The pollen grains of other seed plants grow similar tubes. The megasporangia, which contains the eggs, form tiny female strobili on the tips of special branches on the female tree. The microsporangia, which produce the pollen grains, are in male strobili that hang down like little pine cones on the male tree.
The seed that forms on the female trees is covered with a thick fleshy coat which makes the seed look like a little fruit which it is technically not. So be very careful if you plant one of these wonderful trees and select a male tree!!
Although in fairness to the female tree, its seed is prized in China as a source of medicinal drugs. This odd little group of gymnosperms are mainly xerophytes, plants that are adapted to dry conditions. They share a close common ancestor with flowering plants. Each genera has some species that produce nectar, and attract insects.
It was recently discovered that double fertilization, a trait we thought was unique to flowering plants, also occurs in Ephedra , one of the three surviving genera of gnetophytes. Ephedra , incidentally is the natural source of the alkaloid ephedrin, used to treat hay fever, sinus headaches, and asthma. Its medicinal properties have been known for at least 5, years!
Most gnetophytes are stem plants, like Ephedra, branched photosynthetic stems with no leaves. Gnetum has leaves like those of modern flowers. But the third genus, Welwitschia , is one of the strangest plants on earth. Welwitschia really looks like something out a science fiction novel. It grows in the deserts of southwestern Africa. Most of the plant is deep underground, with a root stretching down to the water table.
The top appears above the soil as a squat cup- shaped stem with two strap-shaped leaves. These are the only leaves the plant will ever grow, and they may live a hundred years or more and reach several meters, usually torn into strips.
Male or female strobili grow from the margins of the upper stem. Division Coniferophyta - sp. The conifers are the largest and most successful group of living gymnosperms.
Many of our familiar forest trees are conifers, including pines, spruces, firs, hemlocks, yews, redwoods and cypress trees. They are an ancient group, dating back mya. They evolved during the Permian, toward the end of the Paleozoic, at a time when the climate was very cool and dry. Their special water conducting cells, called tracheids, allowed them to thrive in these climates and these same adaptations let them continue to dominate in colder and dryer environments today, such as northern latitudes, mountain slopes, and sandy soils.
Because they are superior competitors in such habitats even today, they are the only Division of gymnosperms to successfully compete with the flowering plants. Most conifers are evergreens, with the larch and the bald cypress being notable exceptions. Their needle-shaped leaves are also an adaptation to conserve water. Needles usually occur in small bundles, each bundle emerging from a base that is actually a greatly truncated branch. Conifers have tremendous economic importance, as a source of timber and for byproducts such as pitch, tar, turpentine, and amber and other resins.
Millions are sold each year as Christmas trees. All conifers produce cone shaped strobili, both male cones often called pollen cones and female cones often called seed cones or ovulate cones. Both male and female cones are usually produced on the same tree, but not at the same time, so the trees do not fertilize themselves. Female cones are large and conspicuous, with thick woody scales. Seed cones can persist on the tree for several years after fertilization. A few species, like junipers and the locally common podocarpus front of Richardson , have seeds that are covered with a fleshy coating, and resemble small berries.
The sporangia produced by the sporophytes are located at the bases of the sporophylls, and collected in the strobilus we call a pine cone. The microspore mother cell in the microsporangia produces the haploid pollen grains. Each scale or sporophyll in the male cone has two microsporangia on its lower surface.
Each pollen grain consists of only four cells. When the immature pollen grain finally reaches the seed cone, the megaspore mother cell in the megasporangium produces four haploid megaspores.
Three of these megaspores degenerate, and only the fourth germinates into the female gametophyte. The female gametophyte consists of two or more archegonia, with a single egg in each one.
All eggs are usually fertilized. Each visible scale in the seed cone is really a much reduced lateral branch in itself. So each scale is homologous with the entire male cone.
The megasporangium, which is called a nucellus in seed plants, is covered with a layer of protective cells called an integument, which is open at one end. This tiny opening, the micropyle, marks the point where the male pollen tube will grow into the megasporangium. The megasporangium, together with its integument, makes up the ovule. Seeds develop from ovules. Each scale in the seed cone has two ovules on the upper surface of the scale, and so will ultimately bear two seeds side by side.
The pollen grains formed in the microsporangia of pines have tiny wing on either side. Because they are wind-pollinated? The ovulate cones open to receive pollen, then may close again to protect the developing embryos. When pollen grains land on the ovulate cones, they grow a long pollen tube. By the time this tube reaches the archegonia, about 15 months after pollination, the male gametophyte is fully mature.
The pollen tube enters through the micropyle. The sperm nucleus divides in two, and the pollen tube discharges two sperm. One sperm nucleus degenerates, the other fertilizes the egg. It takes the female gametophyte about 15 months to mature, and about the same time for the pollen tube of the male gametophyte to reach it. The seed develops within the megasporangium.
The seed is the structure containing the embryonic plant and the stored nutrition to support it. A section of the surface of the scale usually detaches along with the seed, giving the seed a little wing to help disperse it farther from the tree. Conifer seeds are very complex little structures, containing cells from three generations of the tree. The nutritive tissues inside the seed are actually the haploid body cells of the female gametophyte.
The seed also contains the developing diploid sporophyte, the little embryonic conifer. The outer wrapping of the seed, the tough and protective seed coat, is formed from the diploid cells of the parent sporophyte. Pine seeds, along with acorns, are the most important source of plant food for North American wildlife. Examine the cycads and cycad frond on display. How do the leaves of cycads differ from those of angiosperms?
The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization. The ovule wall will become part of the fruit. As shown in this diagram of the embryo sac in angiosperms, the ovule is covered by integuments dark green and has an opening called a micropyle.
Inside the embryo sac are three antipodal cells, two synergids, a central cell, and the egg cell. Image credit: OpenStax Biology. The text below was adapted from Openstax Biology The phenomenon of double fertilization , or two fertilization events, is unique to angiosperms and does not occur in any other type of plant or other organism.
As described above, after pollen is deposited on the stigma, it germinates and grows through the style to reach the ovule. The pollen tube cell grows to form the pollen tube, guided to the micropyle by chemical signals from the synergid cells. The generative cell travels through the tube to the egg and divides mitotically to form two sperm cells. One sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm , which serves as a source of nutrition for the developing embryo.
Together, these two fertilization events in angiosperms are known as double fertilization , illustrated below. After fertilization is complete, no other sperm can enter. The fertilized ovule forms the seed , and the ovary become the fruit , usually surrounding the seed. In angiosperms, one sperm fertilizes the egg to form the 2n zygote, and the other sperm fertilizes the central cell to form the triploid 3n endosperm. This is called a double fertilization. After fertilization, the zygote enters a temporary period of development shown below.
It first divides to form two cells: the upper cell, or apical cell , and the lower cell, or basal cell. The division of the basal cell gives rise to the suspensor , which eventually makes connection with the maternal tissue. The suspensor does not become part of the future plant, but instead provides a route for nutrition to be transported from the mother plant to the growing embryo. The apical cell also divides, giving rise to the proembryo the actual embryo that will develop into a plant.
The endosperm accumulates starches, lipids, and proteins, and then nourishes the developing cotyledons embryonic leaves. The cotyledons will serve as an energy store for later embryo development. Once development is reactivated, the developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis. After fertilization, the zygote divides to form an upper terminal cell and a lower basal cell. The basal cell also divides, giving rise to the suspensor.
Plant Systematics: A PhylogeneticApproach. Sinauer Associates Inc. ISBN Other gymnosperms are processed into other products like soap, varnish, and perfumes. Share this comparison:. If you read this far, you should follow us:. Diffen LLC, n. Angiosperms vs. Comparison chart Angiosperms versus Gymnosperms comparison chart Angiosperms Gymnosperms Definition Seed-producing flowering plants whose seeds are enclosed within an ovary. Bare, not enclosed; found on scales, leaves or as cones.
Mostly rely on wind. Uses Medications , food, clothing, etc Paper, Lumber , etc Diversity Hundreds of millions of years ago, gymnosperms were the only kind of plant life on Earth. Examples of Angiosperms and Gymnosperms Examples of angiosperms are monocots like lilies, orchids, agaves known for agave nectar and grasses; and dicots like roses, peas, sunflowers, oaks and maples. Gymnosperm examples include non-flowering evergreen trees such as pine, spruce and fir.
Students compare the reproductive organs of different plants. Gymnosperms are woody plants that bear "naked seeds. A pollen grain is carried by wind currents to the appropriate "egg" where the growth of the pollen tubes through this tissue brings the sperm to the egg.
Gymnosperms are usually of large size with much secondary growth, the leaves are usually evergreen needles or scales. Angiosperms have flowers and bear seeds enclosed in a protective covering called a fruit. Angiosperms are the dominant types of plants today. Angiosperms are further divided into monocots and dicots. Monocots have one seed leaf.
Dicots have two seed leafs. There are at least , species of angiosperms ranging from small flowers to enormous wood trees.
0コメント