Angiosperms

Seeds inside fruits

The angiosperms are the flowering plants. Their pollen grains and ovules develop in special structures, the flowers. The ovules lie inside an ovary, and after fertilisation the ovary becomes a fruit. So the seeds are enclosed.

Angiosperms live almost everywhere, from deserts to ponds. They range in size from Wolffia, a floating plant smaller than a grain of rice, to trees of Eucalyptus over $100\ \text{m}$ tall. They give us food, timber, fibres, fuel and medicines.

Two classes

Angiosperms are divided by the number of cotyledons, the seed leaves of the embryo.

• Dicotyledons have two cotyledons. Their leaves usually have a network of veins, and they have a tap root. Examples: pea, mango, rose.
• Monocotyledons have one cotyledon. Their leaves usually have parallel veins, and they have fibrous roots. Examples: wheat, maize, palm.

The parts that make the gametes

The male sex organ of a flower is the stamen. It has a thin stalk, the filament, and a head, the anther. Cells in the anther divide by meiosis to make pollen grains.

The female sex organ is the pistil, or carpel. It has a swollen ovary at the base, a slender style, and a stigma at the top. The ovules sit inside the ovary.

In an ovule, a megaspore mother cell divides by meiosis into four haploid megaspores. Three of them degenerate. The one that remains divides to form the embryo sac, the female gametophyte.

The embryo sac

A typical embryo sac has 7 cells and 8 nuclei.

• At the end where the pollen tube will enter is the egg apparatus: one egg cell and two synergids.
• At the opposite end are three antipodal cells.
• The large central cell in the middle holds two polar nuclei. These fuse to form a diploid secondary nucleus.

Count the nuclei: $1 + 2 + 3 + 2 = 8$. Count the cells: $1 + 2 + 3 + 1 = 7$.

Double fertilisation

After pollination, a pollen grain lands on the stigma and grows a pollen tube. The tube grows down through the stigma and the style, reaches an ovule, and enters the embryo sac. It releases two male gametes.

1. One male gamete fuses with the egg. This is syngamy, and it forms the diploid zygote.
2. The other male gamete fuses with the two polar nuclei, or the diploid secondary nucleus. This is triple fusion, and it forms the triploid primary endosperm nucleus, the PEN.

Two fusions happen in the same embryo sac, so the event is called double fertilisation. It is found only in angiosperms.

Then:

• the zygote grows into the embryo, with one or two cotyledons;
• the PEN grows into the endosperm, which feeds the growing embryo;
• the synergids and the antipodal cells degenerate;
• the ovule becomes the seed, and the ovary becomes the fruit.

Double fertilisation, counted

Change $n$ and the number of seeds. Watch the chromosome numbers of the egg, the zygote and the endosperm nucleus, and the number of male gametes used.

Try it in Python

The program counts the cells and the nuclei of an embryo sac, and works out the chromosomes in the zygote and in the endosperm nucleus for maize, which has $n = 10$.

Practise with new numbers

Common mistakes

• "Double fertilisation means two pollen grains." One pollen tube brings both male gametes.
• "The endosperm is diploid." It forms from three haploid nuclei, so it is triploid, $3n$.
• Counting 8 cells in the embryo sac. There are 8 nuclei but only 7 cells, because the central cell holds two nuclei.
• "The synergids become part of the seed." The synergids and the antipodal cells degenerate after fertilisation.
• "The ovule becomes the fruit." The ovule becomes the seed. The ovary becomes the fruit.

Remember these

• Angiosperms have ovules inside an ovary, and seeds inside fruits.
• Dicotyledons have two cotyledons, and monocotyledons have one.
• The stamen (filament and anther) makes pollen; the pistil (stigma, style, ovary) holds the ovules.
• The embryo sac has 7 cells and 8 nuclei: the egg, 2 synergids, 3 antipodals and the central cell with 2 polar nuclei.
• Syngamy gives the zygote ($2n$). Triple fusion gives the primary endosperm nucleus ($3n$). Together they are double fertilisation, found only in angiosperms.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which part of a flower becomes the fruit, and which becomes the seed?

2. Of the four megaspores made in an ovule, how many survive, and what do they form?

3. Name the two fusions of double fertilisation, and what each one forms.

4. A seed has one cotyledon. Name the class of its plant, and give one other feature you would expect.

5. Why is double fertilisation described as unique to angiosperms?