SEEDS AND FRIUTS

Seeds

A seed is a structure which develops from the ovule after fertilization. A seed consists of an embryo and one or two cotyledons. It may contain an endosperm or food reserve. The embryo is made up of a radicle or future root system and a plumule or future shoot system. The seed is therefore a potential young plant awaiting certain favourable conditions for germination. Flowering plants have been classified into two major groups according to the number of seed leaves contained in their seeds. Hence plants whose seeds contain two seed leaves are called dicotyledonous plants and those whose seeds contain only one seed leaf are called monocotyledonous plants (mono = one, di = two).

During the development of a seed certain changes take place in the ovule. These are:

1. The endosperm nucleus develops into an endosperm which stores food for the developing embryo.
2. The antipodal and guard cells degenerate.
3. The integuments become the testa or seed coats.
4. The micropyle remains as a tiny hole for absorption of water and diffusion of air during germination.

After the embryo has grown to maturity, the food reserve in the endosperm may or may not have been completely used up. Seeds in which the endosperm is completely used up during development are called non-endospermic seeds. Such seeds have no endosperm in their structure. Food used during germination is stored in their cotyledons. Many dicotyledonous seeds are non endospermic. Examples are cowpea, groundnut, Okra, Pride of Barbados and balsam. Seeds in which the food reserve in the endosperm is not completely used up during development are called endospermic seeds. Such seeds have an endosperm in their structure which contains the food required for germination. There is no food reserve in their cotyledons. Most monocotyledonous seeds are endospermic. Examples are coconut, palmnut, maize, rice and guinea corn. Seeds whether endospermic or non-endospermic are important source of food for man and other animals. The food is stored as proteins, fats or carbohydrates.

 

Structure of a Seed

The seed generally has the following parts:

1. Seed coat or testa: A typical seed has a seed coat which covers an inner embryo. In most seeds, the seed coat is made up of an outer testa and an inner tegmen. The testa is thin, transparent and membranous. Both structures protect the seed.
2. Hilum: On one side of the seed lies a scar known as hilum. The hilum is the point of attachment of the seed to the seed stalk of funicle.
3. Micropyle: Very close to the hilum is a tiny hole called the micropyle. This hole permits air and water into the embryo of the seed.
4. Embryo: The embryo consists of a plumule (the embryonic shoot), a radicle (the embryonic root) and one or two cotyledons (seed leaves). Around the radicle is the coleorhiza which is the sheath that protects the radicle which around the plumule is the coleoptile which is the structure that protects the plumule.

 

Structure of a Monocotyledonous Seeds (e.g. the Maize Grain) – Zea mays)

The maize seed or grain is a one seeded (monocotyledonous) and endospermic seed. It is triangular in shape with a pointed edge where it attaches to the cob. The broad portion bears the remnant of the style. Around the centre of the seed is a ridge marking the position of the radicle and plumule.

A longitudinal section through the maize shows the plumule and radicle beneath the fused fruit and seed coat. A large portion of the seed is the endosperm where food is stored. The lower portion houses the radicle sheath (coleorhiza) while the upper part contains the plumule sheath (coleoptile). The seed is covered by fused pericarp and testa.

 

Structure of a Dicotyledonous Seed (e.g. the Beans or Cowpea Seed)

The cowpea seed is a two-seeded (dicotyledonous) seed. It is curved in shape and protected by the testa or seed coat. It has a scar known as hilum which is the point of attachment of the seed to the fruit. A tiny pore, the micropyle is also present. It permits the entrance of water and air into the seed.

The longitudinal section through the cowpea seed shows the essential features of a typical seed which include the positions of plumule, radicle and the fleshy cotyledon. The radicle will later develop into the root, and the plumule into the shoot. The cotyledons contain starch and protein which nourish the embryo before it germinates and becomes a full-grown seedling.

 

Fruits

A fruit is the structure which develops from the ovary after fertilization. Immediately after fertilization the ovary begins to increase in size as a result of accumulation of food substances. This is followed by gradual changes in colour. The wall of the fruit develops into a pericarp or fruit-coat. At maturity the pericarp is either hard and dry or soft and fleshy. A fruit can therefore be regarded as a mature ovary, containing one or more seeds.

Sometimes a fruit may be so small that it may be mistaken for a seed. Conversely a seed may be so large that it may be mistaken for a fruit. Size is not an identification mark. Fruits and seeds can be identified with the number of scars they possess. A fruit has two scars. These are: the remains of style at one end and the point of attachment to the receptacle at the opposite end. A seed has only one scar or hilum representing the point of attachment of the seed to the placenta of the ovary.

 

STRUCTURE OF A FRUIT

The structure of a typical fruit, e.g. mango is made up of an outer covering called the epicarp, middle layer called the mesocarp and an inner layer called the endocarp. Within the endocarp is the seed(s).

The epicarp, mesocarp and endocarp of a fruit is collectively called the pericarp.

A fruit normally consists of the following parts:

1. the fruit wall or pericarp,
2. the seed or seeds inside the fruit,
3. the remains of the style or scar showing point of attachment of the style,
4. the fruit stalk or scar showing point of attachment of the fruit to the plant.

 

Differences Between a fruit and a Seed

 

Fruit

1. Fruit develops from an ovary and so contains seed(s).
2. It has fruit stalk or a scar (due to receptacle).
3. It bears remains of style or scar left by it.
4. It has no micropyle.
5. Fruit has a pericarp and one or more seeds.

 

Seed

1. Seed develops from an ovule and so contains an embryo.
2. It has a scar (hilum) form ed by attachment of funicle.
3. It does not bear remains of style.
4. It has micropyle.
5. Seed has a seed coat and a single embryo.

 

CLASSIFICATION OF FRUITS

Fruits are classified into different groups depending on their origin and structure. The following are the common ways of classifying fruits:

• True and false fruits.
• Simple, aggregate and composite (multiple) fruit.
• Fleshy and dry fruits.
• Dehiscent and indehiscent fruits.

 

True and False Fruits

1. Meaning of true fruit: A true fruit is the type of fruit that develops solely from a fertilised ovary. It consists of a pericarp and seed or seeds. Examples include mango, orange and cowpea.
2. Meaning of false fruit: A false fruit is the type of fruit that is formed from ovary and other floral parts as well. Examples of false fruits are apple, pineapple and cashew.

 

Simple Aggregate and Composite Fruits

1. Meaning of simple fruit: A simple fruit develops from a flower with a single ovary. The ovary may be monocarpous as in cowpea and maize or syncarpous as in okra, tomato and pawpaw.
2. Meaning of aggregate fruit: An aggregate fruit is one which develops from a single flower with several ovaries. Such a flower has a pistil where the carpels are separated and so have several ovaries. Each ovary develop into a fruitlet. Examples of aggregate fruits are kola, custard, apple and strawberry.
3. Meaning of Composite fruit: A composite or multiple fruit develops from an inflorescence (several flower) or flowers positioned very close to one another. Here, all the fruitlets and the floral parts are fused together to form single large false fruit. Example of this is pineapple.

 

Fleshy and Dry Fruits

Fruits may be classified into fleshy or dry usually according to the nature of their pericarps.

1) Meaning of fleshy fruits: A fleshy fruit is the type of fruit in which the whole pericarp or at least one of its layers is thick, soft and fleshy (succulent) especially when it is ripe.

Types of Fleshy Fruits

a) Drupe: The pericarp of a drupe consist of three layers, namely epicarp, mesocarp and endocarp. The epicarp is thin, the mesocarp may be fibrous, e.g. coconut and oil palm nut or fleshy, e.g. mango while the endocarp is hard and stony. In mango, the fleshy mesocarp is the edible part and the endocarp encloses the seed. Common examples of drupe and mango, coconut.

 

b) Berry: The pericarp of a berry also consists of three layers which are thin epicarp, succulent mesocarp and endocarp. The endocarp in berry is not hard and stony as in a drupe.

It is soft and fleshy. Examples of berries are tomatoes and orange, melon, guava, pepper and pawpaw. In tomato, the seeds are scattered within the mesocarp and endocarp while in orange, it has distinct chambers separated by sheets of endocarp. It has one seed in one chamber.

 

Similarities Between a Drupe and Berry

• Both have fleshy or succulent mesocarp.
• Both have thin epicarp.
• Both have seed(s).

 

Differences Between a Berry and a Drupe

 

Berry, e.g. Tomato

• Berry has many seeds.
• Mesocarp and endocarp are fused.
• Mesocarp and endocarp fuse to form soft pulp.
• Endocarp is edible.
• Mesocarp is edible.

 

Drupe, e.g. Mango

• Drupe has only one seed.
• Mesocarp and endocarp are not fused.
• Drupe has hard endocarp.
• Endocarp is not edible.
• Mesocarp may or may not be edible.

 

c) Pome: Pome is a simple false fruit in which the skin and the fleshy edible part are derived from the receptacle and only the core enclosing the seeds are from the ovary.

Examples of pome are apples and pear.

 

d) Sorosis: Sorosis is a composite false fruit formed from a dense of florescence. Every part of each flower forms part of the fruit while the stalk or axis of the inflorescence swells to form the core or mass. The pineapple and breadfruits are examples of sorosis.

 

2) Dry Fruits

Meaning: A dry fruit is the type of fruit in which the pericarp becomes dry, hard and woody or fibrous when the fruit ripens. There are many types of dry fruits.

 

Dehiscent and Indehiscent Fruits

Fruits may also be grouped into dehiscent or indehiscent depending on whether their pericarps split when ripe or not.

Practically, all fleshy fruits are indehiscent while dry fruits may be dehiscent or indehiscent.

 

a) Dry Dehiscent Fruits

Dry dehiscent fruits are the fruits which split to release their seeds when ripe, leaving the fruit walls on the plant.

Types of dry dehiscent fruits are legumes, follicle capsule and schizocarp.

i) Legume or Pod: A legume or pod is a simple fruit formed from a superior monocarpous pistil. It is usually long and fattened side ways. When the fruit is ripe, the pericarp dehisces longitudinally along both sides to release the seeds inside it. The seeds are usually arranged along one of the margins of the fruit (marginal placentation). Examples of legumes are found in the legume family such as cowpea, crotolaria, soya beans, pride of Barbados and flamboyant.

 

(ii) Capsule: The fruit of a capsule is formed from a single ovary divided into many independent compactments called carpels. The fruit wall is hard when dry. It splits along many sides, that is, along the point of contact of the carpels. The fruits is thus a simple dry dehiscent capsule. Examples of capsule are castor oil fruit. para-rubber fruit, okra fruit and cotton.

 

iii) Follicle: A follicle is a simple fruit formed from a superior monocarpous pistil. The ripe fruit dehisces longitudinally on one side only (compared with pod which splits on two sides. and capsule which splits on many sides Examples of follicle are the silk cotton and kola).

 

iv) Schizocarp: A schizocarp is formed from an ovary made up of one carpel. The dry fruit has several seeds and breaks up into units, each enclosing one seed. Examples of schizocarp are Desmodium and Cassia.

 

b) Dry Indehiscent Fruits

Dry indehiscent fruits are fruits which do not split open when mature or ripe but usually fall to the ground where the pericarp eventually decays to release the seeds. Such fruits are often very small and are produced in large numbers.

Types of dry indehiscent fruits are nut, caryopsis, achene, cypsela and samara.

i) Nut: The nut is a simple true fruit with a hard woody pericarp. It develops from a superior bicarpous or poly carpous ovary and contains one seed chamber with only one seed. An example of nut is the cashew nut where the actual fruit, the nut is on a fleshy edible receptacle.

 

ii) Caryopsis: Caryopsis is a small, simple fruit.Itdevelops from a single pistil consisting of a single ovary. The pericarp and the seed coat are fused to form a covering over the entire seed. Examples of caryopsis are maize grain, rice, millet and guinea corn.

 

iii) Cypsela: Cypsela is a small, simple true fruit. It develops from a superior monocarpous ovary and contains only one seed. The pericarp is free from the seed coat. Each fruit is a triangular in structure and developed from a single ovary. At maturity, the corolla drops off and the hairy calyx becomes very prominent and forms a parachute (pappus). They spread out on top of the fruit and cause it to be easily buoyed in the air and carried from place to place. The seed coat is hard and does not split open on drying. Examples of cypsela are Tridax fruit and Emilia.

 

iv) Achene: Achene is also a small, simple true fruit. It contains only one seed. Achenes develop from an apocarpous pistil. The pericarp is from the seed coat. Examples of achene are strawberry and sunflower.

 

v) Samara: Samara is a simple true fruit in which the pericarp is extended to form one or more wing-like structures.

It develops from a superior ovary made up of more than one carpel and usually contains one or more seeds. Examples are the combretum and pterocarpus (African rose wood).