Plant Reproduction: WJEC A Level Biology Cram

Lesson 1 of 24

The parts of a flower

Fast revision of topic 4.2, Sexual reproduction in plants, in WJEC A level Biology Unit 4. It follows the WJEC specification.

A flower has four , outside to inside: (), (), ( + ), ( + + ).

What each part does
Part	Function
	Protects the bud
	Attracts pollinators
	Holds four ; makes
	Catches ; makes it
	The tube grows down through it
	Holds the ; becomes the fruit
	Secretes as the reward

The half flower below shows where each part sits.

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.

, and together are the .
and together are the .
sit inside the closed .

Lesson 2 of 24

Insect-pollinated flowers

Large, coloured , often with guide markings.
Scent and as the reward.
and inside the flower, so the insect brushes them.
Small, sticky .
Large, heavy, sticky, sculptured , in small quantities.

Lesson 3 of 24

Wind-pollinated flowers

The diagram below shows the 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.

No bright , no scent, no .
hang outside on long .
Large feathery hang outside.
Small, light, smooth in very large quantities.

The comparison in one table
Feature	Insect	Wind
	Large and coloured	Small or absent
Scent and	Present	Absent
	Inside	Outside, on long
	Small and sticky	Large and feathery
	Large, sticky, sculptured	Small, light, smooth
Quantity	Small	Very large

Lesson 4 of 24

The pollen grain

The diagram below labels 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.

: tough outer wall, sculptured.
: thin cellulose inner wall.
: controls tube growth.
: divides into the two male gametes.

Insect is about 60 µm, heavy and spiky. Wind is about 25 µm, light and smooth.

Lesson 5 of 24

Transverse section of an anther

The section below shows the four .

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.

Two lobes, each with two : four in all.
Epidermis outside, then the with thickened walls.
lines each sac and nourishes the developing .
holds the vascular strand; a groove marks where the splits.

Lesson 6 of 24

Making the male gametes

The flow diagram below runs from mother cell to 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.

(2n) → meiosis → of four cells.
Each becomes a with a .
Mitosis → + .
Mitosis again → two male gametes (n).

Lesson 7 of 24

Dehiscence

The dries out, its thickened cells shrink and put the wall under tension, and the wall tears along the between the two of each lobe. The edges curl back and the is exposed.

Lesson 8 of 24

The ovule

The diagram below labels 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.

attaches the to the wall.
Two ; the gap between them is the .
supplies nutrients.
holds eight : + 2 , 3 , 2 that fuse into one .

Lesson 9 of 24

Making the female gamete

The sequence below runs from mother cell to mature .

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.

(2n) → meiosis → four cells; three disintegrate.
The survivor divides by mitosis three times → eight in one cell.
+ 2 at the micropylar end, 3 at the other, 2 in the middle.
The two fuse: one .

Lesson 10 of 24

Pollination

is moving from to . Fertilisation is gametes fusing, later and inside the .

Self and cross
Type	Where the pollen comes from	Result
	The same flower or the same plant	Less ; guaranteed seed
	A different plant of the same	More ; needs a

Lesson 11 of 24

Why cross-pollination helps

Increases genetic and widens the .
Reduces homozygosity, so fewer harmful are expressed.
Some offspring are more likely to survive a change in the environment.

Lesson 12 of 24

Avoiding self-pollination

The diagram below shows four 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.

: and ripen at different times ( or ).
held above the .
Genetic incompatibility: own does not .
Male and female flowers separate, on one plant or on different plants.

Lesson 13 of 24

The pollen tube

The diagram below follows the tube from to .

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.

Sucrose from the makes a compatible grain .
The leads and controls growth; it codes for .
The digest a path down the .
from the guide the tube to the .
The tip breaks down and releases the two male gametes.

Rate of growth = distance ÷ time. 20 cm in 20 hours = 200 mm ÷ 1200 min = 0.2 mm min⁻¹.

Lesson 14 of 24

Double fertilisation

The diagram below shows both 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.

Male (n) + (n) → (2n) → embryo.
Male (n) + (2n) → (3n) → .
No fertilisation no , so no food store is made and no seed forms.

Lesson 15 of 24

What becomes what

The diagram below lists each change.

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.

→ embryo (, , ).
→ , the food store.
→ . → seed.
→ fruit. wall → . → scar.

Lesson 16 of 24

The broad bean seed

The diagram below labels a broad bean with one removed.

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.

outside; is the scar of the ; is the pore beside it.
Two hold the food store: .
points to the ; the is hooked.

Lesson 17 of 24

The maize grain

The diagram below labels a maize grain cut 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.

fused to the : the grain is a one-seeded fruit.
just inside makes the .
is the starchy food store: .
is the single ; sheaths the .

Bean against maize
Feature	Broad bean	Maize
	Two	One ()
Food store
	Separate	Fused to the fruit wall
Class

Lesson 18 of 24

Seed dispersal

Five methods
Method	Adaptation
Wind	Parachute of hairs, or a wing
Animal, carried	Hooks that catch in fur
Animal, eaten	Fleshy, sweet, coloured fruit
Explosive	Fruit wall dries, splits and curls
Water	Buoyant, waterproof fruit wall

reduces with the parent and with siblings, and lets a larger area be colonised.

Lesson 19 of 24

Dormancy

A seed is alive but barely metabolising, at about 10 to 15 % water, with a hard . survives the winter, spreads out in time and allows first.

It is broken by a -specific trigger: cold, light, passing through a gut, fire, or the washing out of an inhibitor.

Lesson 20 of 24

Conditions for germination

The diagram below gives the three conditions and the reasons.

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: mobilises , splits the , transports the soluble products.
Oxygen: , for the used in growth and .
Suitable temperature: raises activity, but heat denatures the .

Light is not needed by most seeds, because the seedling lives on its food store until its leaves reach the light.

Lesson 21 of 24

Germination of a broad bean

The sequence 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 through the ; swell; splits.
Oxygen enters; rises.
→ , → amino acids, → and .
Soluble products move to the growing points for and growth.
first (anchors and takes up water), then the hooked .

The graph below shows during .

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.

falls; seedling rises.
Total falls at first, because loses carbon dioxide.
It rises again once the leaves photosynthesise.

Lesson 22 of 24

Gibberellin and germination

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.

The embryo secretes .
It diffuses to the .
there are switched on, so and are made.
The diffuse into the and the stores.
The soluble products diffuse back to the embryo and are respired for growth.

Evidence: half-grains with no embryo digest no starch, but they do once is added. Dwarf peas grow tall when treated with ; tall peas hardly change.

Lesson 23 of 24

The specified practicals

Three specified practicals: starch and seeds, flower dissection, and a of an .

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 whether increases how much 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 seedsSource of ; soaked 48 hours.
Bottle of 10 % bleach
Bottle of 10 % bleachSurface sterilises the seeds.
Starch-agar plates
Starch-The .
Incubator
25 °C for 48 hours.
Bottle of iodine solution
Bottle of solutionStains the starch that is left.
Pencil and ruler
Pencil and rulerMeasures each halo.

Safety

Bleach and RiskIrritation of skin and eyes.ControlEye protection and gloves; rinse splashes.
ScalpelRiskCuts.ControlCut on a tile, away from your fingers.

Variables

Independent variableTreatment of the seeds

, water, or boiled.

Dependent variableHalo diameter

In mm, across the widest point.

Controlled variablesVariety, age and mass of seed, Soaking time, Concentration and depth of agar, Temperature and time of incubation

Variety, age and mass of seed

One variety, similar masses.

Soaking time

48 hours for every batch.

Concentration and depth of

Same volume of 1 % per plate.

Temperature and time of

25 °C for 48 hours.

Control or controlled variable?

Boiled seeds are a : they show the halo needs a working .
depth, seed variety and temperature are .
Glass beads in place of seeds are not an acceptable .

Method, step by step

1)
Soak the seeds, then surface sterilise them in bleach and rinse.
Why?
also make and would give false halos.
2)
Treat three batches: , water, and boiled and cooled.
3)
Halve the seeds and lay them cut face down on starch , well apart.
4)
all plates at 25 °C for 48 hours, then flood with and measure each halo.

Results and analysis

Example data

halo diameter after 48 hours

Treatment	halo diameter / mm
Live half-seeds	9.6
Live half-seeds in	17.8
Boiled half-seeds ()	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.

What the results show

Boiled seeds gave no halo: the were denatured, so the whole plate stained blue-black.

Live seeds gave a halo of about 9.6 mm, and seeds treated with about 17.8 mm.

therefore increased the amount of released.

Worked calculations

= (17.5 + 18.0 + 17.9) ÷ 3 = 17.8 mm

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

Evaluation

Seeds vary in size and enzyme content.

Effect on resultsHalo diameter varies between plates.

ImprovementUse several half-seeds and take a .

Halo edges are not sharp.

Effect on resultsMeasurements differ between people.

ImprovementOne person measures every plate, on a dark background.

Model answer: describe the method

Describe how you would develop this procedure to find out whether increases production.

Original practice response · 6 indicative marks

One batch of seeds soaked in , one in water.
A third batch boiled and cooled as a .
All halved and laid cut face down on starch .
Same temperature and time for every plate, then flood with .
Measure and compare the halo diameters.
variety, age, mass, soaking time and depth.

What the answer needs to cover

Seeds soaked in
Clear compared
Same variety and mass
Same soaking time
Same
Same temperature and time

Exam practice

Why is there no halo around half-grains with the embryo removed?Show answer

No embryo no , so the makes no and the starch is not .

No
No
Starch not

Specified practical