization mechanism (insects such as bees in the case of the dandelion) to reach another polyploid megaspore to result in a polyploid zygote developing into a sporophyte plant. These barriers found in angiosperms prevent a high level of polyploidy as compared to pteridophytes (Haufler, 2002). Whereas ferns have a higher level of polyploidy to provide genetic variation for intergametophytic selfing, angiosperms do not. Yet both the lady fern and the dandelion are generally known as colonizers. They are both one of the first plants to arrive into a newly created or recently disturbed habitat. According to Haufler (2002), inbreeding may be a trait unique to colonizing species. Inbreeding spreads genetic homozygosity. While ferns use polyploidy to provide genetic variation and prevent evolutionary stagnation, dandelions may use apomictic strategies (producing seeds without sexual reproduction) to prevent homozygosity and evolutionary stagnation from occurring. By creating seeds without sexual reproduction, the dandelion is able to quickly colonize an area with offspring while not permanently depleting genetic variation. Sexual reproduction and recombination through intergametophytic crossing can continue with the dandelions using insect pollinators.

Distribution patterns between fern and dandelion families are similar throughout the world. Yet one study (Kramer, 1993) determined that fern genera are much more widely distributed than angiosperm genera. Out of 150 major angiosperm families, 92.5% are seen in the eastern and western hemispheres. For ferns, 95% of the 20 major families are pantropical. Four reasons for this difference have been determined. 1). Ferns are much older than flowering plants. They evolved during the Devonian period, approximately 365 million years ago. They were the dominant plant life during the Carboniferous period. Angiosperms are thought to have been the dominant plant life later, during the Cretaceous period approximately 130 million years ago. Since ferns have been determined to be much older, they have had more time to spread across the Earth, especially when the continents were closer together. 2). Ferns evolve slower than angiosperms. There is an extremely high selection pressure on angiosperms due to their flowers, fruits and fertilization needs. No such pressure exists for ferns. Without this natural selection pressure, no evolutionary divergence has taken place among ferns. 3). Ferns spread easier across large distances than angiosperms. The high number of extremely small, lightweight spores found in ferns makes them ideal island colonizers. Angiosperms, with larger seeds often do not travel as far. 4). New phylogenetic studies of ferns have made the classification level of genera “fuzzy”. Therefore, distribution data may be off. However, Kramer (1993) concluded that slower evolution and easier dispersal of fern spores are the primary reasons pteridophyte are more widely distributed than angiosperms at the genus level.

Both the lady fern and the common dandelion are colonizers of newly created or disturbed habitat. Yet the habitats they are found in are quite different from each other. The lady fern is most commonly found as the dominant under-story plant of moist woodlands, especially red cedar woods (U.S. forest service website). The lady fern grows rapidly in disturbed areas of the forest where trees have fallen or lightning strikes have created a gap in the canopy. The most active growth period for the lady fern is in the spring and early summer, when the forest canopy is not fully developed and light is abundant. Yet it is somewhat shade tolerant. The fern is best adapted to medium textured soils, as opposed to coarse soil, sand or clay. The soil must have a medium fertility rate and a pH between 4.5-7.0 for optimal growth. Lady ferns have a medium growth rate and long lifespan. Athyrium filix-femina requires the moist soil found in areas with a precipitation range between 30-60 inches a year. The dandelion is also a colonizer, but prefers open fields or other bare areas as a habitat. The dandelion is found in nearly all climates and open areas such as highway roadsides, old fields, clear cut forests and lawns. It has an intermediate shade tolerance. The most active growth period is the spring and fall. The dandelion is adapted to most soil types: coarse, fine and medium textured, as well as sandy soil. It can tolerate poor soil fertility and a wider pH range than the fern (4.8-7.5). The low soil requirements help to explain the dandelion’s broader habitat range versus the lady fern. Dandelions have a much more rapid growth rate and a shorter life span than the fern to allow it to quickly colonize bare urban landscapes or other disturbed areas. The low precipitation requirements of the dandelion (12-55 inches/year) allow it to thrive in the dry fields where it is commonly found.

K-selected plant species tend to grow in fairly predictable climates. They usually have a long life span and development time, along with a generally small amount of seeds. Other plant species follow an r-selected life history pattern, growing in unpredictable climates, having shorter life spans and development times and high seed production. While both the lady fern and dandelion fall somewhere between these extremes, the fern’s physiology, morphology, and growth requirements described above place it a bit closer to the K-selected life history pattern. Conversely, the enormous amount of spores produced by a lady fern is more indicative of an r-selected pattern. The dandelion’s morphology, physiology, and growth requirements, along with the high number of wind dispersed seeds place it much closer to an r-selected life history plant.

The lady fern and common dandelion are both opportunistic colonizers. Both ferns and dandelions reproduce sexually and asexually, although the life cycle of all angiosperms is more complex than pteridophytes. Both plants release large numbers of progeny; ferns use spores, dandelions seeds. Ferns are often polyploid, angiosperms rarely so. Dandelions use cross pollination to add genetic variability while using apomictic seeds to quickly take advantage of new habitat areas. Fern genera are more widely spread globally than angiosperms due to their earlier appearance evolutionarily and the ability of the lighter, more numerous spores to distribute over large distances. The lady fern requires a wet woodland habitat with good soil for optimal growth. The common dandelion can grow, survive and colonize much harsher environments. While both plants lean toward an r-selected life history pattern as far as number of offspring is concerned, the fern’s growth requirements puts it more toward a K-selected plant species. The dandelion’s less stringent needs keep it closer to an r-selected species.

Resources:

Haufler, Christopher H. “Homospory 2002: An Odyssey of Progress in Pteridophyte Genetics and Evolutionary Biology.” Bioscience 52. 12 (2002): 1081-1094.

Kramer K.U. “Distribution patterns in major pteridophyte taxa relative to those of angiosperms.” Journal of Biogeography 20. (1993): 287-291.

Kricher, John and Morrison, Gordon. A Field Guide to Eastern Forests. United States: Houghton-Mifflin, 1998.

Krogh. Biology: A Guide to the Natural World Upper Saddle River: Prentice Hall, 2005.

Mackenzie, Fred and Judith. Our Changing Planet. Upper Saddle River: Prentice Hall, 1995.

University of Alaska Fairbanks Aug 10, 2006 http://www.geobotany.uaf.edu/teaching/biol474/biol474-06_lesson06.pdf

USDA Forest Service. Aug 10, 2006. http://www.fs.fed.us/database/feis/plants.html

USDA Natural Resources Conservation Service. Aug 11, 2006. http://plants.nrcs.usda.gov