Gymnosperms

By: Evan Kendall
Pine cone
Pine cone


Diagnostic Characteristics of Gymnosperms:


Gymnosperms (from the Greek of gymnos=naked and sperma=seed) are conifers and other plants that do not have specialized chambers for their seeds. Gymnosperm means “naked seed” because they don’t actually have enclosed chambers (like ovaries) where the seeds develop. Instead, the seeds for Gymnosperms go through the developmental stage in cones. Gymnosperms evolved from an ancestor of an extinct group of plants known as progymnosperms, and some of those progymnosperms had regular seeds. The first vascular plants with seeds appear in the fossil record at the end of the devonian period and gave rise to the modern-day gymnosperm diversity. Gymnosperms were dominant and thrived during the Mesozoic era and were primarily eaten by dinosaurs. The angiosperm, a relative of the gymnosperm, did not appear until after the gymnosperm. Once the dinosaurs became extinct, the gymnosperms lived on and found a way to survive on Earth.

Phylogeny_of_GS.gif

Habitats of Gymnosperms:

Gymnosperms, primarily conifers, tend to live in the Northern Hemisphere and are found mostly in forests. For example, one type of a Gymnosperm is the Redwood, which is found only in a small coastal piece of Northern California. Another type of Gymnosperm, the Bristlecone pines, is found in California as well, and is considered to be one of the oldest organisms alive.
One tree named Methuselah is over 4600 years old and still produces viable offspring. It grows two miles above sea level in alkaline soil nearly devoid of nutrients with little water.
The conifers (the most popular species of Gymnosperm) dominate large forested regions [in the Northern Hemisphere] because the growing season is fairly brief because of altitude. With approximately 600 living species, this is the most diverse and by far the most ecologically and economically important gymnosperm group.

Major Types of Gymnosperms:

Gymnosperms are only composed of four different phyla. These consist of:
-Phylum Ginkgophyta (Ginkgo)
external image biol_02_img0159.jpg
-Phylum Cycadophyta (Cycads)
external image cycad.gif
-Phylum Gnetophyta (Gnetae)
external image gnetum.jpg
-Phylum Coniferophyta (Conifers)
external image 156030main_Conifers_JPG.jpg
The most common of these phyla is the Phylum Coniferophyta. This phylum consists of conifers (from the Latin conus=cone and ferre=to carry). Conifers are the oldest known living trees and can survive in temperate and extremely cold conditions. Conifers use a cone for their reproductive structure. Many examples of conifers are: Pines, firs, spruces, yews, junipers, cedars, and redwoods. Most of the conifers are fairly large trees, like a pine tree. Conifers are often large and can dominate the plant life in some ecosystems because their stems continue to expand throughout the life of the plant. Conifer trees are needle or scale-like and are connected to the vascular system of the stem. Pine trees are sporophytes (multicellular diploid form in organisms) and are heterosporous (male and female gametophytes develop from different kinds of spores that are produced by separate cones). There are an estimated 800 living species of gymnosperm.

Aside from conifers, the other Gymnosperms are important in their own right as well. For instance, Ginkgo Baloba is the only species in existence for the Phylum Ginkgophyta and it is believed to improve memory.The Ginko biloba has a distinct appearance with fans shaped leaves and is capable of growing up to 30 meters tall. Another defining feature of this plant is that it smells horrible (often compared to the smell of vomit) due to the foul smelling compound called butyric acid found in the integument of the seed. Although this is the only extant species of Ginkophyta, it is very resistant to pollution and pests and is capable of adapting within a large temperature range (from temperate to Mediterranean climates). Cycads, from the Phylum Cycadophyta, look like palms but are actually plants that flower. Cycads can be viewed as beneficial because they can form symbiotic with nitrogen fixing bacteria, but they have also been the subject of extermination programs because they are highly toxic to livestock. Phylum Gnetophyta has three different types of genera which are Welwitschia, Gnetum, and Ephedra. Welwitschia have giant leaves, Gnetum grow in the tropics as vines and trees, and Ephedra is a shrub that exists in many of the deserts in America. Gnetophytes in general are characterized as the only gymnosperms that undergo double fertilization; due to this and the presence of vessel elements in their xylem tissue, it is speculated that gnetophytes may have been ancestral to angiosperms.

A Gymnosperm consists of three major parts: the seed coat, the food supply, and the embryo. The seed coat is the outer layer of the seed that protects the contents inside, otherwise known as the embryo. The food supply provides nourishment and support for the embryo, and the embryo is the new sporophyte forming. See the picture below for a better description of the anatomy of a Gymnosperm.

Seed_GS.gif

Transport of Materials in Gymnosperms:

Materials are transferred in Gymnosperms through the reproductive process. Gymnosperms go through reproduction to create new sporophytes and ultimately end up spreading their seed further so that they can grow and allow for more diverse habitats. Humans use the materials that are created for Gymnosperms as well. Gymnosperms’ wood provides a great material to create paper with and this wood is used all the time to create paper. So, the next time you throw out paper, think twice and recycle it so that Gymnosperms stay on this Earth forever!!!

Reproduction in Gymnosperms:

Reproduction in Gymnosperms is a seven-step process that can be effectively shown through the reproduction in a pine tree. To further your understanding of this process, follow along on the picture for each step to see how reproduction occurs in Gymnosperms.

Steps of reproduction:
1.) Trees contain pollen and ovulate cones.
2.) A pollen cone has hundreds of microsporangia which are in sporophylls (small reproductive leaves). The cells in microsporangia undergo meiosis, creating haploid microspores and then turn into pollen grains (immature male gametophytes).
3.) Pollen falls onto the ovulate cone and goes into the ovule through the micropyle. Pollen grain germinates in the ovule, making a pollen tube that starts to digest through the megasporangium.
4.) In the year following the pollination, fertilization will begin and a megaspore mother cell goes through meiosis to make four haploid cells. Of these four cells created, only one of them grows, but it will divide repeatedly, creating an immature female gametophyte.
5.) A few of the archegonic (female gametangia) that each contain an egg develop within the spore.
6.) The pollen tube has extended to the female gametophyte. Fertilization will begin when one sperm nuclei enters the pollen tube and unites with the egg nucleus. Usually when this happens, only one zygote will actually become an embryo.
7.) The new sporophyte has a basic root and a few embryonic leaves. A food supply surrounds the embryo which provides nourishment. Now the ovule is a pine seed.

Life_Cycle_GS.gif
first_stage_of_gymno.jpg
second_stage_of_gymno.jpg
The inside of a female cone, where each scale of the cone bears two megasporangia, or ovules. Each single cell undergoes meiosis to produce four megasporangia.

third_stage_of_gyno.jpg
Egg cells in the female gametophyte. (AS 22)

The mature seed consists of three generations of tissues: maternal sporophyte tissue (seed coat and central mass of ovule), gametophyte (the sexual form of a plant in the alternation of generations), and daughter sporophyte (embryo). After about two years the mature seeds are shed, restarting the process of reproduction.

The Environmental Adaptations for the Gymnosperm:

Gymnosperms have undergone a few environmental adaptations in order to survive and reproduce more productively. One of these adaptations is that the sperm cells in some Gymnosperms retain the flagellated condition, which traces back to the ancestors of Gymnosperms, yet most of the Gymnosperms now have adapted to not have the flagella in the sperm. The reason that the flagella is not in most Gymnosperm sperm is because it allows reproduction to occur outside of the plant which means that there will be a greater diversity of plants on land.
In addition, Gymnosperms have also adapted to make it easier to live during cold and dry climates. Many of the Gymnosperms have the ability to go through photosynthesis during the winter if given only a little bit of light because they have needle-shaped leaves which reduce water loss in dry conditions because the surface area of these leaves are very small, and do not allow water to leave. Some of the Gymnosperms that have these leaves are evergreens, pines, and firs. The Gymnosperms don’t usually have leaves that drop in the fall, but some do and grow them back early in the spring when the sun comes out more often.
Gymnospsperms that occupy some of the worlds worst conditions (which many tend to do) have developed waxy leaves and and sunken leaf pores to decrease evaporation of water from the leaves. Some conifers have adapted to extremely dry, wildfire-abundant habitats; these plants have evolved so that their cones only open to continue reproduction after exposure to fire.

The Environmental Adaptations for the Gymnosperm:

Review Questions
1) What is the one characteristic that all gymnosperms share? (hint: think seed)
2) What is the most common phylum of gymnosperm and what kind of reproductive structure does it use?
3)What is the most common phylum of gymnosperms? List two characteristics of this phylum.
4) Describe the seven steps of reproduction in gymnosperms. What is it about gymnosperms that separates them from the phyla of plants that need water to reproduce?
5. What characteristics do gymnosperms have in common with angiosperms?
6) In order to adapt to dry and cold climates, gymnosperms have developed what fitness-improving characters?
7) Gymnosperms have adapted to many environmental conditions to improve fitness, one of which includes having a flagella in the sperm. Why is this adaption no longer found in most gymnosperms?
8) What are the small reproductive leaves on a pollen cone called?

Resources:
1) Campbell, Neil A, and Jane B. Reece. Biology. Sixth Edition. San Fransisco: Pearson Education, Inc, 2002.
2) Knee, Michael. "Gymnosperms." Ohio State University. 25 Oct 2009. <http://hcs.osu.edu/hcs300/gymno.htm>.
3)"Introduction to Ginkgoales." UCMP. Web. 24 Oct. 2009. <http://www.ucmp.berkeley.edu/ seedplants/ginkgoales/ginkgo. html>.
4) Kinkead, Gwen. “At Age 4,600-Plus, Methuselah Pine Tree Begets New Offspring.” New York Times. 17 June 2003. 25 Oct. 2009 <http://www.nytimes.com/2003/06/17/science/at-age-4600-plus-methuselah-pine-tree-begets-new-offspring.html>.
5) "Gymnosperms." 26 Oct. 2009. <http://www.geocities.com/heartland/plains/6761/ecycads.htm>
6) "Gymnosperms." 31 Oct. 2009. < http://hcs.osu.edu/hcs300/gymno.htm>.
7) Holley, Dennis. "The Structure and Reproduction of Gymnosperms." Suite101.com. 5 Nov. 2009. <http://botany.suite101.com/article.cfm/the_structure_and_reproduction_of_gymnosperms>.
8) "Gnetophyta." McDaniel College. 5 Nov. 2009 <http://www2.mcdaniel.edu/Biology/botf99/gynopsperms/gnetfolder/gnetophyta.htm>.
9) http://www.life.umd.edu/classroom/BSCI124/lec19.html
Picture: http://www.flowers.vg/flowers/tree-pinecone.htm
Pictures: http://hcs.osu.edu/hcs300/gymno.htm
http://www.crescent.k12.ok.us/staff/jaskew/ISR/botzo/cycad.gif
http://www.biologyreference.com/images/biol_02_img0159.jpg
http://www.nasa.gov/centers/goddard/images/content/156030main_Conifers_JPG.jpg
http://www2.mcdaniel.edu/Biology/botf99/gynopsperms/gnetfolder/gnetophyta.htm