Flower structure

• Flowering plants are known as Angiosperms, and the flowers are their reproductive structures.

  • Most angiosperms are diploid, with meiosis occuring in the reproductive tissues and producing haploid spores which contain gametes.

  • Male spores and pollen grains, and female spores are the embryo sac.

• Most species are hermaphrodite - containing both male and female parts.

• A flower is made up of many structures:

  • At the base is the receptacle, which helps support the flower.

  • The outermost ring of structures is the calyx, which is made up of individual sepals.

    • They are normally green, and protect the flower in bud. They are modified leaves.

    • In lilies, however, sepals are coloured.

  • Inside the calyx is the corolla, a ring of petals.

    • These can range from absent to large, and from pale green to brightly coloured. They are modified leaves.

    • Nectaries, if present, will be at the base of this. They release scented nectars to attract pollinators.

  • Inside the petals are the male parts of the flower, known as the stamen.

    • They consistent of a filament, which supports an anther. This produces the pollen grains.

      • The filament contains vascular tissue, which transports sucrose, mineral ions and water to developing pollen grains.

    • The anther usually contains four pollen sacs arranged in two pairs, side by side. When mature, the sacs dehisce - open up and release pollen.

  • In the centre of the flower is the carpel, which are the female parts of the flower.

    • A carpel is a closed structure in which one or more ovules develop.

    • The lower part of the carpel which surrounds the ovules is the ovary.

    • At it’s tip is the long style, which ends in the receptive surface, the stigma.

Types of cross-pollination

• There are two types of cross-pollination, either done by:

  • Pollinators, such as bees, which are attracted to the large colourful flowers, the scent and their nectar (which is mainly sucrose).

    • They use long tongues to reach the sugary nectar at the base of the flower.

    • This causes the bee to rub against the anther, brushing the sticky and sculptured pollen against the bee.

    • When it enters another flower, the pollen rubs against the ripe stigma, causing cross-pollination.

  • Wind pollinated plants do not have nectar, a scent or bright coloured flowers, relying instead on nature.

    • Their anthers hang outside so the wind can blow any their small, smooth and light pollen.

    • Feathery stigmas hang outside the flowers and provide a large surface area for catching pollen grains.

    • They normally group together in an inflorescence - a cluster of flowers on a branch.
      

Pollination

• This is the transfer of pollen grains from the anther to the mature stigma of a plant of the same species.

• This brings the pollen grains into contact with the female part of the flower, which can result in fertilisation.

Self-pollination

• This is when the pollen from a flower’s anthers is transferred to the mature stigma of the same flower, or another flower on the same plant.

• This is also known as inbreeding.

• It means that genetic variation can only occur via independent assortment, crossing over and mutations.

  • This means they have lower genetic variation, and therefore are more susceptible to natural selection as they are less adaptable.

• This also increases the chances of two harmful recessive alleles from being continued at fertilisation.

• However, it is successful at maintaining successful genomes which are well-suited to stable environments.

• It also requires less energy.

Cross pollination

• This is when pollen from a flower’s anthers are transferred to the mature stigma on another flower on another plant of the same species.

  • Occurs in most Angiosperms.

• This is also known as outbreeding.

• It produces high amounts of genetic variation, as there are two different haploid sets as well as crossing over, independent assortment and mutations.

  • This allows a species a higher chance of survival in a changing environment, as some individuals are likely to have a suitable set of alleles.

• The chance of harmful allele combinations is reduced.

• It requires more energy.

Ensuring cross-pollination

• There is a variety of methods to avoid self-pollination:

  • Dichogamy, which is when the stamen and stigma ripen at different times. There are two types:

    • Protandry - Stamens ripen first, such as in daisies.

    • Protogyny - Stigma ripens first, is rarer, such as in bluebells.

  • The anther is below the stigma to avoid pollen falling on to it, such as in pin-eyed primroses.

  • Genetic incompatibility, where pollen cannot germinate on the stigma of the flower which produced it, such as in red clover.

  • Separate male and female flowers on the same plant, such as maize.

  • Separate male and female plants, such as holly.

Gamete development

Male gametes

Anther structure

• The anther is made up of many structures:

  • The outermost layer is known as the epidermis.

  • In the centre is the vascular strand, which contains xylem and phloem tissue.

  • Around each of the four pollen sacs is multiple layers:

    • The outermost layer is the fibrous layer.

    • Then is the tapetum.

      • This provides nutrients and regulatory molecules to the developing pollen grains.

      • It also has a significant role in the formation of the pollen cell wall, which is chemical resistant and tough.

        • It resists desiccation, so the pollen grains can transfer from one flower to another without drying out.

        • UV light can also not pass through this cell wall, protecting the DNA inside the pollen from mutations. This is especially useful as pollen can be carried at high altitudes.

• In order to release pollen when it is mature, dehiscence occurs:

  • The outer layers of the anther dries out, causing tension in the lateral grooves.

  • This tension then pulls the walls of the anther apart, and the edges of the pollen sacs curl away. This is dehiscence.

  • An opening called the stomium exposes pollen grains, and they are then carried away by pollinators or the wind.

Pollen grains

• This occurs in the pollen sacs of the anther:

  • The diploid pollen mother cell undergo meiosis, forming a tetrad.

    • This tetrad is made up of four haploid cells, which become four pollen grains.

  • These cells then undergo mitosis to produce two nuclei, one generative and one tube nucleus.

  • The generative nucleus then produces two male nuclei by mitosis.

• A pollen grain also has many structures:

  • It has a special cell wall made up of three elements:

    • The outermost layer, the exine. This is a waterproof and tough layer.

    • The innermost layer, the intine. This is a thinner layer.

    • The pits, which are holes in the exine but not the intine. They are used for gas exchange.

  • It has the two nuclei; the generative and tube.

    • The generative splits via mitosis before double fertilisation occurs.

Female gametes

Ovule structure

• There are many structures surrounding the ovule:

  • The ovary wall surrounds the ovary.

  • Attached to this wall by the funicle is the ovule. There can be multiple ovules in one ovary.

  • The ovule is made up of:

    • 2 cell layers known as integuments surround the embryo sac.

      • They have a gap known as the micropyle.

    • The innermost layer is the nucellus, a cell layer which provides nutrients.

    • The centre is the embryo sac.

Embryo sacs

• This occurs in the ovule:

  • A megaspore mother cell, surrounded by the nucellus, undergoes meiosis and makes four megaspores cells.

  • Three of these disintegrate.

  • The remaining cell undergoes 3 rounds of mitosis - from 1 to 2 to 4 to 8. This produces 8 haploid nuclei. They have different names:

    • 2 are polar nuclei, which fuse into 1 polar nuclei, making it a diploid cell. These are in the centre of the embryo sac.

    • 2 synergids. They are located at the bottom of the embryo sac.

    • 1 oosphere, which is the female gamete. It is positioned above and between the synergids.

    • 3 antipodals. They move to the opposite side of the synergids.

Fertilisation

Pollen tube formation

• Pollen tube formation occurs in 6 steps:

  • The pollen lands on the stigma, and germinates in the sucrose. If they are chemically compatible, the tube nucleus begins to make a pollen tube:

  • The nucleus grows the pollen tube out of one of the pits, and leads the pollen tube.

  • The pollen nucleus codes for hydrolases, such as proteases and cellulases to digest through the stigma and the style. Products of digestion are used by the pollen tube.

  • It moves up a gradient chemoattractants such as GABA released by the ovule.

  • It grows through the micropyle and passes into the embryo sac.

  • The pollen tube nucleus then disintegrates, as it has completed it’s function of controlling pollen tube growth.

Double fertilisation

• The pollen tube tip opens, releasing the two male gametes, the generative nuclei, into the embryo sac.

  • One generative nucleus fuses with the oosphere, the female gamete. This forms a diploid zygote.

  • The other generative nucleus fuses with the diploid polar nucleus to form the triploid endosperm nucleus.

    • It then divides rapidly by mitosis, creating endosperm tissue, which takes over from the nucellus in providing nutrition for the embryo.

Fruit and seed development

• The seed develops from the fertilised ovule, and contains an embryonic plant and a food store:

• The zygote:

  • Divides by mitosis into an embryo. This consists of a plumule (the developing shoot), a radicle (the developing root) and one or two cotyledons (seed leaves).

• The endosperm:

  • Develops into a food store for the developing embryo.

• The ovule:

  • The integuments dry out, harden and become waterproof with deposits of lignin.

    • This is known as the seed coat or testa.

  • The micropyle remains as a hole in the seed.

  • The ovule; comprising the embryo, endosperm and testa becomes the seed.

• The ovary:

  • The funicle of the ovule becomes the funcile of the seed.

    • Where it is attached becomes known as the hilum, which is a mark or scar which remains from the funicle.

  • The ovary itself becomes the fruit:

    • It can be sweet, juicy and pigmented like cherries, or dry and hard like almonds.

Fruit and seed structure

Dicotyledons

• Dicotyledons have two seed leaves, or cotyledons, with the embryo lying between them.

  • The endosperm is absorbed into these cotyledons, so therefore it is a non-endospermic seed.

• An example is the broad bean, which has several ovules.

  • After fertilisation, the ovary elongates into a pod, which is the fruit. The broad beans of the seeds.

• Other traits include:

  • A network of leaf veins.

  • Sepals, petals and stamens are in multiples of 4 or 5.

  • Vascular bundles in a ring around stems.

  • Vascular bundles in the centre of roots.

Monocotyledon

• Monocotyledons have one seed leaf, or cotyledon.

  • The endosperm remains as the food store, so it is therefore endospermic.

  • The cotyledon remains small and does not develop further.

  • Seeds then become dormant, with their water content falling below 10% to reduce their metabolic rate.

    • They can remain like this for long time periods and do not germinate until conditions are suitable.

• An example is maize plants, which are similar to other cereal grains and grasses.

  • The testa fuses with the ovary wall, making it a one seeded fruit.

• Other traits include:

  • Leaf veins are parallel.

  • Sepals, petals and stamens are in multiples of 3.

  • Vascular bundles scattered in stems.

  • Vascular bundles are scattered in roots.

Seed dispersal

• The methods by which seeds move away from the parent plant.

• If they germinated too close, the parent plant would outcompete it’s seedling, and it would likely die.

• These have been developed via natural selection, and there is 6 types:

  • Wind:

    • Fruits can either have pores, sails, or parachute-like structures to allow wind dispersal.

    • Seeds are often shaken out when the stem is blown by the wind, and the wind allows them to travel great distances.

    • An example is dandelions.

  

  • Transport:

    • Birds, mammals, reptiles and fish eat seeds, which are dispersed via their faces.

    • Some species can only germinate after scarification has occurred, a process where an animal’s digestive system weakens the testa via acid and enzymes.

    • Tasty fruits have evolved to attract this type of transport, such as cherries.

  

  • Rolling:

    • When the fruit breaks open to release the seed, which falls and rolls across the ground, away from the parent tree.

    • An example is conkers.

  

  • Bursting:

    • When legume pods dry, they split and the seeds scatter.

    • In some species, the pods rotate as they burst open, sending the seeds in many different directions.

    • An example is pea pods.

  

  • Water:

    • Seeds that are developed to be buoyant via air cavities, and float in the water when they fall, and are carried away.

    • An example is coconut trees.

  

  • Carrying:

    • Hooked seeds that attach to animals coats to be carried away.

    • An example is burdock.

Seeds and survival

• There are many evolutionary developments in seeds:

  • Dormancy:

    • Dormant seeds have a low metabolic rate and can survive very cold weather.

    • The water content is reduced below 10% so seeds can survive very dry conditions.

  • Testa:

    • It is chemically resistant, so seeds can survive adverse chemical conditions.

    • It can physically protect the embryo.

  • Dispersal:

    • Seeds can be dispersed great distances to not compete with the parent plant.

    • It allows for the colonisation of new environments.

  • Nutrients:

    • These are provided by cotyledons or the endosperm, and lasts until the embryo can photosynthesise on its own.

  • Inhibitors:

    • Allow germination to occur at a suitable time of year. Vernalisation is the process of these inhibitors being broken down in very cold weather, allowing for germination in the spring.

    • These inhibitors can be in the seeds or the fruits.

Germination

• Germination is the process in which a plant grows from a seed.

  • It lasts until the first photosynthesising leaves are produced, which is when all the endosperm and cotyledon food stores will be used.

• It will occur after a period of dormancy, and when environmental factors are favourable.

Requirements

• A suitable temperature - This is for enzyme action, and varies from species to species, usually between 5-30 degrees.

• Water - Mobilises enzymes for transport via the xylem and phloem, and vacuolates (forms vacuoles) cells, which makes them turgid.

• Oxygen - Aerobic respiration is needed for energy, which is needed for metabolism and growth.

• Lighting - This differs between species, with some seeds needing light to germinate, some needing darkness, and some unbothered.

Dicotyledon/ broad bean germination

• This occurs in 6 steps:

  • When conditions are suitable, water is imbibed (absorbed into pores) rapidly by the seed through the micropyle.

    • The water allows enzyme action and tissues to swell.

  • Food reserves in seeds are insoluble in water, and need to be broken down to be transported to the embryo.

    • Amylose hydrolyses starch to maltose, and protease hydrolyses proteins to amino acids.

  • These soluble products are transported to the embryo.

    • They are also transported to the apical meristems (tissue with unspecialised cells) of the plumule and radicle via the phloem.

    • Rapid cell division then occurs.

    • Some sugars are converted to cellulose for cell wall synthesis.

    • Aerobic respiration releases energy from sugars, and amino acids synthesise proteins.

  • The swollen tissues then rupture the testa, and the radicle emerges.

    • It is positively geotropic (moves in response to gravity) and negatively phototropic (moves in response to light), and therefore grows down into the soil.

  • The plumule then emerges.

    • This is negatively geotropic and positively phototropic, so it grows upwards and through the soil.

  • The part of the plumule between the embryo and cotyledons (which remain below ground) elongates rapidly, pushing the plumule upwards.

    • The plumule is bent over in the shape of a hook as it pushes through, protecting it from damage via soil abrasion.

  • If planted at the correct depth, the plumule hook will straighten and the leaves will unfurl, beginning to photosynthesise.

    • By now food stores in the cotyledons will have been depleted.

Monocotyledons/ barley

• Barley is often used as an example due to the extensive research done by the alcohol industry into its germination.

  • Malting as a term refers to the maltose generated when the starch in barley is digested.

• This occurs in 6 steps:

  • The barley embryo secretes gibberellic acid, a plant growth regulator, which diffuses through the endosperm to the outer aleurone layer.

    • This is a layer of cells which has a high protein content.

  • The gibberellic acid switches on genes in the cells of the aleurone layer, starting transcription and translation, producing enzymes such as protease and amylase.

  • Proteases hydrolyse proteins in the aleurone layer to amino acids, which are used to make amylase.

  • Amylase diffuses out of the aleurone layer to hydrolyse the starch in the endosperm cells.

  • Maltose and glucose produced are diffused through the endosperm to the embryo, therefore the plumule and radicle.

  • These sugars are respired, which fuels biosynthesis and cell division, bringing the seed out of dormancy.