BIO 5 GENERAL BIOLOGY

GYMNOSPERMS

1. The term "gymnosperm" refers to a group of plants that produce "naked" seeds and pollen. The term "naked" seed refers to a seed that does not develop and mature within a protective layer called the fruit. There are several different types of gymnosperms, and these are placed into four divisions within the Kingdom Plantae: Division Coniferophyta (conifers), Division Cycadophyta (cycads), Division Ginkophyta (Ginkgo biloba) and Division Gnetophyta (misc. gymnsperms).

2. Members of the Division Coniferophyta include plants that are familiar to most people. These plants have leaves that are needle-like or scale-like and bear reproductive structures called cones (or strobili). These plants are commonly called conifers (cone-bearers). Included within the Division Coniferophyta are junipers (Juniper_detail_MC.jpg) (The blue "berries" shown in the image are seed cones.), pine (Malebough_lg_MC.jpg) (The cluster of reddish cones are pollen cones), Sequoia (coastal redwood) (Whole_Tree_MC.jpg ) ( Tree_Muir_Woods_MC.jpg) (The California State Tree; difficult to photograph a whole tree), and Sequoiadendron (Big Tree) ( Sentinel_Tree_MC.jpg) (Fire-scarred_trunk_MC.jpg )(redwood found in the Sierra; plants get enormous in size).

3. Members of the Division Cycadophyta (cycads) live in tropical and sub-tropical areas of the world. Some species live in tropical forests and some live in warm deserts. Only one species of cycad is native to the United States (Zamia floridana 130 .jpg ), and this plant grows only in Florida. Like all cycads, the image shows a plant with pinnately compound leaves. Unlike fern leaves, however, cycad leaves are leathery in texture and frequently armed with spines. One species of cycad is planted as a horticultural plant in California. This species is called the Sago palm (Cycas_revoluta_130.jpg ). Cycad individuals produce either seed cones or pollen cones and are commonly referred to as female or male plants, respectively. The image shows a male plant with a pollen cone ( Male_strobilus_MC.jpg). Seed cones (LS_female_cone_RK.jpg ) contain large seeds. The image shows a sporophyll from a seed cone bearing two large seeds ( Cycas_revoluta_ovules.jpg).

4. The Division Ginkgophyta includes only one living species, Ginkgo biloba. Ginkgo plants are trees that grow to quite large size (Tree_lgMC.jpg). Note that the leaves of the plant turn golden in fall. This is one reason that male Ginkgo plants are planted as a horticultural plant in the United States. The leaves are simple and fan-shaped ( Ginkgo_habit_MC.jpg). The leaves are used to make the herbal medicine that is though to increase memory. Female trees produce abundant naked seeds ( Ovules_MC.jpg). Female trees are not prized horticultural trees, however, precisely because of their abundant seed production. The seeds have a fleshy seed coat that smells awful.

5. The Division Gnetophyta includes three genera: Gnetum, Ephedra, and Welwitschia. Gnetum is a tropical shrub or small tree. It grows in moist, tropical forests, is broad-leafed, and has net venation ( Gnetum_montanum.jpg) (Gnetum_sp_leaves_KS.jpg ). Note the white structures on the second image. These are seed cones (megastrobili). Ephedra is a shrub that lives in desert environments. It is, for example, native to the dry regions of California on the east side of the Sierra. Ephedra resembles Equisetum ( Stem_nodes_MC.jpg). Both plants have whorls of non-photosynthetic leaves and photosynthetic stems. Welwitschia is probably the strangest plant found on earth ( Habit_MC.jpg). It is native only to the Kalahari desert of Namibia on the southwest coast of Africa. This region receives less than one inch of rain per year. During its entire life, Welwitschia will have only two leaves that grow continuously longer, reaching lengths of 30-40 feet. The stem is a flat disk that is situated close to the ground, and the plant has an extensive tap root system. Welwitschia plants are either male and produce only microstrobili ( Male_cones_FL.jpg) or female and produce megastrobili (Female_cones_FL.jpg ).

6. Among gymnosperms, probably the most familiar plant is pine, a member of the Division Coniferophyta. Pine trees ( white_pine_MC.jpg) are the sporophyte generation (2N adult) of plant. Below ground is an extensive taproot system. Above ground is a stem with strong apical dominance and leaves that remain on the plant year around. Pine is an example of an evergreen plant.

7. A single individual plant produces two types of reproductive structures early in the spring. Microstrobili or pollen cones ( Malebough_lg_MC.jpg) are generally produced first. These are produced in groups or clusters and appear reddish in color in the image. If a single pollen cone (one of the structures in the group) is viewed with a dissecting microscope ( Male_cone_MC.jpg), the subunits, called microsporophylls, are visible. If we take a dissecting needle and open up the pollen cone or microstrobilus ( Microsporophylls_MC.jpg), the yellow pollen grains are visible as the small, round structures that dust the microsporophylls. If we examine the microstrobilus or pollen cone in longitudinal section using a compound microscope (LS_of_male_cone_MC.jpg ), the entire pollen cone is visible. Note that a microsporangium is visible on the lower surface of each microsporophyll (pollen cone subunit). Meiosis occurs within the microsporangium to form 1N microspores (visible in the microsporangia). Each microspore undergoes mitosis, and a male gametophyte is formed within the microspore wall. This structure is called pollen (pine_pollen_MC.jpg ). Note that pine pollen has two air bladders that allow it to be easily carried by the wind. Also note that four cells are visible within the microspore wall. The largest of these cells, top cell, will form the pollen tube. The second cell from the top will form two sperm cells. When the pollen is mature, the microsporangium breaks open, the microsporophylls separate, and pollen is carried away from the pollen cone and the parent plant by the wind.

8. The pollen is carried to the immature seed cone or megastrobilus ( Young_female_cones_MC.jpg). Note the very small red-colored structures at the very tip of the branch. These are the immature seed cones. One immature megastrobilus is shown in the image ( P_resinosa_Female_cone_MC.jpg). This structure eventually will mature into the structure that we commonly refer to as a pine cone. In the Third image, an immature megastrobilus is shown as it appear under a dissecting microscope (Female_cone_MC.jpg ). The subunits of a pine megastrobilus are called cone scales. Note that the cone appears shiny. This results from the pollination droplets produced by the many unfertilized seeds or ovules of the megastrobilus. The purpose of the pollination droplet is to capture pollen from the air and draw it into the unfertilized seeds.

9. In the longitudinal section of an immature megastrobilus (LS_ovulate_cone_MC.jpg ), the entire megastrobilus is visible. The cone scales bear ovules (unfertilized seeds) on their top surface. The ovules are the swollen areas visible on the right side (top surface) of some cone scales. One of the ovules is shown at high magnification in the image ( Megaspore_mother_cell_130.jpg). Note the pollen that has been drawn into the ovule by the pollination droplet. The outer layer of the ovule is called the integument. The function of this structure is to protect the megasporangium. One the right side of the ovule there is a "C"-shaped clear area. The separates the integument from the megasporangium. Inside the megasporangium, there is a large, green-staining cell. This cell will undergo meiosis and four spores will form. Three cells do not develop. One survives and has a multicellular female gametophyte develop within it.

10. The female gametophyte develops several archegonia, each of which contains one egg cell ( Archegonia_MC.jpg). The image is taken at high magnification. The integument and most of the megasporangium is not visible. At the very bottom of the image, are the cells of the megasporangium. The clear space separates the megasporangium from the megaspore wall. The cells inside of the megaspore wall are part of the female gametophyte (1N). Two archegonia are visible and, inside each archegonium, is one egg. The dark-staining cytoplasm of the egg is visible. The pollen tube grows to the archegonium and sperm are released into the archegonium. The sperm then fuses with the egg and a zygote (2N) is formed. The image (Megagametophyte_130.jpg) shows an ovule at lower magnification than the previous image. In the lower magnification view, the outer layer, the integument and the megasporangium are clearly visible.

11. The zygote undergoes mitosis and forms an embryo. In the image of a dissected pine seed (Dissected_pine_seed_MC.JPG ), The brown-colored remains of the megasporangium is on the left. The tan-colored material is the female gametophte tissue, and the embryo is light yellow in color. The root portion of the embryo points toward the left. The seed leaves are toward the right. If a pine seed is planted and allowed to germinate, a seedling plant emerges from the seed coat and establishes itself ( seedlings_MC.jpg). The seed coats are dark brown, the seed leaves are relatively long, and the first leaves produced by the stem apical meristem are shorter and blue-green. At this stage, the root has begun to absorb water.

12. The image ( Series_red_pine_MC.jpg) shows stages in the development of the pine megastrobilus. A six month old pine cone is visible on the right. The mature megastrobilus is on the left. The other cones show stages of development. Note that the production of mature seed cones or megastrobili takes over two years. Typically, pine trees take their time in reproduction. First, the tree must be several years old before it is capable of producing strobili. Second, it takes several months for the pollen tube to grow to the archegonium after pollination. Finally, it also takes a considerable period of time for the seed to develop after fertilization.