Finally, wind plays an important role in pollination in gymnosperms because pollen is blown by the wind to land on the female cones. Although many angiosperms are also wind-pollinated, animal pollination is more common. Learning Objectives Describe the process of sexual reproduction in gymnosperms. Key Points In gymnosperms, a leafy green sporophyte generates cones containing male and female gametophytes; female cones are bigger than male cones and are located higher up in the tree.
A male cone contains microsporophylls where male gametophytes pollen are produced and are later carried by wind to female gametophytes. The megaspore mother cell in the female cone divides by meiosis to produce four haploid megaspores; one of the megaspores divides to form the female gametophyte.
The male gametophyte lands on the female cone, forming a pollen tube through which the generative cell travels to meet the female gametophyte. One of the two sperm cells released by the generative cell fuses with the egg, forming a diploid zygote that divides to form the embryo. Unlike angiosperms, ovaries are absent in gymnosperms, double fertilization does not take place, male and female gametophytes are present on cones rather than flowers, and wind not animals drives pollination.
Key Terms megasporophyll : bears megasporangium, which produces megaspores that divide into the female gametophyte microsporophyll : a leaflike organ that bears microsporangium, which produces microspores that divide into the male gametophyte pollen. Sexual Reproduction in Gymnosperms As with angiosperms, the life cycle of gymnosperms is also characterized by alternation of generations.
They are adapted to live where fresh water is scarce during part of the year, or in the nitrogen-poor soil of a bog. Therefore, they are still the prominent phylum in the coniferous biome or taiga , where the evergreen conifers have a selective advantage in cold and dry weather. Evergreen conifers continue low levels of photosynthesis during the cold months, and are ready to take advantage of the first sunny days of spring. One disadvantage is that conifers are more susceptible than deciduous trees to leaf infestations because most conifers do not lose their leaves all at once.
They cannot, therefore, shed parasites and restart with a fresh supply of leaves in spring. The life cycle of a gymnosperm involves alternation of generations, with a dominant sporophyte in which reduced male and female gametophytes reside. All gymnosperms are heterosporous.
The male and female reproductive organs can form in cones or strobili. The life cycle of a conifer will serve as our example of reproduction in gymnosperms. Therefore, they are monoecious plants.
Like all gymnosperms, pines are heterosporous and generate two different types of spores male microspores and female megaspores. Male and female spores develop in different strobili, with small male cones and larger female cones.
Each pollen grain consists of just a few haploid cells enclosed in a tough wall reinforced with sporopollenin. In the spring, large amounts of yellow pollen are released and carried by the wind. Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube growth. The pollen tube develops slowly, and the generative cell in the pollen grain produces two haploid sperm or generative nuclei by mitosis. At fertilization, one of the haploid sperm nuclei will unite with the haploid nucleus of an egg cell.
Female cones, or ovulate cones , contain two ovules per scale. Each ovule has a narrow passage that opens near the base of the sporophyll. This passage is the micropyle, through which a pollen tube will later grow. One megaspore mother cell, or megasporocyte , undergoes meiosis in each ovule. Three of the four cells break down; only a single surviving cell will develop into a female multicellular gametophyte, which encloses archegonia an archegonium is a reproductive organ that contains a single large egg.
As the female gametophyte begins to develop, a sticky pollination drop traps windblown pollen grains near the opening of the micropyle. A pollen tube is formed and grows toward the developing gametophyte. One of the generative or sperm nuclei from the pollen tube will enter the egg and fuse with the egg nucleus as the egg matures. Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant.
Although several eggs may be formed and even fertilized, there is usually a single surviving embryo in each ovule. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte tissue that will provide nutrients, and the embryo itself.
Figure illustrates the life cycle of a conifer. Upon maturity, the male gametophyte pollen is released from the male cones and is carried by the wind to land on the female cone. Figure 2. These series of micrographs shows a male gymnosperm gametophyte. Wise; scale-bar data from Matt Russell. Watch this video to see a cedar releasing its pollen in the wind. The female cone also has a central axis on which bracts known as megasporophylls Figure 3 are present.
In the female cone, megaspore mother cells are present in the megasporangium. The megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure. The female gametophyte is contained within a structure called the archegonium. In gymnosperms the cone is the female reproductive part and the pollen is the male reproductive part. Pine trees and other gymnosperms produce two types of cones.
The male cone is called the pollen cone. The larger female cone is the seed cone. A single tree usually produces both pollen and seed cones. Spore-producing structures are found on the scales of cones. Give students the bags of materials. You should have the students keep the material inside the bag and use a hand lens or microscope to look at the materia You may ask student so bring in a flo w er, seed, or pine cone.
This would be excellent to dissect in the lab. You have the following materials in your kit some materials may change :.
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