Reproduction in Flowering Plants.

The flowering plants are a large group of seed plants whose seeds develop with a pericarp. It is then called Angiospermae. They are common in various habitats. They vary in size from small herbs to giant trees. The flower in these plants is the specialized organ for reproduction. It is a short stem with the leaves being modified to various floral parts. The flower arises from the axil of either a green or a scale leaf called bract. In some cases flowers occur without bracts. Flowers may be solitary apical as in Tulip, and so stops the growth of the stem, or may be solitary axillary as in Petunia. In other cases, flowers may be grouped on the floral axis into various aggregations called inflorescence as in Beans and Manthur.

The flower and its parts:

The flower emerges from the axil of a leaf called bract that varies in shape and colour from a plant to the other. The flower in some plants is carried on a pedicle and so becomes stalked. In some other plants, it is sessile.

The structure of a typical (complete) flower:

(as in Beans, Apple, Onion, and Petunia)

There are 4 floral whorls:

1. Calyx: It is the outer whorl of the flower, it is formed of green leaves known as sepals these protect the inner parts of the flower against drought, rain, or wind.

2. Corolla: It is the next whorl inside the Calyx. It is composed of one row or more of petals, that help in protection of the floral sexual parts, and they attrract insects so that, the pollination process occurs.

N.B.: In flowers of most Monocot plants as Tulip and Onion, leaves of the Calyx are hardly differentiated from those of the Corolla and so both whorls are called Perianth.

3. Androecium: It is the male organ that consists of numerous leaves called

stamens. Each stamen consists of a filament which carries an apical

anther that contains 4 sacs of pollen grains.

3. Gynoecium: It is the female organ and is the central whorl of the flower

and consists one or more carpels. The carpel's base is swollen and called

overy which contains the ovules. Carpels may fuse or remain separate

and enclose one or more locules. A thin neck attaches over the ovary

called the style which ends by a sticky disc called stigma where pollen

grains adhere.

N.B.: Leaves of each whorl alternate with that of the next whorl.

Flower functions:

In order for the flower to perform its functions in reproduction and species continutiy, the stamens should prepare for pollen production, and the ovary for ovule formation. Then follows the 2 processes of pollination, and fertilization to produce the fruits, and the seeds.

First: Formation of pollen grains:

If you examine a T.S. in a mature anther of a large stamen as that of Lily plant, you will find 4 sacs of pollen grains. During flower development and before formation of pollen grains, these sacs are full of large nucleated diploid cells called spore mother cells (2N). The following steps take place:

1. Each spore mother cell (2N) is divided meiotically forming 4 haploid microspores (N).

2. Each of these microspores develops into a pollen grain through a mitotic division of its nucleus into 2 nuclei called the tube and the generative nuclei.

3. The wall of the pollen grain then thickens for protection. Meanwhile, the anther matures, and the wall in-between adjacent pollen sacs degenerates.

4. The sacs open releasing the pollen grains that disperse.

Second: Formation of ovules:

During the production of pollen grains in the anther, parallel changes occur in the ovary as follows:

1. The ovule starts to appear as a simple swelling on the interior ovary wall that contains a large spore mother cell (2N).

2. During the ovule growth, a funicle or umbilical cord develops connecting it with the ovary wall (through which food materials reach it). The 2 integuments surround the ovule completely, save a minute hole called micropyle, through which the ovule is fertilized.

3. The spore mother cell (2N) divides meiotically giving a row of 4 haploid cells (N). Three of them degenerate and the fourth grows rapidly forming the embryo sac, that is surrounded by a nutritive tissue called nucellus.

4. The nucleus of the embryo sac (N) is divided mitotically thrice giving 8 nuclei (N), 4 of them migrate to each pole of the embryo sac.

5. From each of these 4 nuclei, one moves to the centre of the embryo sac giving the 2 polar nuclei.

6. Each of the remaining 3 nuclei at both of the sac poles becomes enveloped by some of the cytoplasm and a thin membrane forming distinct cells.

7. The middle of the 3 cells that are close to the micropyle grows forming the egg (N). While the 2 side cells are called synergids. The 3 cells that are distant from the micropyle are called antipodal cells.

The ovule by such way becomes ready for fertilization.

Third: Pollination and Fertilization:

A. Pollination:

This is the process by which pollen grains are transported from the anther to the stigma of the flower.

Types of pollination:

1. Self pollination: The pollen grains are transported from the anther to the stigma of the same flower, or to that of another flower of the same plant.

2. Cross pollination: The pollen grains are transported from the flower anther of a plant to the stigma of another plant of the same species. This type of pollination occurs in plants according to presence of certain factors such as the following:

1. Flowers are unisexual.

2. Organs of one sex mature before those of the other sex (as in early maleness or early femaleness flowers)

3. Hight of anthers is lower than the stigma.

Cross pollination needs a means of transportation for pollen grains from one flower to another such as air, insects, water, or Man.

B. Fertilization:

This process takes place according to the following stages:

1. Pollen grains germination:

When the pollen grains fall on the stigma, they germinate where the tube nucleus (N) forms the pollen tube, which penetrates the stigma and crosses through the style till it reaches the ovule's micropyle. Then the tube nucleus degenerates while the generative nucleus (N) divides mitotically into 2 male nuclei.

1 - One male nucleus (N) fuses with the egg nucleus (N) forming the zygote (2N), which starts to divide mitotically several times forming the embryo (2N).

2 - The second male nucleus (N) fuses with the 2 nuclei of the embryo sac (the polar nuclei) (2N) forming the endosperm nucleus that become triploid (3N).

The last stage No.2 is called the triple fusion. While the two stages together No. 1 & No. 2 are callec double fertilization.

The endosperm nucleus (3N) is divided mitotically several times forming the endosperm tissue that supplies the early developing embryo with food. This tissue may remain outside the embryo occupying a part of the seed.