REPRODUCTION IN FLOWERING PLANTS

INTRODUCTION

A flower is a modified part of the stem or side branch in which sexual reproduction occurs. Most flowers have both female and male reproductive organs.

Structure and Function of a Flower.

i. Pedicel (Stalk): Is the part through which the flower is attached to the plant. If there are more than one pedicel, they join to form one stalk called Peduncle.

ii. Receptacle: Is the swollen tip of pedicel, which form the flower base to which the floral part are attached. The floral parts are normally arranged in 4 or 5 whorls (layers) which divide the flower into the following components.

iii.Calyx: is the outer layer whorl that consists of sepals. Sepals are green and leaf like. They enclose and protect the inner part of the flower when it is in a bud. In some flowers (like cremantis and canna lily) sepals attract insects because it is coloured. Free Sepals are called Polysepalous while fused sepals are called gamosepalous.

iv.Corolla: Is composed of brightly coloured and sweetly scented Petals, especially in insects pollinated flowers. Petals of insect pollinated flowers have granular swelling called nectaries that secrete sugary fluid called nectar. Free petals are called Polypetalous while fused petals are called gamopetalous.

The calyx (sepal) and corolla (petals) together they form Perianth.

v. Androecium: Is the male part of the flower. It is made up of stamens, which is composed of slander stalk (filament) bearing of an anther at the top. The anther consists of 4 pollen grains.

vi.Gynoecium: Is the female part of the flower, also called the pistil. It consists of carpels. Each carpel is composed of ovary, style and stigma.

Ovary: Is an expanded hollow base, which contains ovules (female eggs).

Style: A hollow slander stalk which connects the ovary and the stigma.

Stigma: Is a small swelling at the tip of a style.

vii.Epigynous (Inferior) Ovary: When ovary is located below the other floral parts on the receptacle, like Apple, sunflower, guava and canna lily.

viii. Hypogynous (superior) Ovary: When ovary is located above the other floral parts on the receptacle, e.g. Hibiscus and crotalaria.

ix. Inflorescence: A collection of flowers sharing a common stalk, eg. Sunflower, grasses and dahlia. The inflorescence consists of many small flowers called florets.

Terms in relation to Flowers.

 

i.           Half-flower: Is the longitudinal section that shows the cut surface of a flower.

ii.         Monoecious plant: produces both female and male gametes on the same plant but in different flowers, eg. Maize.

iii.       Dioecious plant. Produces only one of the reproductive gamete, i.e. Female and male gametes are on separate plants, e.g. Pawpaw.

iv.       Haemaphrodite plant. The male and female gametes develop on the same flower, eg. Most of the garden flowers.

v.         Complete flower. Is a flower with all the floral whorls, i.e. Stamen, pistil, calyx and corolla, eg. Hibiscus.

vi.       Incomplete flower. Is a flower with one or more whorls missing, eg. Clemantis.

vii.     Unisexual flower. Is a flower in which either male or female part is missing. Examples:-

• Staminated flower has only male organ (androecium)

• Pistilated flower has only female organ (gynaecium).

viii.   Actimorphic flower: is a regular flower, which is radially symmetrical, ie. Can be divided into two equal halves by vertical section.

ix.       Zygomorhich flowers. Are irregular flowers, which may be bilaterally symmetrical, can be divided into tow equal halves on only one particular lane.

Gametogenesis.

Is the process by which male and female gametes are formed.

Male gametes

Formation of male gametes takes place in the anther. The anther contains four chambered pollen sacs in which pollen grain develops. Each pollen sac contains diploid cell called pollen mother cells. Each pollen mother cell undergoes meiosis to form four haploid young pollen grains. The nucleus of each pollen grain divide by mitosis to form two nuclei that is generative nucleus and tube nucleus.

Female gametes.

Formation of female gametes takes place in the ovary. The ovary contains one or more ovules. The ovule grows and bends to form nucleus, funicle and placenta.

The funicle (stalk of the ovule) is attached to the ovary by the Placenta (a cushion of specialized tissue). Nucleus is a uniform mass of a cell. It is enveloped by protective inner and outer integuments. The integument leaves a small opening called a micropyle. The other end of ovule without microphyle is called chalaza. In the centre of young ovule, the embryo-sac mother cell develops. The cell undergoes meiosis to form a row of four haploid cells. Three of these, nearest to micropyle disintergrate and disappear. The remaining one enlarges to form embryo-sac. The nucleus of the embryo-sac undergoes three successful mitotic divisions to form eight daughter nuclei. Four of these migrate to the chalaza and other four migrate near to the micropyle. One nucleus from each group moves to the centre of the embryo-sac to form polar nuclei. The remaining six nuclei become enclosed by cell membranes, hence separate from one another.

This is now a mature embryo sac with six cells (three cells at each end) and two free nuclei at the centre. One of the cells at the micropyle end becomes functional female gamete (egg). The other two called synergids, ie. Non-functional eggs and they eventually disintegrate. The three cells at the chalaza are called antipodal cells (they play little or no role). They disintegrate after fertilization.

Pollination.

Is the transfer of pollen grains from the anther to the stigma.

Types of pollination.

1.      Self-pollination.

Is the transfer of pollen grains from the anther to the stigma of the same flower, or to the stigma of another flower on the same plant.

Characteristics of self-pollination.

Is common in bisexual flowers where stigma and anther ripen at the same time.

Usually stigma grows up towards the anther. Insects also help.

Advances of self-pollination.

Little pollen production because little pollen get lost.

Disadvantages of self pollination

No exchange (mixing) of genetic materials.

No variation hence less chances of survival in a changing environment.

2.      Cross pollination.

Is the transfer of pollen grains from the anther to the stigma of a flower of another plant but of the same species. Cross pollination enables mixing of genetic qualities between different plants, hence hybrid vigour is obtained.

Characteristics of cross pollination.

Is common in dioecious and also in monoecious plants.

**Plants have developed mechanism to avoid self pollination, that is:-

Female and male reproductive organs on separate plants.

Female and male reproductive organs on different flowers of the same plant.

In bisexual flowers, male and female organs mature at different times.

Male and female organ are separate in the flower (stigma higher than the anther).

Advantages of cross pollination.

Mixing of genetic qualities between different parent, hence more viability and quicker adaptation to the new situation.

Offspring with high productivity, more vigorous and resistant to diseases, ie. Hybrid vigour.

Disadvantages of cross pollination.

Depends on wind, insects or birds.

In case of wind pollination, a lot of pollen get lost therefore pollen has to be produced in large amounts.

Agents of pollination.

Agents of pollination are the carriers of pollen grains from the anther to the stigma. These include.

• Water, for aquatic plants.

• Wind for grasses in Savannah.

• Insects like butterflies and bees.

• Animals like bats and birds.

Characteristics of wind pollinated flower.

They produce abundant, smooth, light, and dusty pollen that are easily carried by air currents.

• Anthers are large with long filaments and hang outside the flower.

• Stigmas are feathery with large surface exposed outside the flower.

• They lack nectar and scent.

• Petals and sepals are often small and green or absent.

• Examples are; Maize, grasses, oaks, hazel, plantain and willows.

Characteristics of Insect (birds) pollinated flowers:

• Have large, brightly coloured petals and sepals to attract insects.

• Pollen grains are large, heavy and sticky.

• Have nectaries that secrete sugary fluid (nectar)

• Often sweet scented nectar.

• Both anthers and stigmas are small and located inside the flower to ensure that insects brush against them as they search for nectar.

          Examples are; buttercup, cherry, horse-chestnut, and deadnettle.

Fertilization.

Fertilization is the process by which nucleus from male gamete fuses with nucleus from female gamete to form a zygote. This happens after pollination and it include the following steps:-

Pollen grain of the same species arrive on the stigma and start to germinate. The pollen grain absorbs sugar secreted by the stigma and the style, and the cytoplasm in the grain grows into a pollen tube through the style to the ovary.

Two nuclei in pollen grain;

Tube nucleus controls the growth of pollen tube.

Generative nucleus divides into two male nuclei which pass along the tube to the ovum.

The pollen tube grows in to the ovary by passing through the micropyle.

The pollen tube releases the two male nuclei into the ovule which also has tow nuclei. Here the double fertilization takes place, that is:-

One of the male gamete fuses with the nucleus of functional egg cell to form a diploid zygote (embryo zygote). The second male gamete nucleus fuses with both polar nuclei of the female gamete to form endosperm zygote which develop into the food store of the seed (cotyledons).

Double fertilization is a unique feature in flowering plants, that is, involving tow male gamete nuclei.

Seed formation.

The fertilized ovule known as zygote undergo mitosis to form embryo. Embryo consists of radicle (young root) and plumule (young shoot). The intergument become the seed coat called testa and the inner seed coat the tergmen. In the seed there is a scar called hilum, the point where the seed has been attached to the funicle.

Fruit formation

While the ovule develops into a seed, the ovary develops into a fruit. The ovary wall becomes fruit wall known as pericarp. The pericarp may enclose one or more seeds. It has three layers, ie. Exocarp, mesocarp and endocarp. The function of fruit is to provide food and protect the seeds and help in dispersal.

Parthenocarpy.

Is the process by which some fruits develop without fertilization. This produces seedless fruits like bananas and pineapple.

Fruit formation can be induced by spraying flower with Gibberellins hormone.

Importance of Fruits.

• Protection of seed

• Help in dispersal of seeds: hence;

• Avoids overcrowding

• Helps colonization of new areas.

Dispersal of fruits and seeds.

1.      By splitting (Self dispersal); seeds flung out by sudden splitting apart (explosive mechanism) and spiraling of the two halves of the pod, eg. Lupin, wallflower, beans.

2.      By wind; parachute of hairs catches wind, e.g. Dandelion and sycamore. Adaptation to this type:-

• Are small and light.

• Have wing-like structures.

• Have feather-like projections.

3.      By Animals

Catching on fur or clothes;

Adaptation: sticky and hooked, e.g. Goosegrass and burdock.

Being eaten;

Adaptation: Succulent and palatable. Seeds pass out in the droppings (faeces) unharmed, e.g. Blackberry and strawberry.

4.      By Water

Adaptation. 
Have fibrous mesocarp,

Air pockets and

Water proof coats.

Types of Fruits.

1). False Fruit (Pseudocarp).  These are fruits that develop not only from the ovary but also from other floral parts like receptacle. Eg. Cashew-nuts, apples and pineapples.

2). True Fruits. These are fruits that develop from ovary, e.g. Beans. Types of True Fruits include Simple, Aggregate, and Multiple/composite Fruits.

i. Simple Fruits. They develop from either one ovary or many ovaries fused together. Simple Fruits are divided into three groups.

 Dry indehiscent Fruits. They become dry at maturity but do not split open (dehisce) to release the seeds, eg. Cashew nuts, sunflower and maize.

  Dry-dehiscent Fruits. They split open (dehisce) when they mature and become dry, e.g. Bean, castor oil and simsim.

  Succulent Fruits. Are fleshy fruits and can be eaten, e.g. Mangoes, avocado, oranges, lemons, etc.

ii.  Aggregate Fruits. They develop from a simple flower. Is a cluster of simple fruits developed from a single flower, all simple fruits are attached to the same receptacle, e.g. Strawberries.

iii. Multiple Fruits. They develop from a group of ovaries originating from different flowers, which are closely packed together, e.g. Pineapples and mulberry.

Maize grain is a fruit and not a seed. Why?

• The seed has only one scar, the attachment to the receptacle, the helium.

• The fruit has two scars, the attachment to the receptacle and to the style. Also maize has no micropyle.