Fruits – SS2 Biology Lesson Note (Third Term)

A fruit is the structure that develops from the matured ovary after fertilisation and contains the seed. Some fruits do not develop from fertilised ovaries and are called parthenocarpic fruits. The wall of a fruit is the pericarp, which has three layers: the epicarp, mesocarp and endocarp.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Define a fruit and name the layers of the pericarp.
2. Classify fruits into simple, aggregate and multiple fruits, and into dry and fleshy fruits.
3. Describe the types of dry dehiscent, dry indehiscent and fleshy fruits, with examples.
4. State the differences between a fruit and a seed, and between a drupe and a berry.
5. Explain why fruits and seeds are dispersed and describe the agents of dispersal.

What are the types of fruits?

Fruits can be classified based on their origin or structure. These include:

1. True and false fruits
2. Simple, aggregate and composite (multiple) fruits
3. Fleshy and dry fruits
4. Dehiscent and indehiscent fruits

• Simple fruits: formed from one flower having either a monocarpous or syncarpous pistil.
• Aggregate fruits: formed from one flower having an apocarpous pistil.
• Multiple fruits: formed from many flowers whose ovaries unite together with other parts of the flower after fertilisation, or from several flowers of an inflorescence, e.g. pineapple and breadfruit.

What are simple fruits?

Simple fruits can be divided into dry and fleshy fruits. A dry fruit is a fruit in which the pericarp becomes dry, hard and woody or fibrous when the fruit ripens. Dry fruits may be dehiscent or indehiscent.

Dry dehiscent fruits

These are fruits with a hard and dry pericarp which splits to release the seeds. There are five types: the follicle, legume, schizocarp, capsule and siliqua.

• Follicle: formed from one carpel and splits along one side only, e.g. kola.
• Legume: formed from one carpel and splits along two sides, e.g. cowpea, crotalaria and all bean species.
• Capsule: formed from fused carpels and splits longitudinally along two or more lines, e.g. okra, castor oil and cotton.
• Schizocarp: a many-seeded fruit which breaks up into one-seeded parts called mericarps, e.g. Mimosa pudica (sensitive plant) and Desmodium.
• Siliqua: a long narrow capsule formed from a bicarpellary ovary. It has two chambers separated by a false septum or replum. It dehisces by the sutures, e.g. Teconia.

Dry indehiscent fruits

These are fruits that do not split to release their seeds. There are five types.

• Achene: a fruit with a dry pericarp that contains one seed, e.g. cashew, nut, sunflower fruit, four o’clock plant, rose and clematis.
• Caryopsis: a one-seeded dry indehiscent fruit in which the pericarp is fused with the testa, e.g. rice, maize and millet.
• Cypsela: a one-seeded dry fruit with a hairy pappus and hairy pericarp, e.g. Tridax, Emilia and goat weed.
• Nut: a dry fruit with a very hard, woody and tough pericarp, e.g. the hard part of cashew fruit, groundnut, palm nut, cola nut, walnut and almond.
• Samara: a fruit with the pericarp extended to form one or more wings, e.g. Combretum, obeche tree and Pterocarpus (African rosewood).

What are fleshy fruits?

Fleshy fruits are juicy, succulent and indehiscent. They store large quantities of water and carbohydrate in their tissue. There are six main types of fleshy fruits, namely:

• Drupe: a true simple fruit with a well-developed pericarp. The epicarp is thin, the mesocarp is fleshy or fibrous and the endocarp is hard and stony. The endocarp encloses one or more seeds. The hard endocarp with its seed is known as the stone of the fruit, e.g. mango, coconut and oil palm fruit.
• Berry: a true simple fruit with a well-developed pericarp. The endocarp is not stony as in a drupe. It is usually a thin membrane closely attached to a fleshy mass made up of the mesocarp and endocarp. Sometimes the whole fruit is eaten, e.g. guava, tomato, banana, lemon, grapes, cucumber and dates.
• Pome: a simple false fruit, e.g. apple and pear. The outer covering and the fleshy edible part are formed by the swelling of the receptacle. Only the core is formed from the ovary.
• Hesperidium: a true simple fruit made up of chambers separated by a sheet of endocarp. The epicarp and mesocarp are fused to form a skin (or rind), e.g. orange and lemon.

What are the differences between a fruit and a seed?

What are the differences between a drupe and a berry?

Why are fruits and seeds dispersed?

After fruits and seeds form and mature, they sometimes fall from their parents to the ground. The number of seeds produced by a single plant is often very large. Most of them perish while some are eaten by animals. Others may fall on ground that is already occupied or unfavourable to the plant.

When seedlings develop close to the parent plant, they soon become overcrowded. They compete among themselves and with the parent plant for light, water, mineral salts and other essential soil constituents, so that only the strongest seedlings finally survive. To prevent this, the fruits and seeds of most plants are adapted so that they can be scattered far away from the parents. This scattering is called dispersal.

Advantages of dispersal of fruits and seeds

1. It prevents overcrowding.
2. It reduces the degree of unhealthy competition between plants.
3. It introduces plant species into new environments where they may be better adapted.
4. It helps to cover barren pieces of land with vegetation.

What are the agents of dispersal?

The common agents that bring about dispersal are wind, animals, water, man and explosive mechanisms.

Wind

Fruits and seeds carried by wind or air currents must be light and small. They have light membranous wings, or long hairy or thread-like structures which act as parachutes in the air.

Wind dispersal is a wasteful process because a great number of seeds may be dropped on sterile or unstable ground, or carried out into the sea. Examples are cotton, poppy, clematis, shorea, asana, tridax fruit and bougainvillea.

Animals

Animals such as birds, fruit bats and monkeys may be agents of dispersal. Many animals feed on soft succulent fruits but either discard the strong protected seed of berries, the stony endocarp of drupes and the achene, or swallow them.

In the latter case, many of the hardened parts are resistant to enzymes and pass through the gut without harm. When they are later dropped, they can germinate successfully, e.g. guava and tomato.

Fruits and seeds may also be carried externally on an animal’s body, because special hooks or spines catch in the fur or wool (the hairy coats of animals), e.g. Desmodium.

Water

The seeds of certain water lilies have air spaces in the pericarp which give the buoyancy required in water. The coconut fruit achieves dispersal of its single seed by floating to a new location.

Man

The agricultural practices of man have aided the dispersal of many fruits and seeds. Guava, sweet potato and cassava are native to tropical America but are now found in all tropical areas of the world. Seeds of weeds are also transported accidentally from one part of the world to another by man’s activities.

Explosive mechanisms

When the fruit wall dries, as in the African oil bean, it splits and twists, throwing the seed out. The legumes of many bean plants (e.g. Acacia and Caesalpinia) burst open when ripe. The drying of the pericarp causes contraction and splitting. Other examples are the fruit of legumes, pride of Barbados and capsules (the fruit of okra, Hibiscus esculentus).

Evaluation

1. Define a fruit and name the three layers of the pericarp.
2. State the difference between dehiscent and indehiscent dry fruits, with one example of each.
3. State three differences between a drupe and a berry.
4. Give two advantages of the dispersal of fruits and seeds.
5. Name the five agents of dispersal and give one example of a fruit or seed dispersed by each.

Assignment

1. Which of the following is not a dry indehiscent fruit? (a) legume (b) cypsela (c) samara (d) caryopsis
2. An example of a berry is (a) tomato (b) coconut (c) apple (d) pineapple
3. Which of the following is a simple dry dehiscent fruit? (a) berry (b) drupe (c) caryopsis (d) achene
4. A dehiscent fruit formed from several fused carpels with many seeds is classified as (a) an achene (b) a follicle (c) a legume (d) a nut
5. Air spaces are characteristics of seeds or fruits dispersed by (a) birds (b) water (c) wind (d) explosive mechanism (e) man