The fruit and the seed

From flower to fruit

After fertilisation the ovules grow into seeds. The ovary grows into a fruit. So a fruit is a mature, ripened ovary.

If a fruit forms without fertilisation of the ovary, it is a parthenocarpic fruit. The banana is an example.

The wall of the fruit

A fruit has a wall, the pericarp, and the seeds inside it. The pericarp may be dry or fleshy. When it is thick and fleshy, it has three layers.

• The epicarp is the outer layer.
• The mesocarp is the middle layer.
• The endocarp is the inner layer.

Mango and coconut: the drupe

Mango and coconut fruits are called drupes. A drupe develops from a monocarpellary, superior ovary, and it holds one seed.

In the mango the three layers are easy to tell apart.

• The epicarp is a thin outer skin.
• The mesocarp is fleshy and edible.
• The endocarp is stony and hard. It is the "stone" that protects the seed.

In the coconut, the mesocarp is fibrous.

You should find that with the starting values the mesocarp makes up most of the wall. That is why a mango has so much to eat and so little skin.

The seed

After fertilisation, an ovule becomes a seed. A seed is made up of a seed coat and an embryo.

The embryo has an embryonal axis and one or two cotyledons. A radicle and a plumule lie at the two ends of the axis. The radicle is the young root. The plumule is the young shoot.

A dicot seed: gram, pea and bean

• The seed coat has two layers: the outer testa and the inner tegmen.
• The hilum is a scar on the seed coat. The developing seed was attached to the fruit through it.
• Just above the hilum is a small pore, the micropyle.
• Inside is the embryo, with an embryonal axis and two cotyledons. The cotyledons are often fleshy and full of reserve food.

What about the endosperm? In bean, gram and pea, the endosperm is not present in the mature seed. Such seeds are non-albuminous. In castor, a part of the endosperm is left over, because the embryo did not use it all. Such seeds are albuminous.

A monocot seed: maize

Monocot seeds are generally endospermic. A few, such as orchids, are non-endospermic. Look at a grain of maize.

• The seed coat is thin and membranous. It is generally fused with the fruit wall.
• The endosperm is bulky and stores food.
• A proteinous layer, the aleurone layer, separates the endosperm from the embryo.
• The embryo is small, and it lies in a groove at one end of the endosperm.
• The embryo has one large, shield-shaped cotyledon, the scutellum, and a short axis with a plumule and a radicle.
• The plumule is enclosed in a sheath, the coleoptile. The radicle is enclosed in the coleorhiza.

Dicot and monocot seeds side by side

• Cotyledons: a dicot seed has two. A monocot seed has one, the scutellum.
• Endosperm in the ripe seed: gram and pea have none. Maize has a bulky endosperm.
• Seed coat: a dicot seed has a testa and a tegmen. In maize it is membranous and fused with the fruit wall.
• Sheaths: the dicot embryo has none. The maize embryo has the coleoptile and the coleorhiza.

Try it in Python

The program keeps the number of cotyledons and the endosperm of some seeds. It names the group and says whether each seed is albuminous.

You should see that castor and maize are albuminous, and gram and pea are not. The wheat line says that it is a monocot seed, and albuminous.

Practise with new numbers

Common mistakes

• Saying that every ripe seed has endosperm. Gram, pea and bean seeds have none, so they are non-albuminous.
• Forgetting that the coat of a maize grain is fused with the fruit wall. It is thin and membranous.
• Mixing up the coleoptile and the coleorhiza. The coleoptile covers the plumule. The coleorhiza covers the radicle. Rhiza means root.
• Calling the coconut mesocarp fleshy. It is fibrous.
• Mixing up the testa and the tegmen. The testa is the outer layer of the seed coat, and the tegmen is the inner one.

Remember these

• A fruit is a mature ovary. A parthenocarpic fruit forms without fertilisation, as in banana.
• The pericarp has the epicarp, the mesocarp and the endocarp, from outside to inside.
• Mango and coconut are drupes: one-seeded, from a monocarpellary superior ovary.
• A dicot seed has two cotyledons, a hilum and a micropyle. A maize grain has one cotyledon, the scutellum.
• Albuminous seeds keep some endosperm, as in castor. Non-albuminous seeds do not, as in gram.

Check yourself

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

1. Write the three layers of the pericarp in order, from the outside to the inside.

2. Why is the banana called a parthenocarpic fruit?

3. In a dicot seed, which part is a scar and which part is a small pore?

4. What is the difference between an albuminous and a non-albuminous seed? Give one example of each.

5. Name the two sheaths of a maize embryo, and say what each one covers.