Biology Reproduction in flowering plants ثانوية عامة

Biology | Reproduction in flowering plants
Reproduction in Flowering plants

• The seeds of flowering plants develop with a pericarp, therefore they called Angiospermae.
• Angiospermae are common in various habitats, they may be small herbs or giant trees.
• The flower is the reproductive organ of Angiospermae.
• The flower is a short stem its leaves are modified into floral parts.
• The flower arises from the axle of either a green or a scale leaf called bract, in some cases flowers occur without bracts.



• The solitary apical flower stops the growth of the stem as in Tulip.
• The solitary axial flower as petunia.
• Inflorescence is a group of flowers on the floral axis in different aggregations as in beans and manthur.



Structure of the flower

• The flower is formed from the a leaf axle called bract.
• Bract is a green or a scale leaf that varies in shape and colour from a plant to the other.
• The Stalked flower is a flower which is carried on a pedicle.
• The Sessile flower has no pedicle.



Structure of the flower

• The typical flower has 4 floral whorls Calyx, Corolla , Androecium and Gynoecium typical flower as in beans, apple, onion and petunia.
• Leaves of each whorl alternate with each other

1) Calyx:
•It is the outer whorl of the flower, it is formed of green leaves

known as sepals
•its functions: 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 coloured leaves called petals, its functions

a) it protect the floral sexual parts
b) they attract insects to carry out pollination process.

3) Androecium:
• It is the male organ, that consists of a group of leaves called stamens. Each stamen consists of a filament which carries anther that contains 4 sacs of pollen grains.



4) The Gynoecium:
• It is the female organ and is the central whorl of the flower
• It consists of one or more carpels,
• Each carpel consists of a swollen part at its base called ovary which contains the ovules, the ovary attaches to a thin neck called the style which ends by a sticky disc called stigma where pollen, grains adhere.
• Carpels may fuse to form many locules (champers) or remain separate to form one locule (champer).





First: Formation of pollen grains
• T.S. in a mature anther of a large stamen as Lily plant.
It consists of 4 sacs of pollen grains.
• The pollin sacs are full of spore mother cells each contain large nucleus {diploid cells (2N) }.
• Each of these cells is divided meiotically forming 4 haploid (N) called micro spores.



The nucleus of microspores divided mitotically into two nuclei called one of them is called tube nucleus and the other is called generative nucleus and change into pollen grains.

• The wall of the pollen grain then thickens for protection.
• At the maturity of the anther the wall in-between adjacent pollen sacs degenerate, then the sacs open releasing the pollen grains .



Second: Formation of ovules
During the production of pollen grains in the anther, production of ovules occurs in the ovary as follows:

• The ovule starts as a swollen part connected to the ovary wall by funicle or umbilical cord to transport food material to it
• The ovary wall contains a large spore mother cell (2N).
• During the ovule growth, two integuments surround the ovule completely, save a minute hole called micropyle through which the ovule is fertilized.
• The spore mother cell (2N) is divided 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.



The formation of embryo sac
1- The nucleus is divided meiotically three times giving 8 nuclei, 4 of them migrate to each pole of the embryo sac.
2- From each of these 4 nuclei, one moves to the centre of the embryo sac giving the 2 polar nuclei.
3- Each of the three nuclei at the poles is surrounded by little amount of cytoplasm and a thin membrane forming cells.
4- The middle of the three cells that near the micropyle grows forming the egg, while the two cells beside the nucleus are called synergids, while the cells that away from micropyle are called antipodal cells.
5- The ovule by such way becomes ready for fertilization


Biology | Reproduction in flowering plants

Third: Pollination and fertilization

A) Pollination
• Pollination is the transferring of pollen grains from the anther to the stigma of the flower.

Types of the pollination:
1- Self pollination:
• It is the transferring of pollen grains 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 anther of plant to the stigma of another plant of the same species.

The factors that need cross-pollination
1- Flowers are unisexual.
2- Organs of one sex mature before those of the other sex.
3- Height of anthers is lower than the stigma.

Methods of cross-pollination:
It takes place by air, insects, water and man.

b) Fertilization:
This process takes place according to the following stages
1- Pollen grains germination:
-When the pollen grain fall on the stigma, it germinates & the tube nucleus control the formation of the pollen tube
- The pollen tube penetrates the stigma and crosses through the style till it reaches the micropyle of the ovule.
- Then the tube nucleus degenerates while the generative nucleus is divided by mitotic division into 2 male nuclei each (N),
-One of them fuses with the egg nucleus (N) forming the zygote (2N), which starts to divide forming the embryo (2N).
-When the pollen grain fall on the stigma, it germinates & the tube nucleus control the formation of the pollen tube
- The pollen tube penetrates the stigma and crosses through the style till it reaches the micropyle of the ovule.
-Then the tube nucleus degenerates while the generative nucleus is divided by mitotic division into 2 male nuclei each (N),
-One of them fuses with the egg nucleus (N) forming the zygote (2N), which starts to divide forming the embryo (2N).



The triple fusion
• It is the fusion of male nucleus (N) with the two embryo sac nuclei (2N) to form the endosperm nucleus (3N).

Double fertilization
• it is the fertilization of egg nucleus (N) by male generative nucleus to form the zygote & the fusion of male nucleus (N) with the two embryo sac nuclei (2N) to form the endosperm nucleus(3N) at the same time.
• The endosperm nucleus is divided to form endosperm tissue; which act as a food of the early developing embryo.

The Formation of fruit and seed:
According the presence of the endosperm tissue the cells divided into two groups Endospermic seeds which contain endosperm and Exendospermic seeds which doesn’t contain endosperm.

Endospermic seeds
• They are the seeds which contains its endosperm tissue occupying a part of the seed outside embryo such as Monocot. seeds.
• In monocot. the ovary and ovule fuse together forming a single seeded fruit called grain, as in maize and wheat.



Exendospermic seeds
- They are the seeds which doesnot contains endosperm because the embryo feed on the endospermic tissue during its development so, the food of the embryo is stored in the two cotyledons.
The integuments of the ovule harden forming the seed testa it is called (seed) as in bean and pea seeds.
- After fertilization, the calyx, the corolla, the endroecium, the style and stigma wither and fall out, only the ovary remains that stores food, and change into fruit due to the hormones (auxins) secreted by ovary.
- The ovary’s wall is transformed into the pericarp, and the seed wall into the coat.
- The 2 synergid cells and antipodal cells disappear while the micropyle remains to allow water to get into the seed during germination.

The Formation of fruit and seed:
There are some fruits which keep some parts of the flower - for example:
1. Leaflets of the calyx and the stamens remain with pomegranate.
2. The calyx may take part in eggplant and dates fruits.
3. The corolla leaflets may stay on marrow fruits.

False fruit:
•It is the fruit in which the receptacle or any part share ovary to store food as in apple where the receptacle may share in fruit formation.
• pollination provides the flower with male cell needed for fertilization.
• The fertilized ovule develops into the seed.
• Fertilization also stimulates the auxins necessary for developing the ovary into a mature fruit.

Parthenocarpy:
It is the formation of fruit from the flower without fertilization.
1- Natural Parthenocarpy:
as in, banana and pine- apple.
2- Artificial Parthenocarpy:
It takes place by spraying the stigmata with the extract of pollen grains.
(Pollen grains powder in ether solution as indole or naphtol acitic acid) to stimulate the ovary to form the fruit.


• The maturation of the fruit and seeds often leads to stop the plant growth or leads to death, especially in annual plants due to consumption of stored food and inhibition of hormones.
• If pollination and fertilization do not take place, the flower withers
and drops off without fruit formation.