Lab 1 3
Seed Plants - Gymnosperms
During the peak of dominance of dinosaurs, the forests and swamps were not scented with the sweet aroma of wildflower blossoms. Instead, the air was likely filled with the copious pollen from the (now extinct) seed ferns and cycads, and perhaps the odor of resins from other cone-bearing species. This landscape was dominated by the gymnosperms which represent a series of evolutionary lines of seed-bearing plants. There are four phyla of gymnosperms with living representatives. Of these, by far the most numerous in terms of species diversity and abundance are the conifers. The gymnosperms today include the tallest and oldest of all trees.
Cycadophyta: The Cycads
ycads are so abundant in the fossil record of the Mesozoic (65 to 230 million years ago) that this era is sometimes referred to as the "Age of Cycads and Dinosaurs." Cycads are widely distributed in tropical and subtropical regions of the world. When cycads thrived on earth, the continents were united into an enormous super continent named Pangaea (approximately 200 million years ago). As the large plates of earth slowly moved, cycads were widely distributed across the globe. Despite their former dominance of the global flora, today many relict cycad populations are seriously threatened with extinction due to extensive collecting and degraded habitats.
Cycad cones can reach 3 feet in length, the largest of all living cone-bearing plants. The most massive seed cones may weigh up to 95 pounds. The cones of cycads are composed of numerous overlapping scales called sporophylls. Sporophylls are actually modified leaves, and in male cones they bear numerous sporangia on their lower surface.
Two large seeds are produced at the base of each sporophyll in female cones of most species. In some species of cycads the cluster of seed-bearing "leaves" superficially resembles a large blossom. In fact, some botanists have suggested that ancient cycad like plants may have given rise to flowering plants. Other botanists believe the connecting link may be extinct cone-bearing species in the Division Gnetophyta, while others say these are merely examples of parallel evolution.
Examine the prepared slide of the cycad Zamia archegonia with a dissecting microscope. Draw what you see. Label the megagametophyte, the archegonium, and the integuments. Can you see the micropyle? Should you expect to see it on every slide? Why or why not?
Cycads have a slow-growing trunk, 3 to 50 feet tall, and a crown of leaves that superficially resembles a palm. Unlike palms, cycad trunks produce rings of xylem surrounding a wide, pithy central core.
Look at the prepared slides of a Zamia stem cross section and draw what you see. Label the xylem cells emanating from the large central pith.
Cycads are unique among gymnosperms because for associations with nitrogen-fixing cyanobacteria living exterior of root outgrowths at the base of the plant. Like bacteria in the root nodules of legumes, the cyanobacteria supply the cycad with inorganic nitrogen by converting inert atmospheric nitrogen into ammonia. Since the photosynthetic cyanobacteria are not exposed to vital sunlight, the cycad supplies its microscopic symbiotic partner with organic nutrients.
Examine the roots on a live specimen and look for a pale greenish layer of filamentous cyanobacteria (usually Nostoc or Anabaena).
Some cycads have striking similarities with ferns, including large leaves that uncoil like a fern. Cycads also produce multiciliate swimming sperm. The later characteristic is especially interesting when you consider that each sperm may have 20,000 to 40,000 spirally arranged cilia and may be up to 400 micrometers (1/60 of an inch) long, visible to the naked eye. This is larger than a grain of table salt and roughly equivalent to an entire plant of Wolffia globosa, the world's smallest flowering plant. Because of multiciliate sperm, cycads illustrate a transitional form between the more primitive spore-producing ferns, which must depend on free environmental water for the transport of their sperm, and the advanced seed plants in which the sperm are non-flagellated and non-motile.
Ginkgophyta: Maidenhair tree
Leaf imprints resembling the present-day maidenhair tree have been found abundantly in sedimentary rocks of the Jurassic and Triassic Periods (135-210 million years ago). The branching pattern of the veins of Ginkgo leaves is primitive. Many extinct members of Gingophyta only known from the fossil record show the same pattern.
What is this pattern called? If you are unsure, look at the leaf veins under the dissecting microscope on the leaves in the acrylic mount.
The leaves of Ginkgo biloba have long been recognized for their benefit as a geriatric drug in China. A tea made from the leaves has been used for a variety of ailments, including asthma, vascular (circulatory) diseases and urinary problems. Ginkgo biloba leaf extract may also be an effective treatment for memory loss and the early stages of Alzheimer's disease in elderly patients. During the past few years Ginkgo has become a popular herbal remedy sold in natural food stores across the United States.
Look at the prepared slide of the Ginkgo male strobilus. Draw what you see.
Does it does differ in any dramatic way from the strobili we saw in some seedless vascular plants?
Once the spores are released, the life cycle is quite different. How?
Look at the prepared slide of Ginkgo pollen. Draw what you see. Can you find the four cells of the male gametophyte generation? Label them.
Female Ginkgo trees are typically not used for landscaping because the seeds are foul-smelling when they fall to the ground. The stench of the seed coats comes from butyric acid, which is also found in rancid butter. Butyric acid (also known as butanoic acid) is a common by-product of many plants and animals, and is responsible for the distinctive odor of Limburger cheese. It is also produced by anaerobic bacteria in the digestive tract of animals, and may be present in vomit. Nevertheless, the large, nutritious seeds of Ginkgo biloba are a popular food in China and Japan.
Look at the longitudinal section of a Ginkgo archegonium. Draw what you see. Label the integument, microspore, and micropyle.
Now look at the prepared slide of the ovule with embryo. Draw what you see. Label the nutritive tissues that feed the embryo. Which generation do these tissues belong to?
Coniferophyta: The Conifers
What the series of life cycle videos on your lap top. Follow along on the gymnosperm life cycle handout from lecture.
Male Reproductive Structures
Get an acrylic mount of male and female conifer cones to examine under the dissecting microscope.
Examine one of the male cones under a dissecting microscope. Note that the microsporophylls are arranged helically along the axis of the cone. Each microsporophyll bears two microsporangia or pollen sacs in which microspore mother cells are formed. Each microspore mother cell eventually undergoes meiosis to give rise to four microspores, which develop into the pollen grains, or immature male gametophytes (microgametophytes).
Look at a prepared slide of a longitudinal section of a Pinus male strobilus. Draw what you see. Label a pollen grain.
Look at the prepared slide of Pinus pollen grain. Identify the cells in the pollen grain diagramed below.
Female Reproductive Structures
Now examine an ovulate cone in acrylic. There are two kinds of female cones in the mount.
Think about the lifecycle of Coniferophyta and provide an explanation for why there are one-year old and two-year old cones on a single tree.
Can you find one-year old and two-year old cones in the fresh samples on the center bench? What are female cones doing outside this time of year?
Take a scale off of a female cone. Each scale is subtended by a sterile bract which is morphologically equivalent to a leaf. The combination of ovuliferous scale and subtending sterile bract is referred to as the seed-scale complex. Dig into the cones and find a seed. At maturity, two seeds are associated with each scale.
Study prepared slides containing longitudinal sections of ovulate cones (longitudinal section Pinus female cone). Find a scale that shows a median or near median section of one of its ovules. Draw what you see and label the ovuliferous scale, sterile bract, integument, the micropyle, and the megasporangium (nucellus).
If you examine the ovules about 15 months after pollination, you will find that they have developed a great deal. After 15 months, the ovules contain a female gametophyte (megagametophyte), which develops from the functional megaspore and which contains two or more archegonia near the “pollen chamber.” Soon after pollination, the pollen grain germinates and the pollen tube begins to form. However, the sperm do not appear until the female gametophyte is mature. Each male gametophyte gives rise to two sperm, one of which may eventually fertilize an egg. The other sperm has no function and disintegrates.
What is happening in the megasporangium in the series of illustrations below?
The fertilized egg, or zygote, develops into an embryo, the young sporophyte. The embryo is surrounded by tissue of the female gametophyte, which serves during germination as source of food for the seed. During embryo development, the integuments develop into the seed coat. A seed, consisting of embryo, seed coat, and stored food, has now been formed.
Get a pine nut from the center bench and gently pry it apart. You should be able to see the mature embryo with cotyledons inside. Draw what you see.
Observe the prepared slide of the Pinus embryo. Draw what you see. Label the cotyledon, root cap, and apical meristem of root and shoot. What is the function of the cotyledons?
Look at the potted pine seedling on the center bench. The browned leaves are the cotyledons and the new leaves are spiraled around the stem. After some growth, some species present their leaves in bundles called “fascicles.” Find a fascicle on the mature branch on the center bench.
Observe the prepared slide of a cross-section of a pine needle. What structures can you identify that confer drought tolerance to these leaves?
Gnetophyta – The Gnetophytes
bserve the two living examples from Gnetophyta on display:
Gentum: tree in the front of the lab
Ephedra: two smaller plants on the center bench.
Welwitschia: small plant with two leaves
Ephedra is a genus of drought-resistant shrubs with jointed stems and whorls of minute scale-like leaves in 2's or 3's at each node. One very unusual characteristic of the Gnetophyta is that many members possess vessels in their xylem tissue, an angiosperm trait that is not found in most gymnosperms.
Look at the prepared slides of an Ephedra root cells. Why do xylem and phloem stain different colors?
Look at the demo slide that shows a close-up xylem cells in Ephedra. There are two kinds of cells, trachieds and vessel elements. Aside from this phylum, vessel elements are only found in angiosperms. Sketch out the difference between a trachieds and vessel element.
Why do you think vessel elements are considered more “advanced?”
Some species of Ephedra have double fertilization, where two sperm are involved in the fertilization process. Double fertilization was once thought to be a strictly angiosperm characteristic. Some older references have suggested that the Gnetophyta may represent a "missing link" in the evolution of flowering plants, but others say that this is an example of convergent or parallel evolution.
The Chinese species Ephedra sinica was the original source of the alkaloid ephedrine, a common decongestant in popular allergy and hay fever remedies. Since ephedrine has a chemical structure similar to epinephrine (adrenalin), it works like a powerful cardiac stimulant that may cause cardiac arrest in infants and heart patients. New synthetic drugs based on the ephedrine/epinephrine ring structure are now marketed as effective and safer bronchodilators. Pseudoephedrine, an isomer of ephedrine, also occurs in species of Ephedra, and may be produced synthetically. Compared to ephedrine, it causes fewer heart symptoms such as palpitation, but is equally effective as a bronchodilator. It is used in over-the-counter drugs such as Sudafed©. E. sinica and other species are also marketed under the name of "ma-huang," a popular herbal stimulant and decongestant.