Gymnosperms Kristin Stuck Maggie Quamme
Unique Characteristics • Adapted for water conservation. • Gametophytes reduced in size, often staying within the parent sporophyte plant. • Swimming sperm replaced by dispersal of the whole male gametophyte (pollen) by wind or insects. • Seeds came into being. • The seeds develop on the surface of the reproductive structures (thus also called the “naked-seed plants”) such as the cones in pine.
Reproduction • Sporophyte is underground network of roots supporting a stem, the trunk, branches. • Sporophyte has two types of reproductive structures referred to as male and female cones. • Each cone is considered to be a branch with a number of leaves, called sporophylls. • Each sporophyll has a sporangium where spores are produced. • Spores and gametes come in two sizes: microspores and megaspores are produced in separate sporangia and develop into male and female gametophytes, respectively.
Reproduction – Male cones • Male cones typically found in clusters at the tips of lower, side branches, and take several years to develop. • Leaves are called microsporophylls. • Each microsporophyll has a microsporangium where the microspores are produced. • Inside the microsporangium, each microspore divides and forms a four-celled male gametophyte, also known as pollen, which contains two sperm nuclei. • Pollination is the transfer of the whole male gametophyte to the female plant. Not fertilization.
Reproduction–Female Cones • Female cones form higher up and take years to develop. • Leaves/scales are called megasporophylls and produce megaspores. • Each scale (megasporophyll) has two areas (megasporangia or ovules) where megaspores can develop. • Each ovule has a micropyle, a small hole in the near end of the ovule wall so the sperm can enter.
Reproduction-Both • Pollination: When a pollen grain goes between the “stem” of the female cone and a megasporophyll. • Fertilization: a tube from the pollen (called a pollen tube) grows into the micropyle, then into the female structures. Causes production of megaspore by the female. The megaspore develops into a female gametophyte and produces a few archegonia, each with an egg in it. The pollen forms two sperm nuclei that go up pollen tube toward the archegonia. A sperm nuclei fertilizes one of the eggs in one of the archegonia in each of the ovules. The other sperm nucleus and all other eggs and archegonia in that female gametophyte disintegrate so one zygote per ovule is left.
Reproduction-Cont. • From the zygote, cells divide and grow into an embryo sporophyte (2n). This forms in the female gametophyte (1n). Typically, parts of the old megasporophyll from the parent pinecone (2n) form a protective seed coat and the “wing” for dispersal. • When the seed is mature or ripe, it separates from the female cone and is carried by the wind to a new location. When the seed germinates, the embryo continues to grow into a new pine tree. Initially, it uses the nutrients provided by the female gametophyte until its first leaves are above ground and large enough to do photosynthesis.
Cycads Cycads have leaves that look like palms, but the plant gets a big cone in the center for reproduction.
Ginkgo Ginkgoes are dioecious (di = two; oecious, eco = house), they come in male and female plants.
Ginkgo A valuable street tree because of its unusual foliage and tolerance of pollution.
Conifers Examples: pines, firs, spruces, yews, junipers, cedars, cypress, and redwoods. Many are evergreen. The leaves are needle-shaped to conserve moisture. Some kinds can be very old and/or large: in California, a number of redwoods are over 100 m tall, there is a sequoia with a trunk diameter of 26 m, and there is a bristlecone pine which is over 4600 years old.