Sexual Reproduction in Plants

Flowering plants are diploid and meiosis takes place within the reproductive tissues to produce haploid reproductive structures or spores.

• Meiosis takes place in the anthers to produce the male spores (pollen grains)
• The female spores are the ovules which are made in the ovary. The female gametes develop within the ovule.
• Flowering plants must transfer the pollen grains from the male anther to the female part of the plant of the same spcies.
• The pollen grain has a tough, resistant wall to prevent it from drying out.
• The design of a flower is related to its method of pollination.

The outermost ring of structures is the sepals. They are usually green and protect the flower in bud.

Inside the sepals is the ring of petals. These are brightly coloured to attract insects. They usually have scent.

Inside the petals are the male part of the plant, the stamens. Each stamen consists of a long filament at the end of which are the anthers which produce pollen grains. The filaments support the anthers and conatin vascular tissue, which transport food materials necessary for the formation of pollen grains. The anther is made up of four pollen sacs arranged in two pairs. When mature, the pollen sacs split to release the pollen.

In the centre of the flower are one or more carpels. These are the female part of the flower. Each carpel is a closed structure inside which one or more ovules develop. The lower part of the carpel is called the ovary and bears at its apex a stalk-like structure, the style. This ends in the stigma.

The transfer of pollen grains from the anther to the stigma of a plant of the same species.

There are two types of pollination:

• Self-pollination

In some species, self-pollination occurs and the pollen from the anthers of a flower need only be transferred to the stigma of the same flower or another flower on the same plant.

• Cross-pollination

In the majority of species, cross-pollination occurs where pollen is transferred from the anthers of one flower to the stigma of another flower on another plant of the same species.

Self-pollination results in inbreeding and a consequent reduction in the degree of variation in the population.

There is also a greater chance of two undesirable recessive alleles being brought together at fertilisation.

However, inbreeding preserves good genomes that may be suited to a relatively stable environment.

• Self-pollination --> Self-fertilisation
• Cross-pollination --> Cross-fertilisation

Self-fertilised species depend on random assortment and crossing over to bring about variation

Outbreeding is of greater evolutionary significance because, in the struggle for survival, some genomes are more successful than others.

Insect pollination:

• Depend on insects for pollination
• Insects feed on nectar found in the nectary at the base of the flower
• As the insect enters the flower, the anthers brush against the insect so the pollen becomes stuck to the insect's body
• When the insect enters another flower, its brushes some of the pollen against the stigma, and cross-pollination takes place.

Wind pollination:

• Anthers hang outside the flower so that the wind can blow away the small, smooth and light pollen
• Feathery stigmas also hang outside the flowers and provide a large surface area
• Pollen grains are blown into the path of the stigma.

Insect:

• Colourful petals
• Scent
• Nectar
• Anthers and stigma within flower
• Small quantities of sticky pollen

Wind:

• Small, green and inconspicuous
• Petals absent
• No scent
• No nectar
• Anthers hang outside flower
• Stigma is large and feathery
• Large quantities of small, smooth, light pollen

Development of the male gamete

In the anther, diploid cells undergo meiosis to form haploid pollen grains.

A pollen gran is surrounded by a tough wall that is resistant to desiccation. This enables pollen grains to be transferred from one flower to another without drying out.

Inside the pollen grain, the haploid nucleus undergoes mitosis to produce two nuclei: a generative nucleus and a tube nucleus. The generative nucleus later gives rise to the two male nuclei.

When the pollen is mature, the outer layers of the anther dry out and tensions are set up in lateral grooves. Eventually, dehiscence occurs and the edges of the pollen sacs curl away, exposing the pollen grains.

Development of the female gamete

The ovules are produced in the ovary with the female gamete or egg nucleus developing inside the ovule.

In the ovule, a mother cell undergoes meiosis to produce a haploid embryo sac, within which eight nuclei form by mitosis. The ovule is contained within the ovary.

The process where a male gamete fuses with a female gamete to produce a zygote.

In flowering plants, the ovule is protected within the ovary. The male gamete is the nucleus contained in the pollen grain and can only reach the female nucleus in the ovule by means of a pollen tube.

• When a compatible pollen grain lands on a stigma, the stigma produces a sugary solution in which the pollen grain germinates, producing a pollen tube.
• The pollen tube grows down the style.
• It secretes digestive enzymes as it goes, digesting its way through the tissues of the style.
• The pollen tube nucleus is positioned at the tip of the tube, with the two male nuclei following behind.
• The pollen tube grows through the gap between the integuments, called the micropyle and passes into the embryo sac.
• The pollen tube nucleus disintergrates.
• The tip of the pollen tube bursts open, releasing the male gametes into the embryo sac.

• One of the male gametes fuses with the female nucleus to form a zygote.
• The other male gamete fuses with both polar nuclei to form a triploid endosperm nucleus.
• Thus, a double fertilisation takes place.

Following fertilisation....

• The diploid zygote divides by mitosis to form the embryo
• The embryo consists of a plumule (developing shoot), radicle (developing root) and one or two seed leaves.
• The triploid endosperm nucleus develops into a food store.
• The integuments become the seed coat/testa.
• The ovule becomes the seed.
• The ovary becomes the fruit.

After a period of dormancy, and when environmental factors become favourable, stored food is mobilised and the seed germinates.

The three requirements for germination are:

• A suitable temperature
• Water
• Oxygen

• Food reserves in seeds are insoluble in water.
• So the reserves must be broken down into relatively simple soluble substances which dissolve in water.
• Water is taken up rapidly by the seed, causing the tissues to swell as wellas mobilising the enzymes.
• The seed coat ruptures as the radicle pushes its way through and grows downwards.
• Enzymes hydrolyse food reserves into smaller molecules and these a transported to growing points.
• When the plumule emerges, it unfurls and begins to photosynthesise. By now, the food reserves in the cotyledons will have been depleted.