REPRODUCTIVE SYSTEMS IN PLANTS

The flower is the reproductive structure of flowering plants. It contains the male and female sex organs which enable them to exhibit sexual reproduction. Fertilization inside the flower often leads to the production of seeds which are capable of germinating into new plants.

 

THE STRUCTURE OF A FLOWER

A flower is a cluster of modified leaves which is borne on a shortened stem, the flower stalk or pedicel.

The flower is made up of four floral parts.

These are:

• the calyx.
• the corolla.
• the androecium.
• the gynoecium.

These floral parts are usually arranged in concentric rings, one above the other, on the receptacle or thalamus. The swollen tip of the pedicel is shown in (Fig 29.1 and 29.2).

 

1) The Calyx: The calyx consists of sepals which are usually small and green. This protects the flower which is in the bud. The outermost whorl of a flower is made up of three to five sepals. They may be separated (polysepalous) or joined to form a cup (gamosepalous).

There may be an epicalyx of bract below the calyx as in Hibiscus flower.

 

2) The corolla: The petals, collectively known as the corolla form the second whorl or floral part inside the sepals. In many plants, they are the eye-catching part of the flower. Most flowers have four to ten petals which may be separated (polypetalous) or joined to form a tube (gamopetalous). Generally, petals are brightly coloured and scented which attract pollen-transfering animals (pollinators). When both the petals and sepals look alike, e.g. lilies, they are collectively known as the perianth.

 

3) Androecium: The androecium are the male reproductive organs of a flower. The whorl inside the petals is a group of stamens collectively known as the androecium.

 

Most stamens have:

• a long slender stalk called the filament.
• a swollen end called the anther.

Flowers may have three to numerous stamens which may be free or united. In some flowers, the filaments are united while the anthers are free. In the Hibiscus, the fused filaments form a staminal tube. In the sunflower, the anthers are fused while the filaments are free. Sometimes, the filaments of stamens are attached to petals (epipetalous).

 

Anther and Pollen Grains

The anther is a 2-4 lobed structure. Pollen grains are produced in these lobes within the region called pollen sacs. When the pollen grains mature, the anther lobes split to release them.

Pollen grains are fine yellowish particles. Each pollen grain is made of:

• two coats- a tough protective outer coat called exine and a thin inner coat called intine of cellulose.
• a haploid generative nucleus which gives rise to two male gametes.
• a haploid tube nucleus.

When a pollen grain lands on the mature stigma of the same type of flower, it germinates to form a pollen tube.

 

4) Gynoecium: The gynoecium are the female reproductive organs of a flower. It is the innermost whorl of the floral parts of the flower.

The gynoecium consists of carpels. It may consist of one, a few or many carpels. A separate carpel or a single structure of several fused carpels is called a pistil.

A pistil with a single carpel is described as monocarpous, e.g. flamboyant, while one with two or more carpels is known as polycarpous, e.g. Hibiscus. When the carpels are free from one another, the pistil is said to be apocarpous, e.g. rose flower, but when they are fused together, it is described as syncarpous, e.g. lilies.

Pistils have the followings

• ovary.
• style.
• stigma.

The ovary contains one or more ovoid structures called ovules. Each ovule houses a female gamete or egg cell. After fertilization, the ovary develops into a fruit while the ovules develop into seeds.

 

Parts of a flower

1. Pedicel – attaches the flower to the stem.
2. Receptacle – carries and holds together the other parts of the flower.
3. Sepals – enclose and protect the other floral parts when the flower is in the bud stage. If brightly coloured, they also attract insects, if green, they make plant food (photosynthesis).
4. Petals (corolla) – attract insects which pollinate flower.
5. Filament – holds or carries the anther.
6. Anther – contains the pollen grains.
7. Pollen grains – produce the male gametes that fertilise the ovules.
8. Stigma – receives pollen grains at pollination.
9. Style – connects the stigma to the ovary, and it is the passage for the pollen tube to reach the ovules.
10. Ovary – contains the ovules, develops into fruit.
11. Ovules – produce the female gametes, develop into seeds.

 

TYPES OF OVARY

The position of an ovary in the receptacle dictates how it is classified. An ovary can be described as superior, inferior and half-inferior.

1. Superior ovary: An ovary is described as superior when it is placed above the other floral parts, namely: the calyx, corolla and stamens on the receptacle, e.g. Hibiscus. The flower having this type of superior ovary is described as hypogynous flower.
2. Half inferior ovary: An ovary is described as half inferior when the ovary lies inside a cup-shaped receptacle and other floral parts appear to be attached slightly above it or almost at the same level, e.g. rose flower. The flower having this type of half inferior ovary is described as perigynous flower.
3. Inferior ovary: An ovary is described as inferior when it is placed below the other floral parts, namely the calyx, corolla and stamens on the receptacle. That is, other floral parts are above it on the receptacle, e.g. cana lily , sunflower. The flower having this type of inferior ovary is described as epigynous flower.

 

TYPES OF FLOWERS

1. Inflorescence: Inflorescence is a group of flowers which attached themselves to a common stalk or axis, e.g. pride of Barbados.
2. Solitary flower: A solitary flower is one that is attached singly either to the leaf axis or to the tip of a branch, e.g. Hibiscus and Pawpaw flower.
3. Perfect flower: A perfect flower is one that has both carpels and stamens in it, e.g. Pride of Barbados.
4. Imperfect flower: An imperfect flower is one in which either stamens or carpels are naturally missing, e.g. maize flower.
5. Complete flower: A complete flower is the type that has naturally all the four floral parts namely; calyx, corolla, androecium and gynoecium, e.g. Hibiscus and Pride of Barbados.
6. Incomplete flower: An incomplete flower is the type which lacks one or more of the floral parts, e.g. maize and pawpaw flowers.
7. Regular flower: A flower is regular if it has all members of a whorl on it, i.e ., petals identical in shape and size, and are evenly arranged on the receptacle. Such a flower can be cut vertically into two similar halves through any one of several vertical places (radial symmetry) and is described as actinomorphic flower, e.g. Hibiscus.
8. Irregular flower: An irregular flower is one in which the members of a whorl, e.g. petals, are not similar either because some parts are fused, some are smaller than others or because one or more parts are missing. The flower can be cut vertically into two similar halves through only one plane (bilateral symmetry) and is described as zygomorphic flower, e.g. pride of Barbados and Delonix.
9. Axillary flowers: Axillary flowers are those which are borne in the axils of leaves.
10. Terminal flowers: Terminal flowers are borne at the end of stems or branches.
11. Bisexual flower: A bisexual flower also called a hermaphrodite has both the carpels (female) and stamens (male) on it, e.g. pride of Barbados and Hibiscus.
12. Unisexual flower: A unisexual flower is the type that has either stamens or carpels as its sexual parts. A flower that has only carpels is a female flower and such flower is described as a pistillate, e.g. maize and pawpaw. On the other hand, any flower that has only stamens is a male flower hence it is described as staminate, e.g. pawpaw, maize.
13. Monoecious flower: When male and female flowers are found on the same plant, the plant is said to be monoecious, e.g. oil palm and maize.
14. Dioecious flower: When male and female flowers are found on different plants, the plant is said to be dioecious, e.g. pawpaw.
15. Essential parts of a flower: The stamens and carpels are regarded as the essential parts of the flower because they produce the gametes required for fertilization to take place resulting in the formation of seeds or fruits.
16. Non-essential parts of a flower: The petals and the sepals are regarded as the non-essential parts of a flower because they are not required for gamete production in flowers.

 

PLACENTATION IN FLOWERING PLANTS

Placentation is defined as the arrangement of the ovules within the ovary. There are various ways in which ovules are attached to the ridges of the ovary. These ovules are attached to the ovary by fleshy structures called placentae (singular placenta) through short stalks called funicles.

 

Kinds of Placentation

The various kinds of placentation are:

1. Marginal placentation: In marginal placentation, the ovules are attached to the placenta along one margin of the ovary. Examples are beans, cowpea, pride of Barbados, flamboyant, cassia, crotalaria and Delonix.
2. Parietal placentation: In this arrangement, the ovules are attached to the sides of a syncarpous ovary having a single chamber, e.g. pawpaw.
3. Free-central placentation: In this type of arrangement, the ovules are borne on a knob which projects from the base of the ovary, e.g. cana lily.
4. Axile placentation: In an axile placentation, the carpels of a syncarpous ovary meet in the centre to form the placenta to which the ovules are attached, e.g. tomato.
5. Basal placentation: In basal placentation, the ovules are attached to the base of a syncarpous ovary, e.g. sunflower.