Unit 4: Reproduction                                                                1   2   3 

3.2. Sexual reproduction in plants

Plants have alternation of generations. In its life cycle it is possible

to distinguish two stages:

- Sporophyte. It is the plant that forms spores. It is the asexual stage.

- Gametophyte. It is the plant that forms gametes. It is the sexual stage.

The gametophyte forms gametes. These gametes join together to form a zygote.

This zygote forms an embryo that develops into a sporophyte.

    The sporophyte forms spores which develop new  gametophytes.

a)   Seedless plants

In Pteridophytes (ferns), gametophyte and sporophyte are different plants.

The sporophyte is the permanent part of the cycle. It is a vascular plant formed by

roots, a rhizome (underground stem) and fronds (leaves).

The spores are produced in sporangia called sori  which are usually on the

underside of fronds.

When spores germinate a gametophyte is produced. It is very small plant

with a very short life. It produces gametes (male and female).

Gametes join together to form the zygote which becomes a new sporophyte.

In Bryophytes (mosses), sporophyte grows as part of the gametophyte.

The gametophyte is the permanent part of the cycle. It is a non-vascular plant

formed by false roots, stem and leaves. Gametophyte produces gametes

that join together to form a zygote.

The zygote becomes a sporophyte.It grows on the top of the gametophyte.

The spores are formed in a sporangium called capsule.

When spores germinate produce new gametophytes.

b)   Seed plants

In Spermatophytes (seed plants), Angiosperms (flowering plants)

and Gymnosperms (conifers), gametophyte is not an independent plant,

but a reduced structure that forms inside the flower.

We can distinguish several phases in the life cycle of Spermatophytes:

- Formation of gametes

The flower is a structure made up of modified leaves. It contains the reproductive organs of the plant where gametes are produced.

In a typical flower we can find:

The receptacle is the base where all the other parts are attached and the peduncle the small stalk that joins the flower to the plant.

Flowers can be:

           - Complete, when it contains all the elements or incomplete,

             when it lacks of any of them(corolla or calyx).

           - Unisexual (they only have stamens or carpels) or hermaphrodites.

     Usually flowers are in groups called inflorescences.

- Pollination

Pollination is the transfer of pollen grains from the anthers of a flower to the pistil of other flower of the same plant (self-pollination) or of other one (cross-pollination).

Pollen grains can be transported in two different ways:

       - By wind. These plants have incomplete and small flowers and they produce a lot of pollen, in order to make easier its release and to assure that at least a part reaches its objective.

   - By insects. These plants have big, coloured and fragrant flowers and usually produce a sweet substance called nectar, in order to attract insects. They do not need to produce a great amount of pollen because insects carry it directly to other flower.

- Fertilisation

Fertilisation consists of the union of the male gamete and the female gamete.

When the pollen grain reaches the stigma, it forms a pollen tube, which grows along the style until it reaches the ovary and the ovule inside.

The male gamete goes down inside its pollen tube. When it arrives to the ovule, join together with the female gamete and the zygote is formed.

- Formation of the seed and the fruit

The seed is formed from the ovule tissues. The seed is composed by three parts:

    - The embryo is produced by the zygote. It is a tiny plant formed by

       the radicle (embryonic root), the plumule (embryonic stem)

       and one or two cotyledons (embryonic leaves) that store nutrients.

        - The  albumen (or endosperm) is a nutritious substance that envelops

       and feed the embryo.

    -The seed coat is a hard protective envelop. 

The fruit is formed from the ovary tissues in Angiosperms. After fertilisation, the flower loses the sepals, petals, stamens, the style and the stigma. The walls of the ovary grow and develop with the seed inside.

The fruit function is to protect the seed and help in its dispersion. There are two types of fruits:

     - Fleshy fruit, such as tomatoes or peaches, where the part around

      the seed is a juicy pulp.

        - Dry fruit, such as sunflower seeds or nuts, where the seed has

      a stony fruit wall.

- Dispersion and germination

Dispersion of seeds and fruits can occur in the following ways:

    - By animals:

            - Animals eat plants, but they can't digest the seeds.

              So these seeds are then released in the animal's faeces

  in different places

- Some fruits are not edible but they have hooks

  that stick to the fur of animals, so they are dispersed as animals move around.

- By wind: Some fruits and seeds are shaped like wings or windmills.

- By water: Some fruits and seeds have waterproof covers that allow them to float.

For germination to take place the seeds must disperse:they must land on suitable ground, at a distance from the mother plant and have enough space, light and nutrients.

Germination begins when the seed absorbs water, which causes its coat to break. The seed opens and the embryo begins to develop to create a new plant.

      In the first stages the new plant feed on the supplies stored in the seed

      until it can carry out photosynthesis and begins to make its own nutrients.

3.3. Label this diagram. What does it represent?

        Then listen the descriptions and relate each one with the described part.

3.5. Identify the phases of this Angiosperm’s life cycle.

3.6. Cut out these pictures and glue them in the right order in your notebook.

        Complete the sentences and relate them with their correspondent image.

        What process is represented?

a.    The ………………………… waits for favourable conditions to ………………………..

b.    The ………………………appears when there is enough moisture in the soil.

c.    The seed ……………..………….. falls and the young ………………….. come out of the soil.

d.    The …………………… feed  the plant while it hasn’t got real leaves to perform  …………………………

e.    Finally, the first real ………………. appear and the cotyledons fall down.