IGCSE Biology Paper 4 (Theory / Extended)

Topic 1: Characteristics and Classification of Living Organisms -- Challenge Exam 1

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0610

Instructions

Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.

Question Navigation

This paper covers the whole of Topic 1. Like a real Cambridge paper, the seven questions range across every sub-topic — characteristics of living organisms, classification and dichotomous keys, and the features of the five kingdoms — and they are deliberately mixed rather than grouped. All three Topic 1 papers do; they differ in the angle they come at it from, not in what they cover.

Question 1 -- Life in Antarctic Rock

Total: 12 marks

A team working in the Dry Valleys of Antarctica drills into sandstone and finds a black band of material 3 mm below the rock surface. Samples of the band are sealed in flasks with a carbon dioxide sensor and kept at −5 °C in the dark. Over 30 days the carbon dioxide concentration in the flasks rises steadily. When the flasks are moved into the light and warmed to 5 °C, the carbon dioxide concentration falls and the samples turn a deep green.

The team wishes to establish whether the black band contains living organisms.

(a) [3]

The rise in carbon dioxide in the dark, and the fall in carbon dioxide in the light, are each evidence for a characteristic of living organisms. Identify the characteristics shown, and state what each observation demonstrates.

Model Answer -- 1(a)

rise in carbon dioxide in the dark shows respiration — chemical reactions in cells breaking down nutrient molecules and releasing energy [1]

the carbon dioxide leaving the organisms is a waste product of metabolism, so its removal is excretion [1]

the fall in carbon dioxide in the light shows nutrition, materials being taken in (by photosynthesis) for energy, growth and development [1]

⚠ If you missed marks here: Each mark needs the characteristic named and linked to the specific observation. Writing out MRS GREN without connecting any of it to the carbon dioxide data earns nothing, and “the organisms are breathing” is refused because respiration is a set of reactions in cells, not a movement of air.

1 mark -- respiration named and linked to carbon dioxide rise in the dark 1 mark -- excretion named for removal of carbon dioxide as a waste product of metabolism 1 mark -- nutrition named and linked to uptake of carbon dioxide in the light

(b) [4]

The team wants to find out whether the organisms in the band grow. Describe how they should obtain valid evidence of growth over a period of eight weeks.

Model Answer -- 1(b)

take several samples of the band of equal area or equal volume at the start [1]

dry each sample in an oven at a low temperature until the mass is constant, then weigh it [1]

repeat with fresh samples taken at intervals over the eight weeks, keeping all other conditions the same [1]

an increase in the mean dry mass per unit area over time is evidence of growth [1]

⚠ If you missed marks here: The whole question turns on the words dry mass and constant mass. Measuring fresh mass would record water absorbed from melting ice rather than new material built, and stopping the drying after a fixed time rather than at constant mass leaves an unknown amount of water behind. Notice also that drying destroys the sample, which is why fresh samples must be taken each time.

1 mark -- take several samples of equal area or volume 1 mark -- dry in an oven and weigh to constant mass 1 mark -- repeat with fresh samples at intervals under the same conditions 1 mark -- increase in mean dry mass over time is the evidence for growth

Explain why the team also seals a flask containing crushed sandstone that has been heated to 200 °C for one hour, and state what result they would expect from it.

Model Answer -- 1(c)

the heated sample acts as a control [1]

heating to 200 °C kills any organisms present, so no living organism remains to respire [1]

no rise in carbon dioxide would be expected; if a rise occurred it would show the gas came from non-living chemical change in the rock rather than from organisms [1]

⚠ If you missed marks here: A control is not just “a second experiment” — the mark is for saying what it rules out. Answers that stop at “to compare” leave the examiner unable to award anything, because comparison is the purpose of every second flask ever set up.

1 mark -- identified as a control 1 mark -- heating kills any organisms present 1 mark -- no rise in carbon dioxide expected, so any rise would show non-living chemical change

(d) [2]

A visiting student argues that because the samples showed no visible change for the first 30 days, the black band cannot have been alive at the start of the investigation. Evaluate this argument.

Model Answer -- 1(d)

the argument is incorrect: the organisms were dormant, not dead [1]

characteristics such as respiration were occurring at a very low rate rather than not at all, which is why carbon dioxide was still accumulating in the dark [1]

⚠ If you missed marks here: This is the dormancy misconception in a fresh setting. A low rate is not an absent rate, and the carbon dioxide data in the stem already contradict the student — the strongest evaluations use the evidence they have been given rather than a general statement about seeds.

1 mark -- argument rejected, organisms were dormant rather than dead 1 mark -- characteristics occurring at a very low rate, supported by the carbon dioxide rise

Question 2 -- Naming and Ranking a Group of Lizards

Total: 12 marks

A survey of rocky islands off the coast of Oman finds four populations of small lizard. Their scientific names, as currently recorded, are shown below.

population	current name	bases differing from population 1 in a 500-base region
1	Pristurus rupestris	--
2	Pristurus carteri	186
3	Asaccus gallagheri	14
4	Pristurus celerrimus	97

Populations 1 and 3 mate readily where their ranges overlap, and their offspring breed successfully. Populations 1 and 2 have never been seen to interbreed.

(a) [2]

Rewrite the name of population 1 as it should appear in a scientific report, and state what each of the two words represents.

Model Answer -- 2(a)

Pristurus rupestris — underlined (or in italics) with a capital P and a lower case r [1]

the first word is the genus and the second is the species [1]

⚠ If you missed marks here: Two separate marks, and candidates routinely score only one by giving a correct name with no explanation or a correct explanation with the capitals wrong. Underlining is the handwritten equivalent of italics and is expected in an exam answer.

1 mark -- name written with capital genus, lower case species, underlined or italic 1 mark -- first word identified as genus and second as species

(b) [3]

Use the breeding evidence to decide whether populations 1 and 3 belong to the same species. Justify your answer.

Model Answer -- 2(b)

they belong to the same species [1]

because a species is a group of organisms that can reproduce to produce fertile offspring [1]

and the offspring of populations 1 and 3 breed successfully, so they are fertile [1]

⚠ If you missed marks here: The mark for the definition is only awarded if the word fertile appears. An answer that says “they can breed together so they are one species” states the conclusion using the incomplete definition and cannot be given the reasoning marks, because that same argument would make a horse and a donkey one species.

1 mark -- same species stated 1 mark -- definition quoted including the word fertile 1 mark -- evidence applied: the offspring themselves breed successfully

Using the DNA data, place populations 2, 3 and 4 in order of relatedness to population 1, most closely related first. Explain what the numbers of differing bases indicate about common ancestors.

Model Answer -- 2(c)

order: 3 (14), then 4 (97), then 2 (186) [1]

the table gives the number of bases that differ, so the smallest number indicates the closest relationship [1]

differences in base sequences accumulate over time [1]

so few differences mean the populations separated from a common ancestor relatively recently [1]

⚠ If you missed marks here: The single most costly error in this topic is reading a differences column as a similarity column, which reverses the whole ranking and takes every mark in the part with it. Say the heading aloud as a sentence before you rank anything. The reasoning marks require the words accumulate over time and common ancestor, not simply “they are more alike”.

1 mark -- correct order 3, 4, 2 1 mark -- recognises that the column shows differences, so fewer means closer 1 mark -- states that differences accumulate over time 1 mark -- links few differences to a more recent common ancestor

(d) [3]

Suggest, with reasons, one change that should be made to the names in the table.

Model Answer -- 2(d)

population 3 should be moved into the genus Pristurus (renamed Pristurus gallagheri) [1]

because classification should reflect evolutionary relationships [1]

and population 3 differs from population 1 at only 14 bases, far fewer than population 2 at 186, yet population 2 is currently placed in the same genus [1]

⚠ If you missed marks here: Three separate things are needed: the change, the principle and the evidence. Answers that give only the numbers score one mark, and answers that only recite “classification reflects evolution” without using the table score one mark. A common misconception is that a name, once given, cannot be altered — in fact DNA evidence has caused a great many groups to be renamed.

1 mark -- change identified: population 3 moved into the genus Pristurus 1 mark -- principle stated: classification should reflect evolutionary relationships 1 mark -- evidence used: 14 differences compared with 186 for a population already in that genus

Question 3 -- Keys for a Rock Pool Survey

Total: 12 marks

A class surveying a rock pool in Goa records five animals.

animal	description
A	shell in two hinged halves; no head; no legs
B	single coiled shell; head with two tentacles; no legs
C	hard chalky exoskeleton; five pairs of walking legs; two pairs of antennae
D	soft body; five arms of equal length; no shell; no legs
E	exoskeleton; four pairs of legs; two body parts; no antennae

A student has begun to write a key for the survey:
“1a lives attached to the rock → go to 2; 1b swims freely → go to 4. 2a shell is pretty → B; 2b shell is plain → A. 3a has lots of legs → C; 3b has fewer legs → E; 3c has no legs → D.”

(a) [3]

Identify three separate faults in the student’s key, explaining in each case why the fault would prevent the key from working.

Model Answer -- 3(a)

step 1 uses behaviour or habitat, which cannot be determined from the specimen itself [1]

step 2 uses a personal judgement (“pretty”), which is not a measurable or visible structural feature and different users would disagree [1]

step 3 offers three choices, so it is not dichotomous; “lots of” and “fewer” are also vague with no number given [1]

⚠ If you missed marks here: Each mark requires the fault and the reason. “Step 1 is wrong” earns nothing. A useful check is the stranger test: could someone who has never seen these animals complete every step from the drawings alone? Anything requiring knowledge, opinion or a comparison with no number fails it.

1 mark -- step 1 fault: uses habitat or behaviour, not visible in the specimen 1 mark -- step 2 fault: subjective judgement rather than a structural feature 1 mark -- step 3 fault: three branches so not dichotomous, and vague leg description

(b) [4]

Construct a dichotomous key that would allow all five animals to be identified.

Model Answer -- 3(b)

step 1 separates the animals with jointed legs from those without, for example: 1a jointed legs present → go to 2; 1b jointed legs absent → go to 3 [1]

the legged animals are separated by a countable feature, for example: 2a four pairs of legs, no antennae → E; 2b five pairs of legs, two pairs of antennae → C [1]

the legless animals are separated in two further steps, for example: 3a shell present → go to 4; 3b shell absent, body with five arms → D; 4a shell coiled, head with tentacles → B; 4b shell in two hinged halves → A [1]

every step offers exactly two choices, all features are visible in the specimen, and each animal reaches one endpoint only [1]

⚠ If you missed marks here: The fourth mark is a quality mark for the key as a whole, and examiners earn it by tracing every animal through your key. The commonest reasons it is lost are an animal that reaches two endpoints, an animal that reaches none, and a step that quietly reintroduces habitat. Trace all five yourself before moving on — it takes twenty seconds.

1 mark -- sensible first step separating legged from legless animals 1 mark -- legged animals separated using leg number or antennae 1 mark -- legless animals separated using shell form and body shape 1 mark -- key is fully dichotomous, uses visible features and identifies all five animals

Animal C and animal E are both arthropods. State the features that place them both in the arthropods, and give one feature that distinguishes them.

Model Answer -- 3(c)

both have an exoskeleton and a segmented body [1]

both have jointed legs and no backbone [1]

they differ in the number of pairs of legs (five pairs in C, four pairs in E) or in the antennae (two pairs in C, none in E) [1]

⚠ If you missed marks here: This part tests whether you can keep the phylum level and the group level apart. Offering “four pairs of legs” as an arthropod feature, or “exoskeleton” as a way of telling the two apart, is the classic error and loses marks in both directions.

1 mark -- exoskeleton and segmented body given as arthropod features 1 mark -- jointed legs and absence of a backbone given 1 mark -- a distinguishing feature given: leg number or antennae

(d) [2]

Explain why a key intended for use in the field must be based on visible structural features.

Model Answer -- 3(d)

the user has only the specimen in front of them, so any feature they cannot see cannot be assessed [1]

features such as habitat, diet or behaviour are not shown by the specimen, and judgements such as “large” vary between users, so the route through the key would become uncertain [1]

⚠ If you missed marks here: The mark is for the reason, not for the rule. Restating “keys must use visible features” simply repeats the question. Note also the second half: reliability matters as much as visibility, which is why comparisons should be made against another part of the same organism.

1 mark -- user has only the specimen, so unseen features cannot be assessed 1 mark -- habitat, behaviour or subjective judgements make the route uncertain

Question 4 -- Five Organisms from a Soil Sample

Total: 12 marks

A soil sample from a coffee plantation in Coorg is examined and five things are described.

	description
P	single-celled; no nucleus; cell wall present, not cellulose; circular DNA loose in the cytoplasm; small rings of extra DNA present
Q	multicellular; body of fine branching threads; cell wall present, not cellulose; no chloroplasts; releases enzymes onto dead leaves and absorbs the products
R	single-celled; nucleus present; no cell wall; no chloroplasts; engulfs bacteria
S	multicellular; cell wall of cellulose; chloroplasts present; stores starch
T	not a cell; a protein coat surrounding genetic material; only increases in number inside the cells of Q

(a) [4]

Name the kingdom of P, Q, R and S.

Model Answer -- 4(a)

P — prokaryote [1]

Q — fungus [1]

R — protoctist [1]

S — plant [1]

⚠ If you missed marks here: Run the questions in a fixed order — nucleus, then number of cells, then wall and chloroplasts — and this part becomes mechanical. The two answers most often wrong are Q, called a plant because it is multicellular with a wall, and R, called an animal because it moves and engulfs food.

1 mark -- P prokaryote 1 mark -- Q fungus 1 mark -- R protoctist 1 mark -- S plant

(b) [4]

Give two features from the descriptions that place Q in its kingdom, and two features that place R in its kingdom.

Model Answer -- 4(b)

Q has a cell wall that is not made of cellulose [1]

Q has no chloroplasts and feeds saprophytically, releasing enzymes onto dead material and absorbing the products [1]

R is single-celled with a nucleus, so it cannot be an animal, which is multicellular [1]

R has no cell wall and no chloroplasts, so it is not a plant or a fungus [1]

⚠ If you missed marks here: Features must be quoted from the description, not from memory. Saying “fungi have hyphae” without pointing at the branching threads in the table is weaker, and saying “Q does not move” is not creditable at all because plants do not move either.

1 mark -- Q: cell wall not made of cellulose 1 mark -- Q: no chloroplasts and saprophytic nutrition 1 mark -- R: single-celled with a nucleus, so not an animal 1 mark -- R: no cell wall and no chloroplasts, so not a plant or fungus

State the two features of T, and name the group of organisms to which T belongs.

Model Answer -- 4(c)

a protein coat surrounding genetic material [1]

T is a virus, and it belongs to no kingdom [1]

⚠ If you missed marks here: The syllabus limits the features of a virus to exactly two, so a long description wastes time and adds nothing. Anything mentioning cytoplasm, ribosomes or a cell wall is wrong, because a virus is not a cell.

1 mark -- protein coat and genetic material given as the two features 1 mark -- identified as a virus, belonging to no kingdom

(d) [2]

Explain why T is not placed in any of the five kingdoms.

Model Answer -- 4(d)

the five kingdoms are defined by cell structure, and T is not a cell [1]

T shows almost none of the characteristics of living organisms and can only replicate inside a host cell [1]

⚠ If you missed marks here: The reasoning has to reach cell structure. “Because it is not alive” is the conclusion rather than the reason, and “because it is too small” cannot be right, since organism P in the same table is comparably small and is classified without difficulty.

1 mark -- kingdoms are defined by cell structure and a virus is not a cell 1 mark -- shows almost none of the characteristics and replicates only inside a host cell

Question 5 -- Four Vertebrates from a Wetland

Total: 10 marks

Four vertebrates are recorded during a survey of a wetland in Kerala.

	body covering	gas exchange	eggs	body temperature
J	wet scales	gills throughout life	soft, no shell, laid in water	varies with surroundings
K	moist, no scales	gills when young, lungs when adult	jelly-coated, laid in water	varies with surroundings
L	dry scales	lungs	leathery shell, laid on land	varies with surroundings
M	feathers	lungs	hard shell, incubated	constant

(a) [4]

Name the vertebrate group of each of J, K, L and M.

Model Answer -- 5(a)

J — fish [1]

K — amphibian [1]

L — reptile [1]

M — bird [1]

⚠ If you missed marks here: Read the body covering first: wet scales, moist and scale-free, dry scales, feathers. Those four rows separate the groups on their own, and the egg column confirms them. K and L are the pair most often swapped, because both are cold-blooded four-legged animals in the same wetland.

1 mark -- J fish 1 mark -- K amphibian 1 mark -- L reptile 1 mark -- M bird

(b) [3]

Compare K and L, giving three differences. Write each difference as a single statement covering both animals.

Model Answer -- 5(b)

K has moist skin without scales whereas L has dry skin with scales [1]

K uses gills when young and lungs when adult whereas L uses lungs throughout life [1]

K lays jelly-coated eggs in water whereas L lays eggs with a leathery shell on land [1]

⚠ If you missed marks here: A comparison mark needs both sides in the same statement, linked by whereas, while or but. Four separate one-sided facts — two about K and two about L — frequently score fewer marks than three properly linked sentences, because the examiner is not permitted to pair them up for you.

1 mark -- skin compared for both animals in one statement 1 mark -- gas exchange compared for both animals in one statement 1 mark -- eggs compared for both animals in one statement

A student watching a dolphin in the sea nearby states that it must belong to the same group as J because it is streamlined, has fins and lives in water. Evaluate this statement.

Model Answer -- 5(c)

the statement is incorrect: the dolphin is a mammal, not a fish [1]

classification uses features rather than habitat or body shape, and a dolphin has hair, uses lungs, keeps a constant body temperature and feeds its young on milk from mammary glands — any two features [1]

a streamlined body with fins is advantageous to any fast-swimming animal, so it can appear in unrelated groups and is not evidence of a close relationship [1]

⚠ If you missed marks here: The third mark is the discriminating one and is rarely given: it requires you to explain why the resemblance exists rather than simply denying it. This is the same reasoning that explains why a cactus and a euphorbia look alike, and it is the reason DNA base sequences are preferred to appearance as evidence of relatedness.

1 mark -- statement rejected and dolphin identified as a mammal 1 mark -- two mammalian features quoted, such as hair, lungs, milk from mammary glands 1 mark -- explains that similar body shape can evolve in unrelated groups living in water

Question 6 -- Arthropods and Plants of a Mango Orchard

Total: 12 marks

A survey of a mango orchard records four arthropods and two plants.

arthropod	pairs of legs	body parts	pairs of antennae	wings
W	3	3	1	2 pairs
X	4	2	0	none
Y	7	2	2	none
Z	28	head plus many similar segments	1	none

Plant 1 has long narrow leaves with veins running side by side, and flowers with three petals. Plant 2 has broad leaves with veins branching from a midrib, flowers with five petals, and produces seeds inside fruits. A third plant, plant 3, has divided leaves called fronds, no flowers, and brown patches on the underside of the fronds.

(a) [3]

State three features shared by W, X, Y and Z that place all four in the arthropods.

Model Answer -- 6(a)

an exoskeleton [1]

a segmented body [1]

jointed legs (and no backbone) [1]

⚠ If you missed marks here: These three features define the phylum, which means they can never be used to separate the four groups from one another. Giving “three pairs of legs” here answers a different question and scores nothing, because it is true of only one of the four animals.

1 mark -- exoskeleton 1 mark -- segmented body 1 mark -- jointed legs or absence of a backbone

(b) [4]

Name the arthropod group of W, X, Y and Z, giving for Y and Z the feature in the table that distinguishes them from each other.

Model Answer -- 6(b)

W — insect; X — arachnid [1]

Y — crustacean [1]

Z — myriapod [1]

Y has two pairs of antennae while Z has one; Z also has legs on many similar segments rather than a body in two regions [1]

⚠ If you missed marks here: Once an animal has more than four pairs of legs, the leg count stops being useful because crustaceans and myriapods both have plenty. The antennae column is doing the work, and the mark scheme will not accept “Z has more legs” as the distinguishing feature.

1 mark -- W insect and X arachnid 1 mark -- Y crustacean 1 mark -- Z myriapod 1 mark -- antennae number given as the feature distinguishing Y from Z

Classify plant 1 and plant 2 into their groups within the flowering plants, giving the evidence for each.

Model Answer -- 6(c)

plant 1 is a monocotyledon, shown by leaves with parallel veins [1]

and by flower parts in a multiple of three [1]

plant 2 is a dicotyledon, shown by leaves with a branching network of veins and flower parts in a multiple of five [1]

⚠ If you missed marks here: Two independent features agreeing makes a classification secure, and the question supplies both for each plant, so quote both. The names refer to the number of cotyledons in the seed, which is not visible here — a reminder that groups are often named after one feature and identified by another.

1 mark -- plant 1 monocotyledon with parallel veins 1 mark -- plant 1 supported by flower parts in a multiple of three 1 mark -- plant 2 dicotyledon with net veins and flower parts in a multiple of five

(d) [2]

Plant 3 is also a member of the plant kingdom. Name the group to which it belongs and explain how it reproduces.

Model Answer -- 6(d)

plant 3 is a fern [1]

it reproduces by spores, produced in the brown patches on the underside of the fronds, and it has no flowers, seeds or fruits [1]

⚠ If you missed marks here: The brown patches are the giveaway and are drawn in exam figures for exactly that reason. Answers saying the fern reproduces by seeds, or that it is not a plant because it has no flowers, are the two standard errors — ferns are true plants with roots, stems and leaves.

1 mark -- identified as a fern 1 mark -- reproduces by spores produced on the underside of the fronds, no flowers or seeds

Question 7 -- Duckweed on a Village Tank

Total: 10 marks

Duckweed is a tiny floating plant. Each plant consists of a small flat frond, and new fronds bud off from the parent and separate. A student floats 20 duckweed plants in a tank of pond water and records the number of plants and the total dry mass of a separate sample of 20 plants harvested each day.

day	0	2	4	6	8
number of plants in the tank	20	34	61	108	140
dry mass of 20 harvested plants / mg	16.0	17.8	19.5	21.0	21.2
fresh mass of 20 harvested plants / mg	310	352	388	402	371

(a) [3]

Identify the characteristic of living organisms shown by each of the first two rows of data, and explain why the third row is the least useful measurement.

Model Answer -- 7(a)

the increase in the number of plants shows reproduction — processes that make more of the same kind of organism [1]

the increase in dry mass per 20 plants shows growth — a permanent increase in size and dry mass [1]

fresh mass includes water, which can be gained or lost without any new material being made, so its fall between day 6 and day 8 does not show a loss of growth [1]

⚠ If you missed marks here: Two rows, two different characteristics — candidates frequently give “growth” for both and lose a mark, because more individuals is reproduction rather than growth. The third mark needs the reason fresh mass is unreliable, not simply the statement that it is.

1 mark -- reproduction identified from the increase in plant number 1 mark -- growth identified from the increase in dry mass, with the definition 1 mark -- fresh mass unreliable because it includes water

(b) [4]

Describe how the student should have measured the dry mass of a sample, and explain why a fresh sample had to be harvested on each day.

Model Answer -- 7(b)

dry the sample in an oven at a low temperature [1]

reweigh at intervals until the mass is constant, showing all the water has been removed [1]

drying kills the plants and removes their water, so the same plants cannot be returned to the tank or measured again [1]

the plants harvested each day must therefore come from the same batch grown in identical conditions, so that the samples are comparable [1]

⚠ If you missed marks here: The phrase to constant mass is the technical mark and is missed by most candidates, who write “dry it in an oven and weigh it”. The second half of the part is about experimental design: because the measurement is destructive, sampling replaces tracking, and the samples must be genuinely comparable for the comparison to mean anything.

1 mark -- dry in an oven at a low temperature 1 mark -- reweigh to constant mass so all water is removed 1 mark -- drying is destructive so the same plants cannot be measured again 1 mark -- samples must come from the same batch grown in identical conditions

Between day 6 and day 8 the number of plants continued to rise but the dry mass per 20 plants rose by only 0.2 mg. Suggest an explanation for this pattern, and state one further measurement that would test your suggestion.

Model Answer -- 7(c)

the individual plants are becoming smaller or thinner, so 20 plants weigh little more than before, even though there are more of them [1]

this is likely because the plants are now crowded and are competing for light or for nutrients in the water, limiting how much new material each can build [1]

a suitable further measurement: the mean area or mean dry mass of a single frond over time, or the concentration of nutrients remaining in the water [1]

⚠ If you missed marks here: This is the part that separates grades. The trap is to say “growth has stopped”, which contradicts the rising plant number in the same table. The dry mass is measured per 20 plants, so a constant value while numbers rise tells you about the size of individuals, not about the population. Any sensible, testable measurement is credited in the final mark, provided it actually tests the explanation you gave.

1 mark -- recognises that individual plants are getting smaller rather than growth stopping 1 mark -- suggests competition for light or nutrients as the cause 1 mark -- gives a suitable further measurement that tests the suggestion

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