Info

2.20 Plants alternate between diploid and haploid life stages.

are called spores, not gametes; the spores undergo one or more mitotic divisions to produce gametes. Although the terms used for this process are somewhat different from those commonly used in regard to animals (and from some of those employed so far in this chapter), the processes in plants and animals are basically the same: in both, meiosis leads to a reduction in chromosome number, producing haploid cells.

In flowering plants, the sporophyte is the obvious, vegetative part of the plant; the gametophyte consists of only a few haploid cells within the sporophyte. The flower, which is part of the sporophyte, contains the reproductive structures. In some plants, both male and female reproductive structures are found in the same flower; in other plants, they exist in different flowers. In either case, the male part of the flower, the stamen, contains diploid reproductive cells called microsporocytes, each of which undergoes meiosis to produce four haploid microspores ( FIGURE 2.21a). Each microspore divides mitotically, producing an immature pollen grain consisting of two haploid nuclei. One of these nuclei, called the tube nucleus, directs the growth of a pollen tube. The other, termed the generative nucleus, divides mitotically to produce two sperm cells. The pollen grain, with its two haploid nuclei, is the male gametophyte.

The female part of the flower, the ovary, contains diploid cells called megasporocytes, each of which undergoes meio-sis to produce four haploid megaspores ( FIGURE 2.21b), only one of which survives. The nucleus of the surviving megaspore divides mitotically three times, producing a total of eight haploid nuclei that make up the female gametophyte, the embryo sac. Division of the cytoplasm then produces separate cells, one of which becomes the egg.

When the plant flowers, the stamens open and release pollen grains. Pollen lands on a flower's stigma — a sticky platform that sits on top of a long stalk called the style. At the base of the style is the ovary. If a pollen grain germinates, it grows a tube down the style into the ovary. The two sperm cells pass down this tube and enter the embryo sac ( FIGURE 2.21c). One of the sperm cells fertilizes the egg cell, producing a diploid zygote, which develops into an embryo. The other sperm cell fuses with two nuclei enclosed in a single cell, giving rise to a 3n (triploid) endosperm, which stores food that will be used later by the embryonic plant. These two fertilization events are termed double fertilization.

Concepts B

In the stamen of a flowering plant, meiosis produces haploid microspores that divide mitotically to produce haploid sperm in a pollen grain. Within the ovary, meiosis produces four haploid megaspores, only one of which divides mitotically three times to produce eight haploid nuclei. During pollination, one sperm fertilizes the egg cell, producing a diploid zygote; the other fuses with two nuclei to form the endosperm.

^ In the stamen, diploid microsporocytes undergo meiosis.

.. .to produce four haploid microspores.

^ Each undergoes mitosis to produce a pollen grain with two haploid nuclei.

^ The tube nucleus directs the growth of a pollen tube.

^ In the stamen, diploid microsporocytes undergo meiosis.

.. .to produce four haploid microspores.

^ Each undergoes mitosis to produce a pollen grain with two haploid nuclei.

^ The tube nucleus directs the growth of a pollen tube.

Pollen Tube

Diploid megasporocytes undergo meiosis.

.to produce four haploid megaspores, but only one survives.

^ The surviving megaspore divides mitotically three times,...

.to produce eight haploid nuclei.

0 The cytoplasm divides, producing separate cells,.

.one of which becomes the egg.

Two of the nuclei become enclosed 1 within the same cell.

.and the other nuclei are partitioned into separate cells.

Embryo (diploid, 2n)

Diploid megasporocytes undergo meiosis.

.to produce four haploid megaspores, but only one survives.

^ The surviving megaspore divides mitotically three times,...

.to produce eight haploid nuclei.

0 The cytoplasm divides, producing separate cells,.

.one of which becomes the egg.

Two of the nuclei become enclosed 1 within the same cell.

.and the other nuclei are partitioned into separate cells.

Embryo (diploid, 2n)

2.21 Sexual reproduction in flowering plants.

(a) Male gametogenesis (spermatogenesis)

(b) Female gametogenesis (oogenesis)

Spermatogonia in the testis can undergo repeated rounds of mitosis, producing more spermatogonia.

Spermatogonium (2n )

A spermatogonium may enter prophase I, becoming a primary spermatocyte.

Spermatogonia in the testis can undergo repeated rounds of mitosis, producing more spermatogonia.

Spermatogonium (2n )

A spermatogonium may enter prophase I, becoming a primary spermatocyte.

^^ Primary spermatocyte (2n)

Each primary spermatocyte completes meiosis I, producing two secondary spermatocytes.

^^ ^^ Secondary spermatocytes (In)

.that then undergo meiosis II to produce two haploid spermatids each.

Maturation

Maturation

Sperm

Spermatids mature into sperm.

Sperm

Oogonium (2n)

Oogonia in the ovary may either undergo repeated rounds of mitosis, producing additional oogonia, or.

Oogonium (2n)

.enter prophase I, becoming primary oocytes.

Each primary oocyte completes meiosis I, producing a large secondary oocyte and a smaller polar body, which disintegrates.

Secondary oocyte (1n )

First polar body

The secondary oocyte completes meiosis II, producing an ovum and a second polar body, which also disintegrates.

Ovum

Second polar body

Was this article helpful?

0 0

Responses

  • silke
    Are eukaryotic cells of flowers haploids?
    8 years ago

Post a comment