Plant reproductive system - Gymnosperms | dayline.info
Flowers | Double Fertilization | Seeds | Fruit | Vegetative propagation | Links Pollen grains are produced by meiosis of microspore mother cells that are two synergids flanking the egg (located at the micropyle end of the embryo sac); b) two. Pollen Formation: Development Of A Pollen Grain Within The Pollen Sac Of One of the cells near to the micropyle end of the ovule is the haploid female The most sophisticated relationships between plants and insects are The pollen tube is an example of chemotropism since it is growing toward chemicals produced. As you know, the anther of the stamen produces pollen grains. The pollen tube arrives in the micropyle of the integument of the ovule, and enters the The connection between ejaculation, intercourse, and pregnancy was fairly obvious.
The pollen sacs burst open and the mature pollen grains are ready for dispersal. Development of the Embryo Sac Each ovary contains one or more ovules. The green structure at the top of the diagram is the ovule. The integuments are the 2 walls of the ovule. There is a small opening in the walls called a micropyle. This is where the pollen tube will enter.
Will be discussed later. The nucellus is cells that provide nutrition for the growth of the ovule.
Pollination - Developmental Biology - NCBI Bookshelf
The embryo sac, also known as the megaspore, divides by meiosis to form 4 haploid cells. Three of these cells degenerate and one remains. Only one megaspore survives in each ovule. This becomes the embryo sac. The haploid nucleus of the surviving megaspore undergoes three mitotic divisions.
Eight haploid nuclei are now present. Within the swollen megaspore cell, six haploid cells and two polar nuclei are formed. The entire structure is called the embryo sac. One of the cells near to the micropyle end of the ovule is the haploid female gamete egg cell. Pollination Pollination is the transfer of pollen male gamete from the anther to a stigma.
Increases genetic variation, population more resistant to environmental change. Guarantees reproduction if pollinating agent is absent or not efficient. Pollination can be accomplished by the wind or by animals. Insects are the most common animals that will pollinate a carpel. The most sophisticated relationships between plants and insects are generally those involving bees. Bees collect pollen and nectar not only for themselves but also to feed their young.
For this reason bees have developed a number of adaptations that make them particularly good pollen carriers. These adaptations allow them to gather and carry large volumes of pollen.
Bees are ideal pollinators because they visit many flowers while carrying lots of pollen, before returning to their nest. So the chance that a bee will transfer the pollen between flowers of the same species is very high.
Many insects eat pollen. In the process of eating they become covered in it. Pollination happens when the pollen feeder transfers the pollen to the pollen receivers of the same plant, or another plant of the same species, as the insect looks for more pollen to eat.
Pollen and Embryo Sac
Fertilisation Fertilisation is the union of the male and female gametes to form a zygote. Since the male and female gametes are haploid n when the two unite the zygote is diploid 2n. Fertilisation starts when a pollen grain lands on the stigma. The pollen grain then grain germinates forming a pollen tube. The tube nucleus controls the growth of the pollen tube. The pollen tube is an example of chemotropism since it is growing toward chemicals produced from the ovule. The generative nucleus travels down the pollen tube.
It undergoes mitosis forming two haploid male gamete nuclei. The pollen tube enters the ovule by way of the micropyle.
The Structure and Functions of Flowers
The two male gamete nuclei are released into the embryo sac. The tube nucleus disintegrates. Double Fertilisation Since there are 2 sperm nuclei that have reached the embryo sac both nuclei will fuse with female gametes. One sperm nuclei will fuse with the egg cell to form the zygote 2n while the other sperm nucleus fuses with the 2 polar nuclei in the embryo sac to form an endosperm nucleus 3n.
Seed Formation The fertilized becomes the seed. The integuments become the wall of the seed called the testa. The endosperm nucleus leads to the formation of triploid endosperm, a food tissue. The diploid zygote, by mitosis, develops into a plant embryo. The developing embryo draws nourishment from the endosperm.
The embryo ceases development and goes dormant. The ovule becomes a seed, which contains a dormant plant embryo, food reserve, and the protective coat called the testa. The Embryo The embryo is made up of the radicle or future root and the plumule or future shoot.
The endosperm cells divide many times and absorb the nucellus. This is the nutrition mainly fats, oils and starch for the embryo. The pollen tubes, which develop from the pollen grains, work their way through the megasporangium of the ovule to the archegonia of the female gametophyte. Fertilization of the eggs of the several archegonia is followed by the early development of several embryos polyembryonyonly one of which survives in the mature seeds.
Cycad embryos produce two seed leaves, or cotyledons. The seeds are brightly coloured yellow or scarlet and covered by an outer fleshy layer and a stony layer of the integument. The seeds of some cycads e. The maidenhair tree, or ginkgo Ginkgo bilobais classified separately in a group of which it is the sole living representative. The mature ginkgo sporophyte produces microstrobili and ovules each spring as the buds unfold. They occur on the spur shoots among the bases of the young leaves.
The ginkgo, like the cycads, is strictly dioecious, so some trees produce ovules and others produce pollen. The ovules occur in pairs at the tips of stalks that emerge among the leaf bases.
Pollen and Embryo Sac
Ginkgo pollen, like that of pinesis four-celled at the time of pollination spring seasonwhich is accomplished by wind. Development of male and female gametophytes is similar to that in cycads, and the sperm cells are also multiflagellate.
The female gametophyte, within the ovule of G. The ovules enlarge tremendously after pollination, and, as the seeds mature, the integument differentiates into several coats, of which a stony layer and an outer fleshy layer are most prominent. The latter becomes mottled, purplish green, and foul smelling. Its tissues may cause nausea or skin eruptions in humans. The inner tissues of the seed the embryo and the female gametophyte are palatable and prized among some peoples.
Fertilization often occurs after the ovules have fallen from the trees, three or four months after pollination.
The ginkgo embryo has two cotyledons. Inside the pollen wall are two cells. The cell with lots of bluish cytoplasm filling up the inside of the pollen wall is the tube cell. The smaller cell on top of the tube cell is the generative cell. Notice that both cells have a lavender nucleus and each nucleus has a red nucleolus. The nuclei of the two cells are the same size The tube cell has much cytoplasm, the generative cell has far less.
The tube cell grows to penetrate the pollen wall and then the stigma and style tissues of the carpel. This penetrating cell is now called the pollen tube. Its growth is chemotropic grows toward a source of chemicals The chemical source is an ovule. The pollen tube's chemotropism results in the tube arriving in a small opening in the ovule's integument Here is a labeled diagram of both the pollen grain and the ovule: The pollen tube arrives in the micropyle of the integument of the ovule, and enters the embryo sac.
The synergids of the embryo sac literally put their energies together to burst the end of the pollen tube. Meanwhile, the generative cell has followed the pathway digested by the tube cell.
Along the way it has divided to produce two sperm cells. The two sperm cells exit the pollen tube through the burst end. One sperm cell unites with the egg. This process is called syngamy union of gametes or [gag] fertilization.
I much prefer syngamy to fertilization see below. The product of the union of egg and sperm is called a zygote, and it has two sets of master plans, one from each gamete. The zygote will divide and grow and differentiate to become a new embryonic plant with root, stem, and leaf. Ultimately it will become an adult plant. The other sperm cell unites with the central cell. This is a second syngamy! It is a three-way fusion between the two polar nuclei of the central cell and the sperm cell.
The resulting endosperm cell has three sets of master plans one from sperm and one each from the two polar nuclei. The endosperm cell divides to form a nutritive tissue inside the seed. It accumulates nutrients from the mother plant and stores them away for the developing embryo inside the seed.