Fertilization and Development of the S eed

After pollination, with the notable exception of a number of crop plants (e.g., peas, apricots), further development of the male gametophyte may not take place unless the pollen grain is (1) from a different plant of the same species or (2) from a variety different from that of the flower receiving it.

Under suitable conditions, the dense cytoplasm of the pollen grain absorbs fluids from the stigma and bulges out through one of the apertures in the form of a tube. This pollen tube then grows down between the cells of the stigma and style until it reaches the micropyle of the ovule. In corn, it may have to grow more than 50 centimeters (20 inches) before it arrives at its destination, but in most plants, the distance is considerably less.

The pollen tube's journey may last less than 24 hours and usually does not take more than 2 days, although there are a few plants in which growth takes over a year. As the tube grows, most of the contents of the pollen grain are discharged into it. The vegetative nucleus stays at the tip, while the generative nucleus (cell) lags behind and divides by mitosis, usually in the tube, producing two nuclei that become sperm cells; no flagella develop. Sometimes, the generative nucleus (cell) divides before the pollen tube has formed. The germinated pollen grain with its vegetative nucleus and two sperms constitutes the mature male game-tophyte (microgametophyte) (Fig. 23.7).

When the pollen tube reaches the micropyle, it continues on to the female gametophyte (megagametophyte), which it enters, destroying a now degenerating synergid in the process; it then either bursts or forms a pore, discharging its contents. Next, an event unique to angiosperms (and a few gneto-phytes), called double fertilization (or double fusion), takes place. One sperm migrates from the synergid to the egg, losing most of its protoplasm along the way. The sperm cell nucleus then unites with the egg nucleus, forming a zygote.

The other sperm cell also migrates from the synergid, and, upon reaching the central cell nuclei, unites with them, producing a 3n (triploid) endosperm nucleus. The endosperm

Chapter 23

pollen grain -

pollen grain -

Mature Male Gametophyte

aperture egetative cell

Figure 23.7 A mature male gametophyte of a flowering plant.

aperture egetative cell

Figure 23.7 A mature male gametophyte of a flowering plant.

nucleus becomes exceptionally active and divides repeatedly by mitosis, with cell cycles being completed every few hours, usually without formation of cell walls. This nutritive 3n tissue, called endosperm, may eventually have hundreds of thousands of nuclei; it surrounds the embryo that develops from the zygote.

In some monocots, such as corn and other grasses, the endosperm tissue becomes an extensive part of the seed, but in most dicots, such as members of the Legume Family (Fabaceae; e.g., peas, beans), the endosperm provides nutriment for the embryo that develops from the zygote but is absorbed into the cotyledons by the time the seed is mature. The integuments harden, becoming a seed coat, and the remaining haploid nuclei or cells (antipodals, synergids, and tube nucleus) degenerate. At the conclusion of these various events, the ovule has become a seed, and at the same time, the ovary matures into a fruit. Seed dispersal and fruits are discussed in Chapter 8.

Oth er Types of (Female) Gametophyte Development

The process of female gametophyte development just described in the section "Fertilization and Development of the Seed" occurs in about 70% of the known flowering plants. The remaining 30% exhibit variations in which the female gametophyte has from 4 to 16 nuclei or cells at maturity, and the endosperm may be 5n, 9n, or even 15n.

One such variation occurs in lilies, a favorite of microscope-slide manufacturers for showing female gametophyte development. When a megasporocyte undergoes meio-sis in a lily, all four of the haploid megaspore nuclei produced remain functional nuclei (not cells). Three of the megaspore nuclei unite, forming one 3n nucleus; the fourth megaspore nucleus remains n. Both the 3n nucleus and the n nucleus

Megaspore Flowering Plants

Figure 23.8 How the megasporocyte of a lily develops. A. The process begins with a diploid (2n) megasporocyte. B. and C. The megasporocyte undergoes meiosis, producing four haploid (n) megaspore nuclei. D. and E. The megaspore nuclei unite, forming a triploid (3n) nucleus; the other haploid nucleus remains separate. F. and G. Both the 3n and n nucleus undergo two consecutive mitotic divisions, resulting in four 3n and four n nuclei. H. Three of the 3n nuclei function as antipodals; the fourth 3n nucleus and one n nucleus function as central cell nuclei. The remaining haploid nuclei function as an egg and two synergids.

Figure 23.8 How the megasporocyte of a lily develops. A. The process begins with a diploid (2n) megasporocyte. B. and C. The megasporocyte undergoes meiosis, producing four haploid (n) megaspore nuclei. D. and E. The megaspore nuclei unite, forming a triploid (3n) nucleus; the other haploid nucleus remains separate. F. and G. Both the 3n and n nucleus undergo two consecutive mitotic divisions, resulting in four 3n and four n nuclei. H. Three of the 3n nuclei function as antipodals; the fourth 3n nucleus and one n nucleus function as central cell nuclei. The remaining haploid nuclei function as an egg and two synergids.

undergo mitosis twice, resulting in a large cell with four 3n nuclei plus four n nuclei. The large cell with eight nuclei becomes the female gametophyte; one of the central cell nuclei is 3n, and the other central cell nucleus is n. During fertilization when one sperm (n) unites with the two central cell nuclei, a 5n endosperm nucleus is produced (Fig. 23.8).

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Responses

  • Paul
    How sperm cell reach to egg and central cell in plants review?
    8 years ago
  • Daniel Isaias
    Which nuclei unties a sperm forming an endosperm nucleus?
    8 years ago
  • liya
    How does the division of megasporocyte in a lily differ?
    8 years ago
  • katrin durr
    What type of division undergoes the megasporocyte of lily?
    8 years ago

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