←Topic 25 · Vascular Plants
Practical 25 Marine Biology CAPS block 48 · Term 3 · 2 hours

Vascular Plants — a Flower, an Estuary and a Mangrove Spear

Take a flower apart and grow its pollen tubes in sugar water. Walk an estuary from the channel to the dry bank and map where eelgrass, cordgrass and glasswort grow. Then test the claim that mangrove seedlings plant themselves like spears — with a weighted pencil and a basin of mud.

What CAPS asks for

Investigations column, block 48: "Mentioned plants and structures should be recognisable from diagrams and photographs, and ideally highlighted for study during a field trip · Flower dissection." Resources: "Videos · Internet · Fresh flowers." The key concepts ask you to "illustrate and discuss reproduction in flowering plants, highlighting the flowers, pollination, mechanism of fertilisation, and production of seeds".

The flower dissection is Investigation 1. Recognising the plants on a field trip — eelgrass and the salt marsh — is Investigation 2, and mangroves are Investigation 3. Investigation 4 is the booklet's "Test your knowledge" questions.

What you need

No estuary near you, and no mangroves? That's fine.

The nearest mangroves to Gqeberha are hundreds of kilometres away, so Investigation 3 is built on described specimens and a kitchen model. Investigation 2 supplies a full estuary transect, and Investigation 1 supplies a dissected lily and a pollen-tube experiment. Every question can be answered from the panels.

Safety — blades, pollen and mud Only cut with the craft knife on a board, cutting away from your fingers, with an adult nearby. Lily pollen stains clothes and is poisonous to cats — keep cut lilies away from pets and wash your hands. In the estuary: go on a falling spring tide with an adult, watch for soft mud you can sink into, and keep out of the channel. Walk on the bare mud between the eelgrass, not on it; take only a strand already washed up. Wash your hands after handling estuary mud.
1 Flower Dissection — and Growing Pollen Tubes CAPS: "Flower dissection" · "highlighting the flowers, pollination, mechanism of fertilisation, and production of seeds"

Aim

To identify the reproductive parts of a flower, and to watch the first step of fertilisation — the growth of a pollen tube.

Method

  1. Count the flower's parts from the outside in: sepals, petals, stamens and the pistil in the middle. (In a lily the three sepals look like the three petals.)
  2. Remove each ring of parts and tape it onto the card in its ring, labelled. Measure a stamen and the pistil.
  3. Look at an anther with the hand lens, then dab it onto dark paper to see the pollen. Touch the stigma: is it sticky?
  4. With an adult, cut the ovary across. Count the chambers and look for the ovules inside.
  5. Pollen tubes. Make sugar solutions of 0, 5, 10, 15 and 20 % (0, 1, 2, 3 and 4 level teaspoons of sugar, roughly, in 100 mL of warm water). Put a drop of each on a dish, dust it with pollen, cover with cling film and keep it warm for an hour. With a microscope, count how many of 50 grains have grown a tube.
Supplied results — a dissected lily, and pollen in sugar

Data set 1.1 — the lily. Six coloured tepals (three outer sepals and three inner petals that look alike). Six stamens: filaments 6 cm long, each tipped with a 1.5 cm anther covered in orange pollen. One pistil in the centre: a sticky, three-lobed stigma on a 7 cm style, above a 2 cm green ovary. Cut across, the ovary had three chambers, each with two rows of small white ovules — about 60 in all.

⚠️ About these numbers The pollen-tube test is a standard school experiment; these counts are constructed for this exercise to show the usual pattern. Use them to practise, not to quote.
Data set 1.2 — lily pollen after one hour at 25 °C (50 grains counted at each strength)
Sugar (%)05101520
Grains with a tube21431269
Mean tube length (µm)106018015040

In plain water several grains swelled and burst. In the 20 % solution most grains looked shrivelled.

The booklet's question 3 diagram

A half-flower: petals around the outside; a stamen on the right whose tip is labelled A, with grains labelled B and, in brackets, C; a pistil on the left with the style, whose tip is labelled D; the swollen base of the pistil labelled E, containing the ovule (female gamete); and a heading F to supply.

Analysis

  1. Draw a labelled half-flower (a flower cut in half from top to bottom), showing petals, sepals, a stamen (anther and filament), the pistil (stigma, style and ovary) and the ovules. 6
  2. The booklet's question 3.1: give the labels for A to E, and a suitable heading for F. 6
  3. The booklet's question 3.2: give the function of A and of D, and of the petals and the style. 4
  4. The booklet's diagram labels pollen "(male gamete)" and the ovule "(female gamete)". Correct both labels. 2
  5. Convert the pollen counts to percentages and plot them against sugar strength. Give the best strength, and explain why both plain water and 20 % sugar gave poor results. 5
  6. Describe, in order, what happens between a pollen grain landing on a stigma and the production of a seed. Say where the pollen tube grows and why it is needed. 4
  7. An eelgrass flower is pollinated underwater. Suggest three ways you would expect it to differ from a lily flower, and why. 3
2 An Estuary Walk — Eelgrass and the Salt Marsh CAPS: "Mentioned plants and structures should be recognisable … ideally highlighted for study during a field trip"

Aim

To recognise the flowering plants of an estuary and find out how their position on the bank is linked to how long the tide covers them.

Method

  1. At spring low tide, lay the rope from the edge of the low-water channel straight up the bank to dry land.
  2. Every 5 m, lay the quadrat and estimate the cover of each plant: eelgrass (thin green ribbons), cordgrass (stiff upright grass), glasswort (fleshy, jointed green stems, also called samphire), rushes (tall, round, sharp-tipped stems), and bare mud. Count the crab holes.
  3. Find a strand of eelgrass already washed up. At home, lay a leaf in a glass of water, squeeze it under water, then cut across it and look at the cut end with the hand lens.
Supplied results — a transect up the bank of the Swartkops estuary
⚠️ About these numbers This transect is a composite built from the zonation typical of an Eastern Cape estuary. It is not one real survey; use it to practise.
Data set 2.1 — percentage cover, crab holes, and how much of the time each point is under water
Distance from the channel (m)051015202530
under water (% of the time)957555402050
eelgrass704550000
cordgrass0106035500
glasswort00104560200
rushes0000105530
bare mud or sand30452520252570
crab holes261824930

Data set 2.2 — an eelgrass strand. Flat ribbon leaves 2 mm wide and up to 25 cm long, growing from a creeping underground stem (rhizome) with tufts of roots at each joint. The leaf floated. Squeezed under water, it let out a stream of tiny bubbles from the cut end; cut across, the hand lens showed rows of small air channels. Tiny flowers were tucked inside a leaf sheath.

Analysis

  1. Draw a kite diagram or bar chart of the cover of the four plants against distance from the channel. 5
  2. Describe the order in which the four plants replace each other up the bank, and link it to how long each point is under water. 3
  3. Suggest two reasons why eelgrass grows only at the low end of the bank. 2
  4. Glasswort is a succulent, with thick, fleshy stems. Suggest how this helps it on the high, salty, often dry part of the marsh. 2
  5. Give two functions of the air channels in an eelgrass leaf. 2
  6. Suggest why the crab holes were most common in the middle of the bank. 2
  7. Give three differences between eelgrass and a seaweed such as sea lettuce or kelp. 3
  8. CAPS and the booklet list Zostera capensis and "Thalassodendron ciliata" as South Africa's estuarine eel grasses. Thalassodendron ciliatum (the usual spelling) is a tropical seagrass of sheltered reef flats and shallow sea floors, found in South Africa only in the far north-east. Which of the two would you expect in the Swartkops estuary, and why? 2
3 Mangroves — Roots, Salt and a Spear in the Mud CAPS mangrove concepts 1–5 · "(Photos and diagrams.)"

Aim

To compare South Africa's three mangroves, and to test with a model the claim that their fruits "stab into the mud".

Three mangroves

White mangrove (Avicennia marina). Grey bark. Hundreds of thin, pencil-like roots stick straight up out of the mud around it. Tiny white salt crystals often coat the undersides of its leaves. Its fruit is a flattened, bean-like capsule about 3 cm long that floats. The most widespread mangrove in South Africa, and the first to colonise new mudflats.
Black mangrove (Bruguiera gymnorrhiza). Dark, rough bark. Its roots loop up out of the mud and back down again, like bent knees. Red flowers. Its seedling hangs from the tree as a cigar-shaped hypocotyl 15–25 cm long before it drops. No salt crystals on its leaves.
Red mangrove (Rhizophora mucronata). Arching prop roots grow out of the trunk above the high-tide mark and curve down into the mud, like the legs of a spider. Its hypocotyl grows up to about 50 cm long on the tree. No salt crystals on its leaves. In South Africa it is found only from the Transkei coast northwards.
Where they stop. CAPS says South African mangroves grow "north of the Nahoon river". The southernmost natural mangroves in Africa are at the Kobonqaba estuary (32°36′S). The small Nahoon stand near East London (32°59′S), about 60 km further south, was planted in 1969 — and it has survived and spread.

Method — the spear model

  1. Tape a coin to the point end of a pencil, so it falls point-first like a heavy hypocotyl.
  2. Fill a basin with thick mud or wet sand. Drop the pencil from 1.5 m, point down, 20 times. Count how many times it sticks upright.
  3. Pour water over the mud to 5 cm deep and repeat; then 20 cm deep (use a deep bucket).
Supplied results — the spear model
Data set 3.1 — pencils that stuck upright (out of 20 drops)
Water over the mud0 cm (low tide)5 cm20 cm (high tide)
Stuck upright1671

With 20 cm of water the pencil slowed sharply as it hit the water, turned sideways and settled on its side. Real red and black mangrove hypocotyls are known to float, and can survive for months in the sea before lodging on a distant mudbank.

Analysis

  1. The booklet's question 4: produce a table comparing the white, black and red mangroves in terms of their breathing roots and their fruit adaptations. 6
  2. Explain why mangrove roots need parts that reach up into the air. 2
  3. Describe the two ways mangroves deal with salt, and say which of the three species uses each. 3
  4. What does the spear model suggest about the booklet's claim that hypocotyls "stab into the mud, when dropped, and take root under the adult"? When would it work, and what happens to the hypocotyls that do not stick? Give one limitation of the model. 4
  5. Explain why the white mangrove is the pioneer on new mudflats. 2
  6. The Nahoon stand was planted 60 km south of the natural limit — and survived. What does that suggest about whether temperature alone decides where mangroves stop? 2
  7. The booklet's question 5: prawn farms are often built on land cleared from mangrove forests. In a paragraph, explain how this could affect the local coastline, giving at least three benefits of mangroves and the consequence of losing each. 6
4 The Booklet's Questions "Test your knowledge" questions 1 and 2 · CAPS: "Relatively few Tracheophyta plants … grow in marine settings"

Aim

To answer the booklet's remaining questions, and to explain what makes these plants different from every other producer in this course.

Analysis

  1. The booklet's question 1: give the term for (a) the transfer of pollen to the pistil of a flower; (b) the division of plants to which all flowering plants belong; (c) when a seed from a flowering plant begins to grow into a seedling; (d) the structures in the ovary of a flower containing the female gamete. 4
  2. The booklet's question 2: define (a) stamens; (b) pollen; (c) prop roots; (d) pneumatophores. 4
  3. Land plants take in water by osmosis through their roots. Explain why that is difficult in seawater, and what a marine flowering plant must be able to do to survive. 3
  4. Seagrasses and mangroves are called vascular plants (tracheophytes). Explain what "vascular" means, and why it places them in a different group from every seaweed in Topics 23 and 24. 2
Memo — worked answers and mark allocation (does not print)

Total: 89 marks — Investigation 1: 30 · Investigation 2: 21 · Investigation 3: 25 · Investigation 4: 13.

Investigation 1 — flower dissection

  • Q1 (6): A neat half-flower drawing (1); any five correctly placed labels: petal, sepal, anther, filament, stigma, style, ovary, ovule (1 each, max 5).
  • Q2 (6): A anther · B pollen (grains) · C male gamete (the pollen carries the male gamete) · D stigma · E ovary · F a heading such as "The structure of a typical flower (half-flower)" (1 each).
  • Q3 (4): A (anther) — produces the pollen (1). D (stigma) — a sticky surface that catches pollen during pollination (1). Petals — attract animal pollinators by their colour and scent (and protect the inner parts) (1). Style — holds the stigma up where it can catch pollen, and is the path the pollen tube grows down to the ovary (1).
  • Q4 (2): Pollen grains are not themselves gametes — each grain carries (produces) the male gamete (1). The ovule is not the gamete — it contains the female gamete (the egg cell) (1).
  • Q5 (5): Percentages: 4, 28, 62, 52, 18 % (1). Graph with labelled axes and accurate points (2). Best strength about 10 % (1). In plain water, water rushed into the grains by osmosis and some burst; in 20 % sugar, water was drawn out of the grains by osmosis, so they shrivelled and could not grow tubes (1).
  • Q6 (4): Pollen lands on the sticky stigma — pollination (1). The grain grows a pollen tube down through the style to the ovary and into an ovule (1); the tube carries the male gamete to the egg, because the gamete cannot swim through dry plant tissue (1). The male gamete fuses with the egg — fertilisation — and the fertilised ovule develops a tough coat and becomes a seed (with the ovary becoming the fruit) (1).
  • Q7 (3): Any three: no large coloured petals; no scent or nectar — there are no insects or birds to attract underwater; small, simple flowers hidden in a leaf sheath; pollen carried by water, so long, thread-like or sticky pollen that drifts and catches on a stigma; large, feathery stigmas to catch drifting pollen.

Investigation 2 — an estuary walk

  • Q8 (5): Kite diagram or bars (1); distance on the horizontal axis (1); cover axis labelled and scaled (1); all four plants accurately plotted (1); key and title (1).
  • Q9 (3): From the channel up: eelgrass (0–5 m) → cordgrass (10 m) → glasswort (15–20 m) → rushes (25–30 m) (2). Each plant grows where it is under water for a particular share of the time — eelgrass where it is almost always covered, rushes where the tide rarely reaches (1).
  • Q10 (2): Any two: its thin leaves dry out quickly when exposed, so it needs to be covered nearly all the time; its flowers are pollinated and its seeds dispersed underwater; it is outcompeted by cordgrass higher up; it needs water to hold its soft leaves up to the light.
  • Q11 (2): The fleshy stems store water, so the plant survives long dry periods between spring tides (1); storing water dilutes the salt it takes in, helping it survive the very salty soil left when seawater evaporates (1).
  • Q12 (2): They make the leaves buoyant, holding them up towards the light (1); they carry oxygen down to the roots and rhizome buried in airless mud (1).
  • Q13 (2): Any two: plenty of food from dead plant material (detritus) from the cordgrass and glasswort; firm mud held together by roots, so burrows do not collapse; covered by the tide often enough to stay moist, but not so long that they cannot come out to feed.
  • Q14 (3): Any three: eelgrass has true roots and a rhizome, a seaweed only a holdfast; eelgrass has vascular (conducting) tissue — xylem and phloem; it produces flowers, pollen and seeds, which seaweeds never do; it is a member of the Kingdom Plantae's flowering plants, while sea lettuce is a green alga and kelp is in Chromista; it takes up nutrients through its roots as well as its leaves.
  • Q15 (2): Zostera capensis (1) — it is the eelgrass of South African estuaries and tolerates changing salinity, whereas Thalassodendron ciliatum is a tropical, marine seagrass of reef flats in the far north-east, not an estuarine plant of the Eastern Cape (1). (For an exam on this topic, the CAPS list remains the expected answer.)

Investigation 3 — mangroves

  • Q16 (6): Table, one mark per cell. White — breathing roots: pencil-like pneumatophores pointing straight up; fruit: a flattened capsule that floats, for wide dispersal. Black — breathing roots: knee-like loops; fruit: a cigar-shaped hypocotyl 15–25 cm. Red — breathing roots: prop roots arching from the trunk; fruit: a long hypocotyl up to about 50 cm.
  • Q17 (2): Estuary mud is waterlogged and has almost no oxygen (1); roots must respire, so the parts that reach above the mud take in oxygen from the air for the buried roots (1).
  • Q18 (3): Some filter out most of the salt at their roots and store any excess in old leaves that are then dropped (1) — the black and red mangroves (1). Others take in salt water and excrete salt through glands on their leaves — the white mangrove, whose leaves carry salt crystals (1).
  • Q19 (4): It works well when the hypocotyl drops onto bare mud at low tide (16 of 20 stuck) (1), but hardly at all through high-tide water (1 of 20) (1). Those that do not stick float away on the tide and may lodge on another mudbank — dispersal, not planting under the adult (1). Any one limitation: a pencil is not shaped or weighted like a hypocotyl; real mud and real heights differ; a real hypocotyl floats, a weighted pencil sinks (1).
  • Q20 (2): Its fruits float and are carried far along the coast by currents, so it reaches new mudflats first (1); once established, its roots trap and stabilise mud, making a place where the other mangroves can follow (1).
  • Q21 (2): If temperature alone set the limit, a stand 60 km further south should have failed (1); because it survived, the natural limit may be set partly by whether propagules can reach suitable estuaries — dispersal and estuary mouths — and not only by cold (1).
  • Q22 (6): A paragraph; for each of three benefits, the benefit (1) and the consequence of losing it (1): roots stabilise mud and shorelines → erosion of the coast and estuary banks; mangroves absorb wave energy and storm surges → coastal flooding and damage; they are nurseries for fish and prawns → fewer wild fish and prawns, hurting fisheries (ironically, the prawns too); they filter sediment and pollutants → muddier, more polluted water; leaf litter feeds crabs and the food web → loss of mangrove animals; they store carbon → more CO₂ released.

Investigation 4 — the booklet's questions

  • Q23 (4): (a) pollination · (b) Magnoliophyta · (c) germination · (d) ovules.
  • Q24 (4): (a) Stamens — the male reproductive structures of a flower (1). (b) Pollen — the tiny grains containing the male gamete in flowering plants (1). (c) Prop roots — extensive roots growing from the stem above the ground, helping the plant grow in unstable substrates (1). (d) Pneumatophores — specialised roots of mangroves that grow upwards into the air to absorb oxygen for oxygen-starved root cells (1).
  • Q25 (3): Water moves by osmosis from a weaker solution to a stronger one (1); seawater is saltier than most plant cells, so water tends to be drawn out of the roots rather than in (1). A marine flowering plant must keep its cells more concentrated than the water around it, or filter out salt at its roots, and get rid of the excess salt it absorbs (1).
  • Q26 (2): Vascular plants have conducting tissue — xylem for water and phloem for food — joining roots, stems and leaves (1). No seaweed has true vascular tissue, roots or flowers, so seagrasses and mangroves are flowering plants (Magnoliophyta) that have returned to the sea, not algae (1).

Marking note. Q4 corrects the booklet's own diagram labels: pollen and ovules carry or contain the gametes; they are not gametes. Q19 tests the booklet's "stab into the mud" claim with a model and expects the learner to name a limitation of that model. Q15 and Q21 flag two places where the prescribed material and current knowledge part ways; the CAPS wording stays the expected exam answer.