Sexual reproduction in flowering plants

The flower and its parts

A flower is a shoot that has been adapted for sexual reproduction, and its parts are arranged in rings on the swollen tip of the stalk, the receptacle. From the outside in they are the sepals, the petals, the stamens and, at the centre, one or more carpels. The sepals enclose and protect the flower while it is a bud. The petals of an insect-pollinated flower are usually large and coloured, and they attract the pollinators; a wind-pollinated grass has small, dull petals or none.

The stamen is the male part. It is a stalk, the filament, carrying an anther at its tip, and the anther is where pollen grains are made. The filament holds the anther where pollen will be picked up by an insect or caught by the wind, and carries water and nutrients to it in its vascular tissue.

The carpel is the female part. At its top is the stigma, the surface that receives pollen; below that is the style, the column a pollen tube grows down; and at the base is the ovary, a hollow chamber holding one or more ovules. Each ovule will contain one egg cell, and after fertilisation each ovule becomes a seed and the ovary becomes the fruit.

Describe the drawing

A flower cut in half from top to bottom. At the centre is the carpel: a stigma at the top with pollen grains on it, a style running down from it, and an ovary at the base containing four ovules, each attached to the ovary wall. On either side a stamen rises from the receptacle: a thin filament carrying an anther at its tip. Petals curve outwards behind the stamens and small sepals sit below them on the receptacle, which joins the stalk. The stamen is the anther and filament; the carpel is the stigma, style and ovary. Pollen is made in the anther and the egg cell in the ovule.

Name the exact structure rather than the whole organ. Pollen is made in the anther, not in the stamen as a whole, and an egg cell is made in an ovule, which is inside the ovary. A flower with both stamens and carpels, like the one drawn, can in principle pollinate itself. Self-pollination moves pollen to a stigma of the same flower or the same plant; cross-pollination moves it to a stigma on another plant of the same species, and gives more genetic variation in the offspring.

Stamen

The male part of a flower: a filament carrying an anther, in which pollen grains are made.

Carpel

The female part of a flower: a stigma, a style and an ovary containing one or more ovules.

Pollination

The transfer of pollen grains from an anther to a stigma.

Micropyle

The small gap in the integuments at one end of an ovule, through which a pollen tube usually enters.

How a pollen grain forms in the anther

A typical anther has four pollen sacs, and each is lined by a layer of cells called the tapetum, which supplies nutrients to the cells developing inside it and contributes material to their walls. Filling the sac are diploid pollen mother cells.

Each pollen mother cell divides by meiosis to give four haploid cells, which stay together for a time as a tetrad. These are the microspores, and the four of them are genetically different from one another and from the parent for the reasons given in meiosis and the sources of genetic variation: crossing over and independent assortment.

The tetrad separates, and each microspore develops into a pollen grain. Its haploid nucleus divides by mitosis into two: a large tube nucleus and a smaller generative nucleus. Both are haploid and both carry identical alleles, because they come from one nucleus by mitosis. At the same time a two-layered wall forms around the grain. The inner layer is the intine. The outer layer, the exine, is made largely of sporopollenin, which is very resistant to decay, and is thin at one or more pores where a pollen tube can later emerge.

Describe the drawing

Two rows. In the anther, a diploid pollen mother cell divides by meiosis into a tetrad of four haploid microspores. Each microspore's nucleus divides by mitosis, giving a pollen grain with a tough outer wall, the exine, a large tube nucleus and a smaller generative nucleus, which later divides by mitosis into two male nuclei. In the ovule, a diploid megaspore mother cell divides by meiosis into a row of four haploid megaspores, three of which degenerate. The survivor's nucleus divides by mitosis three times, giving an embryo sac of eight haploid nuclei: an egg cell and two synergids at one end, two polar nuclei in the middle and three antipodal cells at the other end.

When the pollen is mature the anther wall dries and splits open along a line on each side, which is called dehiscence, and the grains are released. The generative nucleus divides once more by mitosis to give two male nuclei, the male gametes. In many species this happens in the pollen tube after the grain has germinated; in some, the grasses among them, it happens inside the grain before it is released. Either way the result is the same: two haploid male nuclei, genetically identical to each other.

A pollen grain is not itself a gamete. It is a small haploid structure that carries the male gametes and delivers them, which is why a question about the male gamete of a flowering plant is answered with the male nucleus, not the pollen grain.

Tube nucleus

The larger of the two nuclei formed when a microspore's nucleus divides by mitosis; it controls the growth of the pollen tube.

Generative nucleus

The smaller nucleus of a pollen grain, which divides by mitosis to give the two male nuclei.

How the embryo sac forms in the ovule

An ovule is attached to the inside of the ovary wall by a short stalk. Its central tissue is the nucellus, and around that are one or two protective layers, the integuments, which do not quite close at one end. The gap they leave is the micropyle.

One cell of the nucellus near the micropyle enlarges to become the diploid megaspore mother cell (sometimes called the embryo sac mother cell). It divides by meiosis to give a row of four haploid megaspores. Three of them degenerate, and in most flowering plants the one that survives is the one furthest from the micropyle.

The surviving megaspore grows, taking over much of the nucellus, and its nucleus divides by mitosis three times without the cell dividing, giving 2, then 4, then 8 haploid nuclei. Four lie at each end at first, and then one from each end moves to the centre. Cell membranes and walls form around six of the others, and the result is the mature embryo sac:

So the embryo sac has eight haploid nuclei in seven cells, since the two polar nuclei share the central cell. This pattern is found in most flowering plants and is the one the specification expects; a minority of species build their embryo sac a little differently. In some species the two polar nuclei fuse before fertilisation into a single diploid nucleus, which changes nothing about the endosperm being triploid in the end.

Compare the two pathways side by side. Both begin with one diploid mother cell dividing by meiosis, and both then use only mitosis. The difference is in what is kept: in the anther all four products of meiosis become pollen grains, while in the ovule three of the four are lost and the fourth produces a whole embryo sac.

Embryo sac

The structure inside an ovule, formed from one megaspore by three mitotic divisions, holding the egg cell, two synergids, two polar nuclei and three antipodal cells.

Pollination and the pollen tube

Pollination is the transfer of pollen from an anther to a stigma. It is not fertilisation, and the two are separated by the growth of a pollen tube that, in a long style, can take hours or more.

A pollen grain that lands on a stigma of the same species absorbs water and takes in the sugary fluid the stigma secretes, and it germinates: a pollen tube grows out through one of the pores in the exine. The tube grows only at its tip, and it grows through the stigma and down the style towards the ovary. Pollen on the stigma of a different species usually fails at this stage, because recognition between the grain and the stigma does not happen.

Four parts of the pollen grain each have a role in getting the male nuclei from the stigma to the embryo sac.

The pollen tube is the route. It is an outgrowth of the pollen grain's own cell, so the nuclei and cytoplasm move down inside it, protected from drying out, and it can be many times longer than the grain it came from.

The tube nucleus stays near the growing tip and controls the tube's growth. It is the nucleus whose genes are transcribed to make the proteins the tube needs, the hydrolytic enzymes among them. Once the tube has reached the embryo sac its work is done and it degenerates.

The enzymes are secreted from the tip of the tube. They are hydrolytic enzymes, including enzymes that break down pectin, and they digest a path through the tissues of the style. The products of that digestion are absorbed by the tube and used, with the sugars taken up from the stigma, as respiratory substrates and as raw material for new tube wall.

The generative nucleus follows behind the tube nucleus and, if it has not already done so, divides by mitosis to give the two male nuclei. Neither male nucleus moves itself: they are carried down by the tube's growth.

The tube's direction is set by chemicals. Chemicals released from the ovule, from the synergids in particular, attract the tip, so the tube grows towards them, which is chemotropism (see plant responses). The tube reaches the ovary, enters an ovule through the micropyle, passes through the nucellus and enters one of the two synergids. Its tip then bursts and releases the two male nuclei into the embryo sac.

Double fertilisation

Both male nuclei are used, and each fuses with a different partner. That is what the word double refers to: two fusions, in one embryo sac, at about the same time.

Describe the drawing

An ovule in section, wrapped in two integuments that leave a gap at the top, the micropyle, and filled with nucellus tissue. Inside it lies the embryo sac: two synergids and the egg cell at the micropyle end, two polar nuclei in the middle and three antipodal cells at the far end. A pollen tube grows down through the micropyle and into one synergid. The tube nucleus is near the tip of the tube, with the two haploid male nuclei just behind it. A box below gives the two fusions: one male nucleus with the egg nucleus makes a diploid zygote, which becomes the embryo, and the other male nucleus with the two polar nuclei makes a triploid endosperm nucleus, which becomes the food store.

One male nucleus fuses with the nucleus of the egg cell. Haploid plus haploid gives a diploid zygote, and the zygote divides by mitosis to become the embryo of the seed.

The other male nucleus fuses with the two polar nuclei in the central cell. Three haploid nuclei fuse, so the result is a triploid endosperm nucleus, with three sets of chromosomes. It divides repeatedly by mitosis to form the endosperm, a tissue that stores food, mainly starch, and in many seeds oil and protein as well, for the embryo to use as it grows.

The synergids and the antipodal cells take no part in either fusion and usually degenerate. The endosperm forms only once a male nucleus has reached the central cell, and the two male nuclei arrive together, so endosperm normally develops only in an embryo sac whose egg is also being fertilised. A plant does not build food stores in ovules whose eggs were never fertilised.

Double fertilisation

The fusion, in the embryo sac of a flowering plant, of one male nucleus with the egg nucleus to form a zygote and of the other with the two polar nuclei to form a triploid endosperm nucleus.

Endosperm

The triploid food-storing tissue of a seed, formed by mitosis from the nucleus produced when a male nucleus fuses with the two polar nuclei.

From ovule to seed, and ovary to fruit

After fertilisation the parts of the flower take on new roles. The zygote divides by mitosis into an embryo with a young root, the radicle, a young shoot, the plumule, and one or two seed leaves, the cotyledons. The endosperm grows around it. The integuments harden into the seed coat, the testa, and the micropyle survives as a tiny pore in it, through which water can enter when the seed germinates. The whole ovule is now a seed, and the ovary wall develops into the fruit around it. The petals, stamens, stigma and style usually wither.

Seeds differ in where the food finally ends up. In cereals such as maize, wheat and barley the endosperm persists and is the main store in the ripe grain, which is why a germinating barley grain digests endosperm starch with amylase made under the control of gibberellin. In peas and beans the endosperm is absorbed by the growing embryo before the seed is ripe, and the food is stored in two large cotyledons instead.

Core practical: sucrose concentration and pollen tube growth

Pollen germinates and grows a tube in a sucrose solution on a microscope slide, which makes the growth of the tube easy to watch and to measure. The core practical asks how the concentration of sucrose affects it.

A typical method: make up a series of sucrose solutions, for example 0, 5, 10, 15, 20 and 25 g per 100 cm³ of water. Place one drop of each on its own labelled cavity slide. Transfer pollen from a freshly opened anther to each drop by touching the anther to the surface or with a fine paintbrush, and add a coverslip to limit evaporation, which would raise the concentration. Keep all the slides at the same temperature, and examine each under the microscope at fixed times, for example every 15 minutes for an hour.

There are two dependent variables to choose from. For germination, count the grains in a field of view and the number of those with a tube; a common rule is that a grain has germinated once its tube is longer than the grain's diameter. Express the result as a percentage and count several fields per slide. For growth, measure the length of a set number of tubes with an eyepiece graticule calibrated against a stage micrometer, and calculate the mean.

Control variables: the species, and the age of the flowers the pollen came from; the temperature; the volume of each drop; the time at which each slide is examined; and the criterion for counting a grain as germinated. Some recipes add a small amount of boric acid or a calcium salt, which pollen tubes need; if yours does, every solution must contain the same amount.

The usual result is an optimum. In distilled water or very dilute sucrose, the water potential outside is higher than inside the grain, water enters by osmosis, and many grains or tube tips burst. At high concentrations the outside water potential is lower, water leaves, and tubes grow little or not at all. In between, the tube takes in water without bursting and the sucrose is also a respiratory substrate for its growth. Where the optimum falls varies between species, so do not quote a figure as a general answer.

Safety. Pollen can trigger allergies, so a student with hay fever or asthma should avoid handling it directly, and everyone should wash their hands afterwards. Microscope slides and coverslips break into sharp fragments. Boric acid is classified as toxic to reproduction, so if the medium contains it, use the solution prepared for you rather than the solid, and follow the CLEAPSS Hazcard and your school's risk assessment.