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| 1. What are the main divisions and representing species of the gymnosperms? This group of plants can be divided into conifers (pine, sequoia, cypress), that have flowers known as strobiles (cones), cycads (very ancient gymnosperms, like the cycads used in garden architecture, that also form strobiles), gnetaceae (gnetum) and ginkgos (the known species is Ginkgo biloba). Image Diversity: conifers cycads gnetaceae ginkgos 2. How different are gymnosperms from bryophytes and pteridophytes? Gymnosperms are not cryptogamic as bryophytes and pteridophytes are. They are phanerogamic and so they form flowers and seeds. 3. What is the evolutionary importance of the emergence of seeds in the plant kingdom? The evolutionary importance of the seed is related to the plant capability of distant colonization and to the protection of the embryo. Embryo-containing seeds can be carried by water, wind and animals and germinate in different environments. This fact contributes to the exploration of a variety of ecological niches and for the diversity of plant species. Seeds in addition protect the plant embryo against external aggressions and they also provide germination under more adequate conditions (inside the seed). These features contribute to the evolutionary success of the phanerogam. 4. In which areas of the globe is gymnosperm abundance noteworthy? These plants are the typical vegetation of cold regions like the taiga, or boreal forest, of the northern hemisphere, or the araucaria forests of the southern hemisphere. Image Diversity: boreal forest araucaria forest 5. What is the life cycle of the gymnosperms? As all plants they present a diplobiontic life cycle, i.e., alternation of generations with diploid and haploid stages. The lasting (final) stage is the diploid one. Image Diversity: conifers life cycle 6. What is pollen? Pollen grains are the male gametophytes of the phanerogamic (flowering) plants. Therefore within the pollen grains the male gametes of these plants are formed by mitosis. Image Diversity: pollen grain 7. How are the male gametes of gymnosperms formed? What is the relationship between the pollen grains and the concept of alternation of generations? In the male strobiles (cones) there are microsporophylls (fertile leaves of the strobile) that contain microsporangia. Within the microsporagium spore mother cells undergo meiosis and generate haploid microspores. The microspore wall develops into winglike projections (to facilitate its aerial propagation) and mitosis occurs producing two cells, the generative cell and the tube cell. The final structure containing these elements is the pollen grain. The pollen grain relates to the alternation of generations since it is the male gametophyte (the haploid stage of the life cycle). 8. How are the female gametes of gymnosperms formed? What is the relationship between this process and the concept of alternation of generations? In the female strobiles (cones) there are megasporophylls (fertile leaves of the strobile) that contain megasporangia. In the megasporangium the spore mother cell undergoes meiosis generating four haploid cells of which three regress and one gives birth to the functional megaspore. The functional megaspore by several mitosis forms the female gametophyte that contains the oospheres (female gametes) of the plant. The female gametophyte is located within the megasporangium that has a small opening, the micropyle, through which the pollen tube enters. The process is related to the diplobiontic life cycle (alternation of generations) since the functional megaspore generates the haploid stage of the plant (the female gametophyte). 9. How do fecundation and zygote formation occur in these plants? Do these processes depend on water? The microsporangia in the male strobile rupture at the right period of the year releasing thousands of pollen grains. Since their pollen grains are “winged” they can be transported by the wind over distances. When the pollen grains fall into the female strobiles they pass the micropyle and enter the pollen chamber. This process is called pollination. Within the pollen chamber the generative cell nucleus divides forming two gametic nuclei and the tube cell elongates forming the pollen tube. The pollen tube penetrates the female gametophyte and the gametic nuclei (also known as sperm nuclei) pass through the tube; one of them unites with an oosphere (the female gamete) and forms the zygote (2n). Generally fecundation occurs one year after pollination and during this time interval the maturation of the male and of the female gametes takes place. The fecundation in these plants is independent from water. The gametophytes however are entirely dependent on the sporophyte (the adult plant) for nutrition and hydration. 10. How are the gymnosperm seeds formed? What are the ploidies of the structures that compose the seeds? Their seeds are formed from differentiation of the megasporangia in the female strobiles (cones). After an oosphere is fecundated the female gametophyte originates the haploid (n) primary endosperm (nutritive tissue that covers the embryo) and the covering of the megasporangium turns into the diploid (2n) seed shell. |
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GYMNOSPERMS When you are finished with this page, try the Gymnosperm Quiz. Gymnosperms are a type of plant that reproduce by a seed that is not enclosed as with angiosperms who have seeds contained flowers. Many of these plants are conifers or cone bearing plants. Gymnosperms are found throughout the world. They have roots and stems. Their leaves are in the form of needles in conifers. At the time of pollination, the pollen or male pollen grains fall among the cone scales until they fall on to the ovules. As the cone grows up, it gets bigger and the seeds that are created by the male pollen and female parts are seen. Most conifers are evergreens or always green and they do not shed their leaves as deciduous plants do but retain their needles for quite a while. When the needles fall off, they are quickly replaced. Other groups within the gymnosperms include the gingkos and cycads. Gymnosperms Quiz Click on the radio button in front of the correct answers. 1. These are a type of plant that reproduce by seeds not contained in a flower angiosperms gymnosperms ferns Correct: gymnosperms 2. Cone bearing plants are called fungi conifers angiosperms Correct:conifers 3. The leaves of conifers are in the form of needles fronds mycelium Correct:needles 4. Conifers shed their needles frequently. True False Correct: True 5. Other plants in the gymnosperm group include gingkos and cycads maples and elms tulips and roses Correct:gingkos and cycads |