Sexual Reproduction in Plants

The flowering plants or angiosperms are the most sucessful of all terrestrial plants. The flower is the organ of reproduction and usually contains both male and female parts. In angiosperms the female part, the ovule, is never exposed but is enclosed within a modified leaf, the carpel. A key feature of the success of flowering plants is their relationship with animals.

Pollen grains have no power of independent movement and have to be transferred to the female part of the flower to ensure fertilisation. Flowering Plants have evolved the strategy of attracting animals, particularly insects, to their flowers. Some plants are pollinated by the action of wind.

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

• Meiosis takes place in the anther to produce the male spores or pollen grains which contain haploid gametes
• The female spores are the ovules, which are made in the ovary. The female gametes develop inside the ovule.

Flowering plants must transfer the pollen grain from the male anther to the female part of a plant of the species. This is called Pollination. A pollen grain has a tough resistant wall to prevent it from drying out during this transfer. When the male and female gametes fuse it is called Fertilisation. This Fertilised ovule becomes the seed.

A flower is made up of 4 sets of modified leaves arranged from the outside to the centre:

• Outermost ring is Sepals. They are usually green and protect that flower in bud
• Inside the sepals is the ring of Petals. These are brightly coloured to attract insects. They usually have a scent and may produce nectar to attract insects
• Inside the petals are the male parts of the plant, the Stamens. Each stamen consists of a long filament at the end of which are the Anthers which produce pollen grains. As well as supporting the anther the filament contains vascular tissue which transports food materias necessary for the formation of pollen grains. The anther is usually made up of 4 pollen sacs arranged in two pairs, side by side. When mature the pollen sac split to release the pollen
• In the centre of the flower are one or more Carpels. These are the female parts of the flower. Each carpel is a closed structure inside which one or more ovules develop. The lower part of the carpel, which surrounds in the Ovules, is called the Ovary and bears at its apex a stalk-like structure, the Style. This ends in a receptive surface, the Stigma.

Pollination is the transfer of pollen grains from the anther to the stigma of a plant of the same species. Pollination is necessary so that the pollen grains, containing the male gametes, are brought into contact with the female part of the flower so that fertilisation can be achieved. This means that pollen grains must be transferred from the ripe anther to the receptive stigma.

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 a large number 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.

Flowers are highly adapted for cross-pollination by either wind or insects.

Insect - Pollinated Flowers                               Wind - Pollinated Flowers

Brightly Coloured Petals                                      Dull Green Petals

Scented                                                             Not Scented

Nectar                                                               No Nectar

Large Petals                                                      Small Petals 

Small Quantity of Pollen Produced                       Large Quantity of Pollen Produced     

Sticky and Large Pollen                                      Smooth and Small Pollen            

Anthers and Stigma enclosed in Petals                Anthers and Stigma enclosed in Petals

Stigmas are Small and Round                             Stigmas are Large, Long and Feathery

In Insect Pollination, for example Bees:

• Feed on suagry nectar using their long tongues to reach the nectaries at the base of the female part of the flower.
• As bee enters the flowers, anthers brush against the back of the bee leaving sticky pollen behind.
• When the bee enters another flower, it brushes some of the pollen against the ripe stigma and cross pollination has taken place.

• In Wind Pollinated Flowers :
• The Anthers hang outside the flower so that the wind can blow away small, smooth and light pollen.
• The feathery stigmas hang outside the flowers and provide a large surface area for catching pollen grains that are blown into their path.

Self-pollination results in in-breeding 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 fertilistation.

However, there are advantages to inbreeding because it can preserve good genomes which may be suited ti a relatively stable environment.

Therefore the two forms of pollination have very different genetic consequences:

• Self -pollination leads to self -fertilisation, cross -pollination to cross- fertilisation.
• Self- fertilised species depend on random assortment and crossing over during meiosis, and on on mutation to bring about variation in the genomes of male and female gametes.
• Self -fertilised species display less genetic variation than cross- fertilised species that are produced from gametes from two different individuals.
• Outbreeding is of greater evolutionary significance because in the struggle for survival some genomes are more succesful than others.

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 the stigma, the stigma produces a sugary solution in which the pollen grain germinates, producing a pollen tube.
• The pollen tube grows down the style,secreting enzymes as it goes, digesting its way through the tissues of the style. It may also gain nutrients from the digested products.
• The pollen tube nucleus is positioned at the tip of the tube, with the two male nuclei close behind.
• The pollen tube grows through the gap between the integuments, called the microphyle, and passes into the embryo sac.
• The pollen tube nucleus disintegrates presumably having completed its function of controlling the growth of the pollen tube.
• The tip of the pollen tube bursts open releasing the male gamete into the embryo sac and the two male nuclei enter.
• One of the male gamete fuses with the female nucleus to form a zygote.
• Other male gamete fuses with both polar nuclei to form a triploid endosperm nucleus.
• Thus, a double fertilisation occurs, a process unique to flowering plants.

Following fertilisation, the development of the dedd and fruit takes place.

The seed develops from the fertilised ovule and contains an embryonic plant and a food store.

• The diploid zygote divides by mitosis to form the embryo,
• Consisting of a plumule ( devolping shoot )
• A radicle ( developing root ) and
• One or two seed leaves or cotyledons
• The triploid endosperm nucleus develops into a food store to provide reserves for the developing embryo
• The Integuements become the seed coat or Testa
• The Ovule becomes the seed
• The Ovary becomes the fruit

The Broad Bean is classed as a dicotyledon(has two seed leaves or cotyledons) whereas the maize is classed as a monocotyledon (has only one cotyledon). In the Broad Bean the food store has been absorbed into the coteyldon. In the maize,typically of cereal grains,the food store surrounds the seed leaves. The maize is a fruit and not a seed.

After a period of dormancy and when environmental factors are favourable, stored food will be mobilised and the seed will germinate.

The three main requirements for successful germination are :-

• A Suitable Temperature - Optimum Temperature for germination is the optimum for the enzymes involved in the process of germination. The temperature varies from species to species.
• Water - Mobilisation of enzymes, vacuolation of cells and for transport .
• Oxygen - Aerobic Respiration makes energy, in the form of ATP, avaliable for metabolism and growth.

• Food reserves in seeds are insoluble in water and cannot as such be transported in the seedling
• The reserves must be broken down into relatively simple soluble substances which dissolve in water and are then transported to the growning apices of the young shoot or plumule and the young root radicle
• Water is taken up rapidly by the seed in the initial stages, causing the tissues to swell as well as mobilising the enzymes
• The seed coat ruptures as the radicle pushes its way through first. The radicle will grow downwards and the plumule upwards.
• The enzyme, amyalse, hydrolyses starch into maltose, which is transported to growing points.
• During Germination the cotyledons of the broad ben remain below ground.
• The plumule is bent over in the shape of a hook as it pushes its way up through the soil. This protects the tip from damage by soil abrasion.
• If the seed has been planted at the corrrect depth in the soil, when the plumule emerges it unfurls and begins to make food for itself by photosynthesis. By now the food reserves in the cotyledons will have been depleted.