Plant Reproduction: WJEC A Level Biology Notes

Lesson 1 of 24

What are the parts of a flower and what does each one do?

These notes cover topic 4.2, Sexual reproduction in plants, in WJEC A level Biology Unit 4: , Inheritance and Options. Each lesson follows the WJEC specification, and the exam tips come from BioWelsh analysis of WJEC past papers.

A flower is a shoot whose leaves have been modified for sexual reproduction. It makes gametes, brings them together and then protects and feeds the embryo that results. Like an animal, a flowering plant makes gametes and fuses them to give a . Unlike an animal, it cannot move, so getting the male to the female is the problem the whole flower is built to solve.

The parts of a flower are arranged in four rings, called , attached to the swollen top of the flower stalk. Working from the outside inwards they are the , the , the and the .

: the outer ring of . are usually small and green and protect the flower while it is still a bud.
: the ring of inside the . In an insect-pollinated flower these are large and coloured.
: the male parts. Each is an on a stalk called a .
: the female part, in the centre: a on top of a , with a swollen at the base.

The diagram below shows a generalised insect-pollinated flower cut in half vertically, so you can see inside the .

Labelled half flower. At the bottom a green stalk widens into a green oval receptacle, with two green sepals curving down and outwards from its sides. Two large pink petals sweep upwards and outwards from the receptacle to form a wide open bowl. A small gold oval sits at the base of each petal. Above the receptacle a green ovary holds two cream ovules, each containing a purple oval, joined to the base by short green stalks. A narrow green style runs up from the ovary to a dark pink oval stigma fringed with short hairs. Four gold anthers on curving gold filaments stand inside the ring of petals. Labels name the stigma, style, anther, filament, petal, sepal, ovary, ovule, nectary and receptacle. — A generalised insect-pollinated flower cut in half vertically. Original BioWelsh diagram.

The at the bottom is the swollen end of the flower stalk that all four are attached to.
Each contains four . are made inside them.
The is the sticky, receptive surface that catches . The holds it up and the grows down through it.
The is a closed chamber. Inside it are the , and inside each is the that holds the female .
A at the base of the secretes , a sugary solution that rewards the pollinator.

The parts of a flower and what each one does
Part	Function
()	Protects the flower while it is in bud
()	Attracts pollinators by colour, scent and shape; gives the insect a landing platform
	Contains four , in which are made
	Holds the in position and carries sucrose and water to it
	Catches ; its secretions make a compatible
	The tissue the grows down on its way to the
	Contains the ; becomes the fruit after fertilisation
	Contains the and the female ; becomes the seed
	Secretes , the reward that brings the pollinator to the flower

The photograph below shows a broad bean flower, which is pollinated by bumblebees.

A flower with white petals growing from a thick green stem among broad green leaves. One petal stands upright behind, and two lower petals each carry a large dark brown patch. Small pale green sepals are visible where the flower joins the stem. More flowers with the same dark patches are out of focus behind. — A broad bean flower, which is pollinated by bumblebees. Photo: Daniel Felic, Public domain.

The large white are the . The dark patch on each side is a marking that guides insects in.
The small green structures at the base are the , which made up the outside of the bud.
The and are hidden inside the , so an insect has to push in to reach the and picks up as it does so.

Lesson 2 of 24

How is an insect-pollinated flower adapted to its pollinator?

An insect-pollinated flower does not scatter at random. It pays an animal to carry it. Everything about the flower is built round making that transfer reliable, so relatively little is needed.

Large, brightly coloured that an insect can see from a distance, often with lines or patches that point towards the centre.
Scent, which carries further than colour and works at night for moths.
at the base of the , secreting a sugary solution. The insect has to push past the and to reach it.
and inside the flower, positioned so that they brush against the insect's body.
A sticky with a small surface area, because is delivered directly on to it.
Large, heavy, sticky with a spiky or sculptured surface that catches in the hairs of the insect.
Small quantities of , because so little is wasted.

Making and large costs the plant energy and carbon. It is worth it because an insect that has learned to feed on one usually visits others of the same next, so the arrives on a that can use it. Scattering on the wind would need far more to get the same result.

Lesson 3 of 24

How is a wind-pollinated flower adapted to the wind?

A wind-pollinated flower does not pay anything to be visited. It puts its resources into instead, and relies on chance. Because the chance of any one grain landing on a of the right is tiny, it makes enormous quantities of very small, light grains.

The diagram below shows a generalised wind-pollinated flower.

Labelled diagram of a green stem with a small green structure at the top in place of petals, above which sits a green ovary. From the ovary a short stalk rises and divides into two long pink stigmas that curve upwards and outwards, each fringed with fine side branches. Four gold anthers hang well below and to the sides on long thin gold filaments. Small gold circles drift away from each anther. Labels name the feathery stigma, the anther on a long filament, the pollen grains, the ovary and the small green perianth. — A generalised wind-pollinated flower. Original BioWelsh diagram.

There are no large coloured and no scent or . The whole flower is small and green or brown.
The hang outside the flower on long, thin, flexible , so the slightest breeze shakes the out.
The are large and feathery and also hang outside, giving a big surface area to catch grains from the air.
The are small, light and smooth, with no spines to catch on anything.

The photograph below shows cock's-foot grass in flower.

A green flowering grass head held between finger and thumb against a deep blue sky. Dozens of pale yellow-green anthers, each shaped like a narrow bar, dangle from the green spikelets on threads so fine they are barely visible. There are no petals. Green fields and a tree are blurred in the background. — Cock's-foot grass in flower, with the anthers hanging outside. Photo: Harry Rose, CC BY 2.0. Resized.

The pale yellow-green dangle from the spikelets on so thin that they bend under the weight of the .
There are no at all, so nothing blocks the as it leaves.
Each is free to swing, which is what shakes the into the air.

The photograph below shows a ribwort plantain spike, another wind-pollinated flower.

A green conical flower spike on a ribbed green stalk against a pale blue sky. Round the lower part of the spike is a dense ring of cream anthers on fine white filaments, with white thread-like structures among them. The upper part of the spike is still tightly closed and green with brown tips. Two more spikes are blurred behind. — A ribwort plantain flower spike, showing the ring of anthers and the feathery stigmas. Photo: Harry Rose, CC BY 2.0. Resized.

A ring of cream on long sticks out all round the spike.
The white, feathery structures among them are the .
The flowers open a ring at a time, working up the spike, so the and the of one flower are ripe at different times.

Comparing insect-pollinated and wind-pollinated flowers
Feature	Insect-pollinated	Wind-pollinated
	Large, brightly coloured, often with guide markings	Small and green, or absent
Scent and	Present	Absent
	Inside the flower, firmly attached	Hanging outside on long
	Small, sticky, inside the flower	Large and feathery, hanging outside
	Large, heavy, sticky, sculptured	Small, light, smooth
Quantity of	Small	Very large

Lesson 4 of 24

What can a pollen grain tell you about pollination?

A is not the male . It is a tiny structure that contains two and carries them safely from one flower to another. The two male gametes are made inside it.

The diagram below shows a mature .

Labelled circular diagram. The outer ring is gold and edged with short spikes all round. Inside it is a narrow pale ring, then a cream interior. A purple oval lies above and left of centre and a smaller pink oval below and right of it. A short dashed arc near the bottom of the outer ring marks a gap. Labels name the exine, the intine, the tube nucleus, the generative nucleus and the pore. — The structure of a mature pollen grain. Original BioWelsh diagram.

The outer wall is the . It is made of , an extremely tough and chemically polymer, and it is the sculptured part you see under the microscope.
Inside the is a thin inner wall of cellulose, the .
The controls the growth of the when the grain germinates.
The divides by mitosis into the two male gametes.
There is a thinner region, or pore, in the where the emerges.

The of an insect-pollinated grain and that of a wind-pollinated grain look completely different. The diagram below compares them at the same scale.

Two gold circles side by side on a white background. The left circle is large, ringed with about twenty short spikes, and has several small paler patches on its surface. The right circle is less than half the width and has a smooth outline with one small pale patch. Text under the left circle reads large, heavy and sticky, sculptured surface with spines, made in small quantities, about 60 micrometres across. Text under the right reads small, light and smooth, no spines to catch on an insect, made in very large quantities, about 25 micrometres across. — Pollen from an insect-pollinated and a wind-pollinated flower, drawn to the same scale. Original BioWelsh diagram.

The insect-pollinated grain is large, roughly 60 µm across, and covered with spines and ridges that hook into the hairs of an insect.
The wind-pollinated grain is much smaller, roughly 25 µm across, and smooth, so nothing slows it in the air.
Size matters as well as texture. A smaller, lighter grain stays airborne for longer.

The scanning electron micrograph below shows lily , from an insect-pollinated flower.

Grey scanning electron micrograph of three oval grains on a smooth grey background. Each grain is covered with a honeycomb network of raised ridges enclosing shallow rounded pits. The grain on the right has collapsed inwards along a deep groove running down its length. A scale bar at the bottom left is labelled 20 micrometres. — Scanning electron micrograph of lily pollen grains. Photo: No specific author, Public domain.

The surface is covered in a network of ridges around shallow pits, which is typical of sculptured .
The scale bar at the bottom left reads 20 µm, so each grain is roughly 80 µm long.
One grain has collapsed inwards along a groove, showing that the is a shell around a living cell.

The scanning electron micrograph below shows the of one grain at a much higher .

Grey scanning electron micrograph filling the whole frame. The surface is built from thousands of short club-shaped rods standing on end, which join near their tips into rings that enclose dark circular gaps. The pattern repeats evenly across a curved surface that falls away into shadow at the lower right. — Scanning electron micrograph of the sculptured exine of a pollen grain. Photo: Lukas Herold, CC BY 4.0. Resized.

The is not solid. It is built from club-shaped rods that support a patterned outer layer.
The pattern is the same for every grain of a , which is why can be used to identify a plant.
resists decay, so survive in peat and lake sediments for thousands of years.

Lesson 5 of 24

What do you see in a transverse section of an anther?

Cut an across and it looks like a butterfly. Each wing is one lobe, and each lobe contains two , so there are four in every . The two lobes are joined by a block of tissue called the , which carries the vascular strand.

The diagram below shows an in transverse section.

Labelled diagram shaped like a butterfly. Two rounded lobes are joined in the middle by a narrow band of pale tissue holding a green circle with a blue and a pink dot inside it. Each lobe contains two cream ovals, each ringed by a thick pale gold band and containing seven small gold circles. A gold band follows the inside of each lobe's outer wall, and a V-shaped notch cuts into the middle of that wall between the two ovals. Labels name the epidermis, the fibrous layer, the pollen sac, the tapetum, the vascular strand, the lateral groove and the parenchyma of the connective. — A transverse section of an anther. Original BioWelsh diagram.

The whole is covered by an epidermis, a single outer layer of cells.
Just inside the epidermis is the , whose cells have thickened walls. Drying of these cells is what pulls the open.
Each of the four is a cavity in which the develop.
The lines each . Its cells break down as the matures and supply nutrients to the developing grains.
The in the middle contains parenchyma and the vascular strand, which brings sucrose, mineral ions and water from the .
A groove runs down the outer side of each lobe, between its two . This is where the splits open.

The layers of an anther and what each one does
Layer	Function
Epidermis	Protects the and reduces water loss
	Thickened cells that shrink as they dry and put the wall under tension, opening the
	Nourishes the developing and supplies material for the
	The cavity in which develop; there are four in each
(parenchyma)	Joins the two lobes and holds the vascular strand
Vascular strand	Brings sucrose, mineral ions and water to the developing

Lesson 6 of 24

How are the male gametes made?

Inside each is a layer of . Everything that follows is one round of meiosis and then two rounds of mitosis.

The diagram below follows one all the way to two male gametes.

Flow diagram of four stages joined by downward arrows labelled meiosis, mitosis and mitosis. The top row is one purple oval with a white centre, labelled pollen mother cell 2n. The second row is four gold circles with white centres, labelled a tetrad of four haploid cells, each becoming one pollen grain. The third row is one large cream circle containing a purple oval and a smaller pink oval. The fourth row is the same cream circle containing a purple oval and two small pink ovals. Text at the bottom reads that meiosis halves the chromosome number once and everything after that is mitosis. — Development of the male gametes in a pollen sac. Original BioWelsh diagram.

Each (2n) divides by meiosis.
Meiosis gives four cells held together in a group called a . Because they came from meiosis, all four are genetically different.
The four cells separate. Each one develops a tough , fed by the , and becomes a .
The of each grain divides by mitosis into a and a .
The divides by mitosis again into two male gametes. In many this second division happens only after the grain has landed on a .

The light micrograph below shows inside a .

Pale olive-green light micrograph. Four or five rounded cells are grouped near the centre, and each contains four separate darker reddish-brown bodies packed together inside a common boundary. The surrounding field is covered with many small pale circles. A single darker golden-brown body with a ridged surface lies alone at the left. — Light micrograph of tetrads in a pollen sac. Photo: Massimo Vischi, CC BY 4.0. Resized.

Each pale cell contains four darker rounded bodies. Those four are the products of one meiosis.
The four are still held inside the wall of the original mother cell, which is why they are grouped like this.
A of four cells is the clearest evidence in a slide that meiosis has taken place.

Lesson 7 of 24

How does an anther release its pollen?

Releasing the is called . There is no muscle and no active process: the opens because its own tissues dry out.

The diagram below shows the three stages.

Three panels joined by arrows. The first, headed before dehiscence, shows a closed oval anther containing two cream sacs full of gold circles, with a dashed line down the middle. The second, headed walls dry out, shows the same anther with four arrows pressing inwards from the sides and the word tension above it. The third, headed stomium splits, shows the two halves of the wall curled back like open lips, with gold circles escaping upwards on both sides and the word stomium below. — How an anther opens and releases its pollen. Original BioWelsh diagram.

In the closed the are full and the outer walls are moist.
As the matures it loses water. The thickened cells of the shrink and pull on the wall, putting it under tension.
The wall finally tears along the line of weakness between the two of each lobe. This line is the , and the groove you can see from the outside marks it.
The torn edges curl back, so the two sacs of each lobe open into a single exposed surface and the is free.

The photograph below shows lily at the moment of .

Close-up of five orange anthers hanging from pale cream filaments below a yellow petal. Each anther is a long narrow oval with a pale line running down its whole length where it has begun to split. A small round purple-pink structure on a green stalk sits at the bottom centre. The background is blurred green foliage. — Lily stamens at the moment of dehiscence. Photo: Nadiatalent, CC0.

Each orange has a paler line running down its length. That line is the split opening along the .
The are long and hold the clear of the , so an insect brushing past must touch them.
The small purple structure at the bottom of the picture is the of the same flower.

Lesson 8 of 24

What is inside an ovule?

The sit inside the , attached to its wall by short stalks. Each is a small package with several layers, and the female is right at the centre of it.

The diagram below shows a mature .

Labelled oval diagram. Two green layers form the outside, with a cream layer inside them and a pale lilac oval at the centre. A horizontal channel with green walls cuts through both green layers on the right. Inside the lilac oval are a pink circle near the channel, two purple circles above and below it, a larger blue circle in the middle, and three grey circles in a vertical line at the left end. A thick green stalk curves away from the lower right. Labels name the outer integument, the inner integument, the nucellus, the embryo sac, the three antipodal cells, the synergid, the oosphere, the polar nucleus, the micropyle and the funicle. — A mature ovule. Original BioWelsh diagram.

The is the stalk that attaches the to the wall and carries its vascular tissue.
Two wrap around the . The outer one becomes the of the seed.
On one side the do not meet. The narrow canal they leave is the , and the grows in through it.
Inside the is the , a layer of cells that supplies nutrients to the developing .
At the centre is the , and inside it are eight : the with two at the micropylar end, three at the other end, and two in the middle that fuse into one .

In a section of a whole you see several at once. The diagram below is a of a transverse section of an .

Labelled diagram of a large green ring enclosing a white space. In the centre is a small green oval. Three larger ovals lie inside the ring, two on the left and one on the upper right, each made of a green outer layer, a cream middle layer and a pale lilac centre, and each joined to the central oval by a short curved green stalk. Labels name the ovary wall, the integuments, the nucellus, the embryo sac, the ovule, the funicle, the placenta and the cavity of the ovary. — Low power plan of a transverse section of an ovary. Original BioWelsh diagram.

The wall forms the outside. After fertilisation it becomes the fruit wall.
The are attached to a ridge of tissue called the , which runs down the middle of this .
A shows only the outline and position of each tissue. No individual cells are drawn.
Because the are attached at different heights, a single section may cut some through the middle and others near the edge.

The light micrograph below shows a transverse section of a lily .

Circular field of view, stained pale green with small red dots throughout. Six hook-shaped bodies made of densely packed dark red cells curve inwards from the tissue, arranged in pairs around the centre, each separated from the next by a pale empty space. The tissue at the upper left contains a denser patch of small dark cells. A straight black line runs from the top of the field down to the centre, which is the pointer of the microscope. — Light micrograph of a transverse section of a lily ovary. Photo: Ilse Anahi Carrasco, CC BY 4.0. Resized.

The pale tissue round the outside is the wall, with darker vascular bundles in it.
The is divided into three chambers, and the are attached where the three meet in the centre.
Each appears as a darker, hook-shaped body projecting into a chamber, because the curves back on itself as it develops.

Lesson 9 of 24

How is the female gamete made?

The female side follows the same rule as the male side: one round of meiosis, then mitosis. The difference is that three of the four products of meiosis are thrown away, and the survivor divides three times without any cell division, so the ends up as one large cell with eight in it.

The diagram below follows the whole sequence.

Five stages in a row, joined by arrows labelled meiosis, mitosis three times, and two polar nuclei fuse. Stage one is a tall lilac oval containing one purple circle. Stage two contains one gold circle and three grey circles. Stage three contains a single gold circle. Stage four contains eight gold circles in two rows of four. Stage five contains a pink circle low down with a small purple circle just above it on each side, a large blue circle in the middle and three grey circles in a row at the top, with labels naming the oosphere, the two synergids, the polar nucleus and the three antipodal cells. — Development of the female gamete in the ovule. Original BioWelsh diagram.

A (2n) in the divides by meiosis.
This gives four cells. Three of them disintegrate and only one survives.
The survivor grows and its divides by mitosis three times, giving eight . The cell does not divide, so all eight share one cytoplasm.
The eight arrange themselves: one (the female ) with two at the micropylar end, three at the far end, and two in the middle.
The two fuse, giving one . The is now mature and ready for fertilisation.

It is worth putting the two sides next to each other, because examiners often ask about both in one question. The diagram below compares them.

Two columns of four coloured boxes, separated by a grey vertical line, headed in the anther and in the ovule. Each column runs downwards through arrows labelled meiosis, mitosis and mitosis. The left column reads pollen mother cell 2n, four cells in a tetrad n, tube nucleus plus generative nucleus, and two male gametes from the generative nucleus. The right column reads megaspore mother cell 2n, four megaspores with three disintegrating, eight nuclei after three mitotic divisions, and one oosphere plus one polar nucleus from two fusing. A box across the bottom states that both gametes are nuclei and that meiosis happens once in each. — Cell division in the making of the male and female gametes. Original BioWelsh diagram.

Both start from a mother cell and both use meiosis exactly once.
Everything after meiosis is mitosis, on both sides.
Both gametes are , not whole cells.
The male side keeps all four products of meiosis; the female side keeps only one.

Lesson 10 of 24

What is pollination, and how does it differ from fertilisation?

is the transfer of from an to a . That is all it is: a journey. Fertilisation is the fusion of a male with a female , and it happens much later, deep inside the . can happen without fertilisation following, for example if the is from the wrong .

Two kinds of pollination
Type	What happens	Genetic result
	lands on the of the same flower, or of another flower on the same plant	Offspring are less varied; come from one parent only
	is carried to the of a different plant of the same	Offspring are more varied; come from two parents

Most flowers can do both. is a safety net: it guarantees seed even when no pollinator arrives and no other plant of the is nearby. That is why self-pollinating are often good colonisers of new ground. But if a plant always self-pollinates, its offspring gain very little new genetic .

Lesson 11 of 24

Why does cross-pollination matter?

brings together from two different plants, so the offspring carry combinations of that neither parent had. cannot do that: it can only reshuffle the that one plant already carries.

It increases genetic in the offspring, and so widens the of the .
that if the environment changes, some individuals are more likely to have that let them survive and reproduce.
It reduces homozygosity. Repeated makes more and more , so harmful are more likely to be expressed.
It avoids inbreeding depression, the loss of vigour that comes with that homozygosity.

This is why is such an advantage to a colonising new ground. The conditions in a new are unknown, so a varied set of offspring is far more likely to include some that can cope with them.

Lesson 12 of 24

How do flowers make self-pollination less likely?

Because has real advantages, many flowers have features that make difficult. None of them makes it impossible, which keeps the safety net.

The diagram below shows four of these devices.

Four panels separated by grey vertical lines. The first, headed protandry, shows two small flowers side by side: the left with gold anthers raised and a grey tip, the right with a dark pink tip and grey anthers. The second, headed stigma above anther, shows one flower with a dark pink oval held well above two gold anthers, with a gap marked between them. The third, headed genetic incompatibility, shows a gold circle with an arrow to a pink stigma crossed out in red above the words same plant, and the same pair without the cross and with a green tube growing, above the words different plant. The fourth, headed separate flowers, shows one stem with a gold oval labelled male on one branch and a pink oval labelled female on another. — Four ways a flower makes self-pollination less likely. Original BioWelsh diagram.

: the and the of one flower ripen at different times. If the ripen first it is called ; if the ripens first it is .
Position: the is held above the , so cannot simply fall on to it.
Genetic incompatibility: carrying the same incompatibility as the fails to , or its tube stops growing in the .
Separate flowers: male and female flowers are on different parts of the plant, or on separate plants altogether.

Some go further. In a primrose, half the plants have a long with the at the mouth of the flower and the low down, and half have a short with the at the mouth. A bee feeding in one type picks up on a part of its body that touches the of the other type, so mostly moves between the two forms.

Lesson 13 of 24

How does the pollen tube reach the ovule?

A that lands on a compatible of the same absorbs water and sucrose from the 's secretions and germinates. A tube grows out through a pore in the and burrows down through the .

The diagram below follows a from the to the .

Labelled vertical diagram. A dark pink oval stigma sits at the top of a narrow pale green style. Below the style is a large green ovary with a white cavity, containing an ovule made of two green layers, a cream layer and a pale lilac centre. A vertical channel cuts through the green layers at the top of the ovule. Four gold circles rest on the stigma, and from one of them a thick purple tube runs down through the style, through the channel and into the lilac centre, ending beside a pink circle. Two small pink circles and one purple circle lie inside the lower part of the tube. Labels name the pollen grains, the style, the pollen tube, the micropyle, the ovary wall, the two male gametes, the oosphere, the polar nucleus and the chemoattractants. — Growth of the pollen tube from stigma to embryo sac. Original BioWelsh diagram.

The sits at the tip of the growing tube and controls its growth. It codes for the the tube secretes.
Those digest a path through the tissue of the . The products are also absorbed and used by the growing tube.
The two male gametes follow behind the .
The secretes , and the tube grows down the towards the .
The tube enters the through the , and the tip breaks down so that the two male gametes are released into the .

can be germinated on a microscope slide in sucrose solution, which is a common way to study tube growth. The graph below shows how the concentration of the sucrose solution affects the length of the tube after three hours.

Line graph with the concentration of sucrose solution as a percentage from 0 to 30 on the horizontal axis and the mean pollen tube length after three hours in micrometres from 0 to 360 on the vertical axis. A single purple curve starts at about 50, rises to a peak of about 340 at 12.5 per cent, then falls steadily to about 10 at 30 per cent. Text near the peak reads longest at about 12.5 per cent, and text to the upper right reads water leaves the grain by osmosis. — Pollen tube length in different sucrose solutions. Original BioWelsh graph.

In pure water almost no tube grows. The grain takes in water rapidly by and bursts.
Tube length rises to a maximum at about 12.5 % sucrose, which is close to the inside the grain.
Above that, the external solution has a lower than the grain, so water leaves the grain by and growth slows.

Lesson 14 of 24

What happens in double fertilisation?

Flowering plants are the only group that fertilise twice in one , which is where the name comes from. Both male gametes are used: one makes the embryo and the other makes its food supply.

The diagram below shows the two fusions.

Labelled diagram of a large pale lilac oval. A thick purple tube enters from the left carrying two pink circles. Inside the oval are a large pink circle marked n below and left of centre, a larger blue circle marked 2n above and right of centre, two small purple circles and three grey circles at the top. Two pink arrows lead from the gametes, one curving down to the pink circle and one running right to the blue circle. Two boxes below are headed Fusion 1 and Fusion 2. The first reads male gamete n plus oosphere n gives a zygote 2n, which becomes the embryo. The second reads male gamete n plus polar nucleus 2n gives the primary endosperm nucleus 3n, the endosperm. — Double fertilisation: two fusions in one embryo sac. Original BioWelsh diagram.

The tip of the breaks down inside the and releases both male gametes.
The first male (n) fuses with the (n). This gives a (2n), which divides to form the embryo.
The second male (n) fuses with the (2n). This gives a (3n), which divides to form the .
The is the food store of the seed. In many seeds it is later absorbed into the .

Making the food store only after fertilisation saves the plant a great deal of energy. If an is not fertilised there is no , so there is no point in filling it with starch, protein and lipid. This is why a poorly pollinated maize cob has gaps: the that were never fertilised simply do not develop into kernels.

The photograph below shows well filled sweetcorn cobs.

Five husked corn cobs lying on a metal tray, overlapping each other. Each cob is completely covered in tightly packed, glossy, rounded kernels in straight lengthways rows. The kernels are a mixture of pale yellow and creamy white. A few fine pale threads are caught between the cobs. — Sweetcorn cobs, each kernel formed from one fertilised ovule. Photo: Nutrition, Food Safety and Health, CC0.

Every kernel on the cob came from one in which took place.
The kernels are in straight rows because the are arranged in rows along the cob.
A cob with gaps in the rows is one in which some were never fertilised, so no food store was laid down in them.

Lesson 15 of 24

What does each structure become after fertilisation?

Once the two fusions have happened, the rest of the flower either develops or is discarded. The , , and wither and fall. Everything that is left turns into part of the seed or the fruit.

The diagram below lists what each structure becomes.

Seven pairs of boxes in two columns, joined by arrows pointing right. The left boxes are coloured and read zygote 2n, primary endosperm nucleus 3n, integuments, ovule, ovary, ovary wall and funicle. The white boxes on the right read embryo with plumule, radicle and cotyledons; endosperm, the food store; testa, the seed coat; seed; fruit; fruit wall or pericarp; and the stalk of the seed, attached at the hilum. A line at the bottom reads that the seed comes from the ovule and the fruit from the ovary. — What each structure becomes after fertilisation. Original BioWelsh diagram.

The (2n) divides by mitosis into the embryo: , and one or two .
The (3n) divides into the , the food store.
The harden into the , the seed coat.
The whole becomes the seed, and the whole becomes the fruit.
The wall becomes the fruit wall, the .
The leaves a scar on the seed called the , next to the old .

The two sentences worth memorising are: the seed comes from the , and the fruit comes from the . A fruit is the ripened whether it is fleshy like a tomato, dry and splitting like a pea pod, or a single hard grain like maize.

Lesson 16 of 24

What is inside a broad bean seed?

A broad bean is the standard example of a , dicotyledonous seed. the has been absorbed during development, so the food store is in the two instead.

The diagram below shows a broad bean seed with one taken away.

Labelled oval diagram. A brown outer ring encloses a large cream area. A thick green band starts near the centre, curves left and hooks upwards, ending in two small pale green leaf shapes. From the same starting point a second green band runs down and to the right, tapering as it goes, and reaches a small brown rounded rectangle set in the edge of the brown ring. A tiny white oval sits just below that rectangle. Labels name the testa, the cotyledon, the plumule, the radicle, the hilum and the micropyle. — A broad bean seed cut open with one cotyledon removed. Original BioWelsh diagram.

The is the tough coat formed from the . It protects the seed and keeps it dry.
The is the scar on the edge where the was attached.
The is a tiny pore beside the , left over from the 's entry. Water is taken in through it at .
The two fill most of the seed. They hold the food store, mostly starch with some protein and lipid.
The embryo lies between the : a pointing towards the and a hooked pointing the other way.

The photograph below shows dried broad bean seeds.

A small heap of about fifteen flattened oval seeds on a white background. Each is very dark, almost black, with a smooth slightly wrinkled surface that catches the light. A few show a paler band along one edge. They lie at different angles, some overlapping. — Dried broad bean seeds. Photo: Ural-66, CC0.

The seeds are hard and dark because the has dried out and hardened.
Each seed is flattened, which is the shape of the two pressed together inside.
A dry seed contains only about 10 to 15 % water, which is why it can survive for years.

Lesson 17 of 24

How is a maize grain different from a broad bean seed?

Maize is a , and a maize grain is : the is not absorbed, so the food store stays as a separate tissue. The grain is also not a seed at all in the strict sense. The is fused to the wall, so the whole grain is a one-seeded fruit.

The diagram below shows a maize grain cut in half lengthways.

Labelled diagram of a rounded shape that tapers to a point at the bottom. Three coloured bands follow the outline: a deep gold outer band, a paler gold band and then a large cream interior. To the right of centre is a pale green leaf-shaped region bounded on its left by a thick gold line. Inside the green region a dark green band curves upwards inside a paler green sheath, and a second dark green band runs downwards from a small pale circle. Labels name the pericarp fused to the testa, the aleurone layer, the endosperm, the scutellum, the coleoptile, the plumule and the radicle. — A maize grain cut in half lengthways. Original BioWelsh diagram.

The outside is the , the fruit wall, fused to the beneath it.
Just inside is the , a layer of protein-rich cells that makes the at .
The fills most of the grain. It is packed with starch.
The embryo lies to one side. Its single is the , a shield-shaped structure pressed against the , which absorbs the digested food.
The is sheathed in a protective , and the points the other way.

Comparing a broad bean seed and a maize grain
Feature	Broad bean	Maize
Class
	Two	One (the )
Food store	In the ()	In the ()
	Separate from the fruit wall	Fused to the fruit wall
What you plant	A seed inside a pod	A one-seeded fruit
Protection of the	Bent into a hook	Sheathed in a

The photograph below shows a young maize cob with its silks.

A maize plant photographed in sunlight among its long strap-shaped green leaves. A cob wrapped in smooth pale green husks points upwards from the stem, and a large tuft of fine cream-coloured threads emerges from its tip and hangs down one side. A second, smaller tuft of threads shows lower down the stem. — A young maize cob with its silks. Photo: Alex Lomas, CC BY 2.0. Resized.

Each pale thread is the of one , and it runs all the way from the tip of the cob down to its own .
Maize is wind-pollinated, and the mass of silks gives a huge surface area for catching from the tassels above.
A that lands on one silk grows a tube down that whole , which is why the maize is one of the longest known.

Lesson 18 of 24

How are seeds dispersed, and why does it matter?

A seed that germinates directly under its parent is competing with the parent, and with all its siblings, for the same light, water and mineral ions. moves seeds away from the parent plant and spreads the over a larger area.

Methods of seed and fruit dispersal
Method	Adaptation	Example
Wind	Parachutes of fine hairs, or wings, that slow the fall	Dandelion, sycamore
Animal (carried)	Hooks on the fruit that catch in fur or clothing	Burdock, cleavers
Animal (eaten)	Fleshy, sweet, brightly coloured fruit; the seeds pass out in the faeces	Blackberry, holly
Explosive	The fruit wall dries, splits and curls, flicking the seeds out	Pea, Himalayan balsam
Water	A buoyant, waterproof fruit wall	Coconut, yellow flag iris

The photograph below shows a dandelion-type seed head.

Close-up against a pale blue sky. A single fruiting head carries about a dozen remaining fruits, each hanging from a spreading disc of fine white hairs on a slender stalk. Below them a ring of narrow dried sepals curves back and downwards from a dark purple centre. Dry brown stems and buds are blurred around it. — A dandelion seed head, dispersed by wind. Photo: Adesolive, CC0.

Each fruit hangs below a ring of fine hairs called a , which acts as a parachute.
The greatly increases air , so the fruit falls slowly and is carried further by the wind.
The fruits are arranged in a ball so that the wind can take them from any direction.

The photograph below shows lesser burdock.

A burdock plant with large oval green leaves and several flower heads on green stalks. Each head is a ball of green bracts, and every bract narrows to a fine point that curves over into a hook at the tip. A tuft of purple-pink florets with pale stamens sticks out of the top of each ball. Grass is blurred behind. — Lesser burdock, whose fruits are carried on animal fur. Photo: Robert Flogaus-Faust, CC BY 4.0. Resized.

Each flower head sits on a ball of green bracts, and every bract ends in a hook.
When an animal brushes past, the hooks catch in its fur and the whole head is carried away.
The seeds are released later, wherever the head is finally dislodged, which may be a long way from the parent.

Many use more than one method. Himalayan balsam flicks its seeds up to six metres with an explosive fruit, but it grows on river banks, so seeds that land in the water are then carried much further downstream, and others are carried on animals or people. Extra methods spread the seeds over a larger area and reduce .

Lesson 19 of 24

Why do seeds not germinate straight away?

Most seeds go through a period of , in which they are alive but not growing. The seed dries down to about 10 to 15 % water, its metabolism almost stops, and the hardens.

lets the seed survive conditions the parent plant could not, such as a cold winter or a dry season.
It spreads out in time, so a single bad spell does not kill a whole generation.
It gives the seed time to be dispersed before it starts to grow.

is broken by a specific trigger, which differs between : a period of low temperature, exposure to light, passing through an animal's gut, fire, or simply enough time for a chemical inhibitor to be washed out of the . Some fruits contain inhibitors that stop the seeds while they are still inside the fruit, which is why seeds are washed before they are used in a experiment.

Lesson 20 of 24

What does a seed need in order to germinate?

begins when the embryo starts to grow again and ends when the seedling can photosynthesise for itself. Three external conditions are needed, and each has a clear biological reason.

The diagram below gives the three conditions and the reason for each.

Three coloured circles in a row with headings and text beneath. The blue circle contains a white water drop and is headed Water, with text saying it mobilises the enzymes, swells the tissues so the testa splits and carries the soluble products to the embryo. The pink circle contains the symbol for oxygen and is headed Oxygen, with text saying it is needed for aerobic respiration, which releases the ATP for growth and for active transport. The gold circle contains a thermometer and is headed Suitable temperature, with text saying usually five to thirty degrees Celsius, because warmth speeds up diffusion and raises enzyme activity but heat would denature the enzymes. — The three conditions a seed needs in order to germinate. Original BioWelsh diagram.

Water. It is taken in through the . It swells the tissues so that the splits, it activates the , and it is the in which the soluble products are transported to the embryo.
Oxygen. It is needed for , which releases the for growth, protein synthesis and . (If you are not sure why a cell needs rather than glucose, read the ATP lesson on why cells need ATP.)
A suitable temperature. Warmth speeds up and raises the rate of -controlled reactions. Too high and the are denatured.

Light is not on the list. Most seeds perfectly well in the dark, because they are living on their own food store until the first leaves reach the light. A few small-seeded do need light, as a signal that they are near the surface.

Water and oxygen are taken up in a definite order. The graph below shows both during the first two days.

Line graph with time after the dry seed was placed in water in hours from 0 to 48 on the horizontal axis. A blue solid line for water uptake rises very steeply from zero to about 50 in the first six hours, levels off near 60 between 12 and 24 hours, then rises steadily again to about 82 by 48 hours. A red dashed line for oxygen uptake rises from about 4 to about 13 in the first twelve hours, stays flat until about 26 hours, then rises steadily to about 66 by 48 hours. Text near the top reads rapid imbibition on the left and cell division and growth on the right. — Water uptake and oxygen uptake by a germinating broad bean. Original BioWelsh graph.

Water uptake is very fast at first. This stage is : the dry tissues simply soak water up, and it does not need the seed to be alive.
Water uptake then levels off while the seed is fully hydrated but before much growth has happened.
Oxygen uptake rises as soon as the tissues are hydrated, because the of become active.
Both rise again after about a day, when cell division and growth begin in the .

Lesson 21 of 24

What happens during the germination of a broad bean?

A broad bean germinates below ground. The stay where they are and shrink as their food store is used up, while the and then the grow out.

The diagram below shows the five stages.

Five seeds in a row below a brown ground line, on a pale brown background, numbered 1 to 5 above. Seed 1 is a plain cream oval with three blue arrows pointing into its left side. Seed 2 has a curved line along its lower edge where the coat has split. Seed 3 has a green root growing down from the split and branching near its tip. Seed 4 also has a green shoot growing up from the top of the seed and arching over to the right above the ground line. Seed 5 has a straight green stem rising well above the ground with two green leaves at the top, and a longer branched root below. Captions below read the seed imbibes water through the micropyle, the testa splits, the radicle emerges and grows down, the plumule follows bent into a hook, and the hook straightens and the leaves unfurl. — Germination of a broad bean. Original BioWelsh diagram.

Water is through the . The swell and the softens and splits.
Oxygen can now reach the embryo, so increases sharply.
in the the stores: breaks starch down into , break protein down into amino acids, and break lipids down into and .
The soluble products are transported to the growing points. There they are respired for , or used to build new cell walls and proteins by mitosis and growth.
The emerges first, through the , and grows down. The follows, bent into a hook so that the soil is pushed aside by the bend and not by the delicate growing tip.

The always comes out first, for two reasons. It anchors the seedling in the soil, and it starts taking up the water and mineral ions the growing shoot will need.

The photograph below shows soaked beans, one of which has begun to .

Close-up of many swollen red-brown seeds packed together and wet. One seed near the centre has split open along its side, and a smooth white radicle has pushed out and curves downwards. The pale cream tissue of the seed's interior is exposed where the coat has opened. — Soaked bean seeds, one of which has begun to germinate. Photo: Judgefloro, CC0.

The of the seed has split along one side.
A white has pushed out through the split and is growing away from the seed.
The pale tissue exposed inside the split is the , which is the seed's food store.

Following the of a seed shows where the food is going. The graph below separates the from the seedling.

Line graph with time after the start of germination in days from 0 to 20 on the horizontal axis and dry mass in grams from 0 to 0.6 on the vertical axis. An orange line starts at about 0.41 and falls steadily to about 0.03 by day 20. A green line starts near zero and rises steadily to about 0.50 by day 20. A blue dashed line for the total starts at about 0.41, dips to a minimum of about 0.31 near day 11, then rises to about 0.53 by day 20. Text reads total dry mass falls at first and photosynthesis begins. — Dry mass during the germination of a broad bean. Original BioWelsh graph.

The of the falls steadily as their stores are and moved to the embryo.
The of the seedling rises as those materials are built into new tissue.
The total falls at first. That is because releases carbon dioxide, so carbon is being lost from the system.
After about two weeks the total rises again, because the first leaves are photosynthesising and fixing carbon faster than loses it.

Lesson 22 of 24

How does gibberellin start germination in a cereal grain?

In a cereal grain the food store is in the , which has no of its own and no living connection to the embryo. Something has to tell the to make the , and that something is a plant growth substance called , also written .

The diagram below shows the sequence in a barley grain.

Labelled diagram of a large gold oval with a thick gold ring just inside its edge. A pale green oval labelled embryo lies inside the left end. A numbered purple dashed arrow runs from the embryo up to the gold ring, labelled gibberellin. A numbered gold arrow curves from the ring into the cream interior, labelled amylase and protease. Text in the interior reads starch to maltose and protein to amino acids. A numbered green arrow curves back from the interior to the embryo, labelled soluble sugars and amino acids diffuse back to the embryo. A box below lists the five numbered steps. — How gibberellin starts germination in a barley grain. Original BioWelsh diagram.

Water is and the embryo becomes active. It secretes .
The diffuses outwards to the .
In the aleurone cells it switches on the for , so and are synthesised.
The diffuse into the and the stores: starch to , protein to amino acids.
The soluble products diffuse back to the embryo, where they are respired for and used for growth.

The evidence for this comes from a simple experiment. If the embryo is cut off a barley grain and the rest is placed on starch , no starch is digested. If is added to that embryo-free half, starch is digested again. So the embryo's role is to supply , and the come from the .

also controls stem growth, which is easy to see in dwarf varieties. The graph below compares tall and dwarf peas.

Line graph with time after planting in days from 0 to 28 on the horizontal axis and mean height of seedlings in millimetres from 0 to 90 on the vertical axis. A green solid line for the tall variety with no gibberellic acid rises in an S shape to about 82. An orange dashed line for the dwarf variety with gibberellic acid follows a very similar S shape slightly below it, reaching about 76. A red dotted line for the dwarf variety with no gibberellic acid rises only to about 22. — Height of tall and dwarf pea seedlings, with and without gibberellic acid. Original BioWelsh graph.

The tall variety grows normally without any treatment.
The untreated dwarf variety stays short, because a stops it making enough .
Treating the dwarf variety with makes it grow almost as tall as the tall variety, which shows that the missing was the cause.
Treating the tall variety with extra makes very little difference, because it already makes enough.

starts , but that does not appears first. The graph below shows and activity in seeds soaked in .

Line graph with time after soaking in hours from 0 to 120 on the horizontal axis and a shared scale from 0 to 100 on the vertical axis. A green solid line for germination as a percentage stays near zero to about 20 hours, rises steeply to about 90 by 66 hours and then levels off near 96. An orange dashed line for amylase activity stays near 4 until about 40 hours, then rises gradually to about 37 by 120 hours. Text reads seeds germinate first and amylase rises later. — Germination and amylase activity after soaking in gibberellic acid. Original BioWelsh graph.

The percentage of seeds that have germinated rises steeply from about 24 hours and levels off by about 72 hours.
activity only begins to rise after most of the seeds have already germinated.
So these data do not support the hypothesis that stimulates synthesis which then causes . The seeds germinated before the rose.
In exam terms, you would reject that hypothesis, and the reason is that happened before the increase in .

Lesson 23 of 24

The specified practical work for this topic

WJEC lists three pieces of specified practical work for this topic. Each one is set out in full below: the digestion of starch by seeds, the dissection of wind-pollinated and insect-pollinated flowers, and a of an with a calculation.

Specified practical

Investigating the digestion of starch agar by germinating seeds

Unit 4, section 2, specified practical 1 (BioWelsh entry number)

Aim To show that seeds release , and to find out whether soaking the seeds in increases the amount of they release.

Five pale purple cards in a row joined by purple arrows. The cards show a beaker of water holding three cream seeds, a white bottle, a cream agar plate with three half seeds and pale rings around them, an incubator showing 25 degrees Celsius with trays of seeds, and a bottle of brown solution. The captions read soak broad beans for two days, surface sterilise and boil a control set, cut in half and lay cut face down, 48 hours at 25 degrees Celsius, and flood with iodine and measure the halos. — Digestion of starch agar by germinating seeds, at a glance. Original BioWelsh diagram.

Equipment

Broad bean seeds
Broad bean seeds30, soaked 48 hoursThe source of the ; they must be alive and .
Bottle of 10 % bleach
Bottle of 10 % bleachIn a beakerSurface sterilises the seeds so that do not digest the instead.
Beaker of distilled water
Beaker of distilled water250 cm³Soaks the seeds and rinses off the bleach.
Starch-agar plates
Starch-6, 1 % starchThe . Any clear area shows where starch has been .
Boiling tube
Boiling tube1Holds the seeds that are boiled to make the .
Water bath
Water bath100 °CBoils one batch of seeds to denature their .
Incubator
25 °CKeeps every plate at the same temperature while the acts.
Scalpel
ScalpelSharp bladeCuts each seed cleanly in half so the cut surface touches the .
White tile
White tileA firm, washable surface to cut on.
Forceps
ForcepsPlace the half-seeds without touching them.
Bottle of iodine solution
Bottle of solutionStains the remaining starch blue-black so the clear halos show up.
Pencil and ruler
Pencil and ruler30 cm rulerMeasures the diameter of each halo in millimetres.
Eye protection
Eye protection solution and bleach are irritants.

Safety

10 % bleachRiskIrritation of skin and eyes; bleaching of clothing.ControlWear eye protection and gloves, keep the bleach in a labelled beaker and rinse spills with plenty of water.
ScalpelRiskCuts to the fingers when halving seeds.ControlCut on a tile, cut away from your fingers, and hold the seed with forceps rather than your hand.
Boiling water bathRiskScalds from the water or from steam.ControlLower the boiling tube in with a test tube holder and keep hands away from the steam.
solutionRiskIrritation of skin and eyes, and staining.ControlWear eye protection, pour it carefully over the plate and wash any splashes off at once.
Microbial growth on the RiskUnknown growing on a warm starch plate.ControlSurface sterilise the seeds, keep lids on except when adding or removing seeds, and the plates before disposal.

Variables

Independent variableTreatment of the seeds

Soaked in , soaked in water, or boiled.

The boiled seeds are the and the water-soaked seeds are the untreated comparison.

Dependent variableDiameter of the clear halo

Measured in millimetres across the widest point after is added.

A wider halo more diffused out and more starch was .

Example: 17.8 mm with against 9.6 mm without

Controlled variablesVariety and age of seed, Mass of seed tissue on each plate, Soaking time, Concentration and depth of the starch agar, Temperature of incubation, Time of incubation

Variety and age of seed

Use broad beans of one variety from the same packet.

Different varieties store different amounts of starch and make different amounts of .

Mass of seed tissue on each plate

Choose seeds of similar mass and use three half-seeds per plate.

A bigger piece of tissue holds more , so a bigger halo would not be caused by the treatment.

Soaking time

Soak every batch for the same 48 hours.

A longer soak the seed is further into and has made more .

Concentration and depth of the starch

Pour the same volume of the same 1 % into every plate.

A deeper or more concentrated layer takes longer to clear, so the halo would be smaller.

Temperature of

All plates at 25 °C in the same .

activity rises with temperature up to its optimum.

Time of

Exactly 48 hours for every plate.

The halo widens with time, so plates left longer would look more active.

Control experiment or controlled variable?

The boiled seeds are a : a whole extra set-up that differs in one thing, whether the are working. It tells you that the halo is caused by an .

The variety of seed, the depth of the and the temperature are : things kept the same in every set-up so that only the treatment differs.

Boiling the seeds is not the same as boiling the extract. Examiners have seen candidates offer glass beads in place of seeds, which controls the shape of the object but not the , and is not accepted.

Method, step by step

1)
Soak 30 broad bean seeds in distilled water for 48 hours.
Why?
A dry seed is . activates the , so only a soaked seed releases .
2)
Surface sterilise all the seeds in 10 % bleach for 5 minutes, then rinse them three times in distilled water.
Why?
Bacteria and fungi on the also make , and would produce halos that had nothing to do with the seed.
3)
Soak ten of the seeds in 0.1 mmol dm⁻³ , ten in distilled water, and boil the last ten for 10 minutes and cool them.
Why?
The three treatments give the test, the untreated comparison and the .
4)
Cut each seed in half with a scalpel on a white tile, cutting away from your fingers.
Why?
Cutting exposes the , so the can diffuse straight into the .
5)
Use forceps to lay three half-seeds on each starch-, cut face down and well apart.
Why?
Cut face down puts the in contact with the . Spacing them out stops the halos merging.
6)
Label the base of each plate with the treatment, and all the plates at 25 °C for 48 hours.
Why?
Every plate must be at the same temperature, because temperature affects the rate of the .
7)
Remove the half-seeds with forceps and flood each plate with solution for one minute, then pour the excess away.
Why?
stains starch blue-black. Where the starch has been there is nothing to stain, so a clear halo appears.
8)
Measure the diameter of each halo in millimetres across its widest point.
Why?
Diameter is easier to measure repeatably than area, and the plates can be compared directly.
9)
Calculate a halo diameter for each treatment.
Why?
Seeds differ, so a from several halos is a better estimate than any one measurement.

Results and analysis

Example data

halo diameter after 48 hours on starch , for three treatments

Treatment	Halo 1 / mm	Halo 2 / mm	Halo 3 / mm	/ mm
Live half-seeds, no	9.0	10.5	9.3	9.6
Live half-seeds soaked in	17.5	18.0	17.9	17.8
Boiled half-seeds ()	0.0	0.0	0.0	0.0

Two dark blue-purple circles side by side. The left circle, headed control boiled seeds, contains three small cream half-discs with no change in the colour around them. The right circle, headed live seeds, has a large pale cream circle around each of its three half-discs. A pink measuring bracket above one pale circle is labelled halo diameter. Text below reads that the agar stains blue-black everywhere on the control because boiling denatured the enzymes, and that a clear halo around each live seed shows amylase has diffused out and digested the starch. — The two starch-agar plates after iodine is added. Original BioWelsh diagram.

Bar chart with five pale blue bars. The vertical axis is mean halo diameter in millimetres from 0 to 20. From left to right the bars are 4.2 at a concentration of zero, 9.6 at 0.01, 14.1 at 0.10, 17.8 at 1.00 and 18.4 at 10.0 millimoles per cubic decimetre. — Mean halo diameter on starch agar at five gibberellic acid concentrations. Original BioWelsh graph.

What the results show

The boiled seeds gave no halo at all. Boiling denatured their , so no starch was and the whole plate stained blue-black. This is what shows that the halos are caused by an from the seed and not by the seed simply absorbing the .

The live seeds gave a clear halo of about 9.6 mm, so seeds do release .

The seeds soaked in gave a much wider halo, about 17.8 mm. therefore increased the amount of released, which supports the idea that is the signal that starts production.

The halo diameter is not proportional to the amount of . diffuses outwards in all directions, so doubling the diameter roughly four times the area of cleared.

Worked calculations

halo diameter = (17.5 + 18.0 + 17.9) ÷ 3 = 17.8 mm

Increase caused by = 17.8 − 9.6 = 8.2 mm

Percentage increase = (8.2 ÷ 9.6) × 100 = 85 %

Area cleared = πr² = 3.14 × 8.9² = 249 mm² for the treated seeds

Evaluation

Seeds vary in size and in how much starch and enzyme they contain.

Effect on resultsThe halo diameter for one treatment varies from plate to plate.

ImprovementUse several half-seeds per treatment, choose seeds of similar mass and take a .

Halo diameter is a measure of how far the enzyme has diffused, not directly of enzyme concentration.

Effect on resultsTwo treatments can be compared but the amount of cannot be read off the plate.

ImprovementRun a set of known concentrations on the same and use them as a calibration curve.

The edge of a halo is not sharp.

Effect on resultsDifferent people measure the same halo differently.

ImprovementAgree where the edge is, measure on a dark background, and have the same person measure every plate.

Microorganisms can grow on a warm starch plate.

Effect on resultsA halo may be produced by a fungus rather than by the seed.

ImprovementSurface sterilise the seeds and set up an uninoculated plate to check that the itself stays clear of growth.

Going further

Other ways to carry out this practical

Use half-grains of barley with the embryo removed. With no embryo there is no , so no halo forms unless is added.
Grind germinated seeds in to make an extract, then time how long a volume takes to clear a starch solution followed with .
seeds for 1, 2, 5, 8 and 10 days and compare the activity of extracts made from equal masses of tissue.
Test a of concentrations and plot halo diameter against concentration.

Why the same mass rather than the same number of seeds?

Seeds of the same variety still differ in mass, and a larger seed holds more tissue and so more .

Using the same mass of tissue for each extract the activity can be stated per gram, so the results are comparable and any change can be put down to the treatment rather than to seed size.

Model answer: describe the method

Describe how you would develop this procedure to find out whether treating seeds with increases the amount of they produce.

Original practice response · 6 indicative marks

Soak one batch of seeds in solution and an otherwise identical batch in distilled water.
Boil a third batch and cool it, to give a in which the are denatured.
Cut all the seeds in half and lay them cut face down on starch-.
every plate at the same temperature for the same time, then flood each with solution.
Measure the diameter of each clear halo and compare the for the two live treatments.
the variety, age and mass of the seeds, the soaking time, and the concentration and depth of the .

What the answer needs to cover

Some seeds soaked in
Clear measured and compared
Same variety, age, size or mass of seed
Soaked for the same time
Same concentration and depth of starch
Same temperature and time of

Exam practice

State the independent and dependent variables in an experiment comparing half-grains with and without an embryo.Show answer

The is the presence or absence of the embryo. The is the presence, or the diameter, of the clear halo.

Independent: embryo present or absent
Dependent: presence or diameter of the halo

A plate of half-grains with no embryo gave no halo. Use your knowledge of plant hormones to explain this.Show answer

Without the embryo there is no , so the is not stimulated to make . With no , the starch in the is not , so the whole plate stains blue-black.

No embryo no
normally starts synthesis
No no starch

Explain why the seeds are surface sterilised before they are placed on the agar.Show answer

Bacteria and fungi on the also secrete . If they were not removed, they could produce clear areas that were not caused by the seed, so the results would not be valid.

also release
They would produce halos not caused by the seed

Using your knowledge of germination, explain why amylase activity falls once the seedling has leaves.Show answer

Once the leaves are photosynthesising the plant makes its own glucose, so it no longer needs to the starch store and less is made.

provides glucose
Less need to break down the stored starch

Specified practical

Dissecting wind-pollinated and insect-pollinated flowers

Unit 4, section 2, specified practical 2 (BioWelsh entry number)

Aim To take an insect-pollinated flower and a wind-pollinated flower apart by , to identify and count the parts of each, and to relate the differences between them to the way each is pollinated.

Five pale purple cards in a row joined by purple arrows. The cards show a gold flower with brown anthers and a pink tip, a white tile, a scalpel, a microscope slide with a pink specimen on it, and a green flower spike with pale anthers round its base. The captions read take an insect-pollinated flower, remove each whorl in turn on a white tile, slice the carpel lengthways to find the ovules, mount pollen and ovules in iodine, and repeat with a wind-pollinated flower. — Dissecting wind- and insect-pollinated flowers, at a glance. Original BioWelsh diagram.