Sexual Reproduction in Plants

- In flowering plants, the flower is the reproductive organ which is a specialised shoot consisting of a modified stem and leaves.
- The stem-like part is the pedicel and receptacle, while modified leaves form corolla and calyx.

Structure of a Flower

Parts of a Flower

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- A typical flower consists of the following parts:

a) Calyx

- Made up of sepals.
- They enclose and protect the flower when it is in a bud. Some flowers have an outer whorl made of sepal-like structures called epicalyx.

b) Corolla

- Consists of petals.
- The petals are brightly coloured in insect-pollinated flowers.

c) Androecium

- Is the male part of the flower. It consists of stamens.
- Each stamen consists of a filament whose end has an anther.
- Inside the anther are pollen sacs which contain pollen grains.

d) Gynoecium (pistil)

• Is the female part of the flower.
• It consists of one or more carpels.
• Each carpel consists of an ovary, a sty le and a stigma.
• The ovary contains ovules which become seeds after fertilisation.
• A monocarpous pistil has one carpel e.g. beans.
• A polycarpous pistil has many carpels.
• If the carpes are free, it is called apocarpous as in rose and Bryophyllum.
• In carpels that are fused it is called syncarpous as in Hibiscus.
• A complete flower has all the four floral parts.
• A regular flower can be divided into two halves by any vertical section passing through the centre. e.g. morning glory.
• Irregular flower can be divided into two halves in only one plane e.g. crotalaria.

Pollination

Definition of Pollination

- This is the transfer of pollen grains from the anther to the stigma.

Types of Pollination

• Self pollination is the transfer of pollen grains from the anther of one flower to the stigma of the same flower.
• Cross-pollination is the transfer of pollen grains from the anther of one flower to the stigma of a different flower, of the same species.

Agents of Pollination

• Agents of pollination include wind, insects, birds and mammals.
• Insect pollinators include bees, butterflies and mosquitoes.

Mechanisms that hinder self-pollination

• Stamens ripen early and release their pollen grains before the stigma, mature. This is called protandry e.g. in sunflower.
• The stigma matures earlier and dries before the anthers release the pollen grains.
• This is called protogyny and is common in grasses.
• Self sterility or incompatibility.
• Pollen grains are sterile to the stigma of the same flower, e.g. in maize flower.
• Shorter stamens than pistils.

Fertilisation in Plants

• The pollen grain contains the generative nucleus and a tube nucleus.
• When the pollen grain lands on the stigma, it absorbs nutrient and germinates forming a pollen tube.
• This pollen tube grows through the style pushing its way between the cells.
• It gets nourishment from these cells.
• The tube nucleus occupies the position at the tip of the growing pollen tube.
• The generative nucleus follows behind the tube nucleus, and divides to form two male gamete nuclei.
• The pollen tube enters the ovule through the micropyle.
• When the pollen tube penetrates, the ovule disintegrates and the pollen tube bursts open leaving a clear way for the male nuclei.
• One male nucleus fuses with the egg cell nucleus to form a diploid zygote which develops into an embryo.
• The other male gamete nucleus fuses with the polar nucleus to form a triploid nucleus which forms the primary endosperm.
• This is called double fertilisation.

Changes in a Flower After Fertilisation

• The integuments develops into seed coat (testa).
• The zygote develops into an embryo.
• The triploid nucleus develops into an endosperm.
• The ovules become seeds.
• The ovary develops into a fruit.
• The ovary wall develops into pericarp.
• The style, dries up and falls off leaving a scar.
• The corolla, calyx and stamens dry up and fall off.
• In some the calyx persists.

Fruit formation

• Fruit development without fertilisation is called parthenocarpy
• e.g. as in pineapples and bananas.
• Such fruits do not have seeds.

Classification of Fruits

• False fruits develops from other parts such as calyx, corolla and receptacle,
• e.g. apple and pineapple which develops from an inflorescence.
• True fruits develop from the ovary, e.g. bean fruit (pod).
• True fruits can be divided into fleshy or succulent fruits e.g. berries and drupes and dry fruits.
• The dry ones can be divided into Dehiscent which split open to release seeds and indehiscent which do not open.

Types of fruits

Type of fruit	Structure	Example
Berry Fleshy	Ovary fleshy, thin skinned juicy with many seeds	Tomato, orange
Drupe fleshy	Outer layer fleshy, inner layer hard, endosing one or more seeds	Mango, plum
Pod Dehiscent (dry)	Ovary wall thin, contains many seeds. Splits	Bean, pea
Schizocarp Dehiscent (dry)	The ripe fruit breaks up into small one seeded	Castor oil
Caryopsil Dry	Pericarp and seed coat are fused to form thin covering	Maize grain
Cypsela Dry indehiscent	One seeded fruit. The calyx persists	Bidens, Tridax
Pome	Outer fleshy layer develops from calyx	Pear, apple
Multiple fruit	Formed from several flowers in a cluster	Pineapple
Achene	Ovary wall separated from seed	Sunflower

Placentation

- This is the arrangement of the ovules in an ovary.

a) Marginal Placentation:

- The placenta appears as one ridge on the ovary wall e.g. bean.

b) Parietal Placentation:

- The placenta is on the ridges on ovary wall.
- Ovules are in them e.g. pawpaw.

c) Axile Placentation:

- The placenta is in the centre.
- Ovary is divided into a number of loculi. e.g. orange.

d) Basal Placentation:

- The placenta is formed at the base of the ovary e.g. sunflower.

e) Free Central Placentation:

- Placenta is in the centre of the ovary.
- There are no loculi e.g. in primrose.

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Methods of Fruit and Seed Dispersal

1. Animal Dispersal of Seeds

• Fleshy fruits are eaten by animals.
• Animals are attracted to the fruits by the bright colour, scent or the fact that it is edible.
• The seeds pass through the digestive tract undamaged and are passed out with faeces. E.g. tomatoes and guavas.
• Such seeds have hard, resistant seed coats.
• Others have fruits with hooks or spines that stick on animal fur or on clothes.
• Later the seeds are brushed of or fall off on their own e.g. Bidens pilosa (Black jack).

2. Wind Dispersal of Seeds

• Fruits and seeds are small and light in order to be carried by air currents.
• A fruit that is a capsule e.g. tobacco split or has pores at the top e.g. Mexican poppy.
• The capsule is attached to along stalk when swayed by wind the seeds are released and scattered.
• Some seeds have hairy or feather-like structures which increase their surface area so that they can be blown off by the wind e.g. Sonchus.
• Others have wing-like structures e.g. Jacaranda and Nandi Flame.
• These extensions increase the surface area of fruits and seeds such that they are carried by the wind.

3. Water Dispersal of Seeds

• Fruits like coconut have fibrous mesocarp which is spongy to trap air, the trapped air make the fruit light and buoyant to float on water.
• Plants like water lily produce seeds whose seed coats trap air bubbles.
• The air bubbles make the seeds float on water and are carried away.
• The pericarp and seed coat are waterproof.

4. Self Dispersal (Explosive) Mechanism

• This is seen in pods like bean and pea.
• Pressure inside the pod forces it to open along lines of weakness throwing seeds away from parent plant.