extension 4.1 - 4.4

8/6/2019 Extension 4.1 - 4.4

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Edexcel Biology for AS Dynamic Learning CD-ROM © Hodder Education 2008

4 Biodiversity and natural resources


■ A* Extension 4.1: Exploring the plant kingdom

Green plants – the varietyThe green plants make up one of the five kingdoms of living things. Green plants include the

mosses and liverworts, the clubmosses and horsetails, the ferns, the conifers, and the floweringplants. In the long history of life, green plants evolved about 500 million years ago, from aquatic,

single-celled organisms called green algae.

A green plant is an organism with:

■ a wall around each cell whose chief component is cellulose

■ cell organelles called chloroplasts, the site of photosynthesis

■ an unusual life cycle, with two distinct stages or generations that alternate – one generation

produces spores, and the other produces gametes (sex cells).

Flowering plants became the dominant group of terrestrial green plants early in their

evolutionary history – about 100 million years ago. In successfully adapting to terrestrialconditions, flowering plants evolved:

■ ground tissues that support stems and leaves in the air

■ xylem vessels for the internal transport of water

■ phloem elements for the internal transport of nutrients e.g. sucrose, amino acids

■ mechanisms for reducing water loss from their stems and leaves.

developed 145 myadiversified 65 mya

(1 hour = 146 my)

origin oflife 3500 mya

first prokaryote

fossils (bacteria)

O2 productionby photosynthesis(cyanobacteria)

plantsinvadethe land

fossilsbecomeabundant

liverwortPellia(0.5–2 cm)

horsetailEquisetum(20–80 cm)

male fernDryopteris(15–150 cm)

Scots pinePinus(30–50 m)

living examples of the major groups of plants that have evolved

clubmossLycopodium(5–50 cm)

origin of eukaryotes

500–435 mya

360–290 mya200–145 mya

humansappear

12 12

3

4

5

6

7

8

910

111212

3

4

5

6

7

8

910

11

age of mammals beginsage of reptiles ends

age of reptiles begins

insects and amphibiansinvade the land

oak treeQuercus(20–40 m)

wild carrotDaucus(30–100 cm)

dandelionTaraxacum(2–15 cm)

flowering plants

first liverworts

ferns and conifersgiant clubmossand horsetails

history of life on the 24-hour clock

8/6/2019 Extension 4.1 - 4.4


2 BIODIVERSITY AND NATURAL RESOURCES – A* EXTENSIONS


■ A* Extension 4.2: The histology ofmonocotyledonous plantsThe grasses are the largest family within the monocotyledons, and grasses are the most abundant

and widely spread of all flowering plants. With the aid of their highly developed fibrous root

system and vigorous leaf growth, grasses cover much of the soil, and so help prevent its erosionby wind and water.

A grass plant has a leaf blade that is characteristic of monocotyledons (with parallel veinsrunning its length). However, the leaf blade is circular, surrounding the stem for some distance

above its point of attachment. The paths of the leaf trace bundles, from their point of origin on

the outermost ring of vascular bundles until they pass into a leaf base, fill the stem with scatteredbundles when seen in cross-section. These features are illustrated here in the stem of maize (Zea

mays). This is a quite different arrangement from that seen in the cross-section of the sunflowerstem, for example.

circular leaf baseouter ring ofsmall bundles

leaf trace bundlesin the pith

leaf blade showingparallel veins

stem

point of attachmentof leaf blade

point of attachmentof leaf blade

circular leaf base cutto show bundles thatpass to leaf blade

■ A* Extension 4.3: Defences must fail – in thelong term, at least!Herbivores must overcome defence mechanisms of green plants – or fail to survive themselves, asall food webs testify. All livings things are dependent of plants, directly or indirectly, for it is only

the green plants that are able to harness the energy of the Sun and manufacture elaborated food

molecules. Either defence mechanisms are overcome during the life-time of the plant, or ondeath of the plant its nutrients enter the food chain. But plants themselves respond to the

ravages of browsing in different ways, as shown in the diagram on the next page, which

illustrates predation, parasitism and recovery in the holly leaf.

8/6/2019 Extension 4.1 - 4.4




protective spines effective against large herbivores,e.g. cows, but the leaf is vulnerable to attack froma tiny insect parasite with an appetite for mesophyllcells, e.g. holly leaf-miner (below)

adult lays eggs on midribon underside of leaf

larvae burrow into midrib

tissue of leaf and feed larva pupates andadult fly is formed

adults

mate

now each larva moves intomesophyll tisssue and feeds,creating its large, irregularleaf mine

leaf mine reachesmaximum size, larvacuts a hinge flap(escape hatch)in the epidermis

fly escapes frompupa and fromleaf via flap

the structure of a healthy holly leaf

the holly leaf-miner ‘fly’ (×10)

life cycle of the leaf-miner

June: July: September: March: May / June:

holly leaf with newly formedmesophyll in mine spaces

holly leaf with mines

recovery of the holly leafafter the fly escapes the mine is often refilled by mesophyll cells, formedby divisions of the remaining cells around the edge of the mine –

externally, the leaf has a light green, slightly bulging area where theleaf mine was

epidermis

with cuticle

palisade mesophyll cells

(this leaf section was

prepared and stained

to show the nucleus ineach cell; chloroplasts

in the palisade cells are

hardly visible)

bundle sheath

of parenchyma

xylem

phloem

fibres

TS of leaf (40)

epidermis

contains stomata

8/6/2019 Extension 4.1 - 4.4


Suggested activityHolly trees may be heavily parasitised by the leaf-miner insect. Taking a suitable holly tree near

your school or home, assess the incidence of parasitism of leaves on different aspects of the tree(N, S, E and W), working at a height above ground of approximately 1.5 m. To do this, select

branches with about 25 adjacent leaves as survey samples. Express your results as a percentage.

Do the results differ significantly on trees exposed to different levels of atmospheric pollution?

■ A* Extension 4.4: Primary and secondarysuccessions – an outline

Primary succession – pioneer plants and soil formation New land is formed on the Earth’s surface at river deltas, at sand dunes, and from cooledvolcanic larva, for example. At these sites, all that is initially present is the bulk of the material

for the mineral particles of a soil. Organic matter, living or otherwise, is absent.

The mineral component of soil is formed partly by erosion of parent rock. Erosion is thebreaking of solid rock into smaller particles by the effects of extremes of temperature, by the

action of wind and water, and by chemical reactions that occur, such as when slightly acid rainfalls. Mineral particles are also blown or washed in from elsewhere. The resulting mineral

skeleton is of particles of a wide range of sizes from small stones and coarse sand to the finest clay

particles.Soil, when it has fully formed, has organic matter known as humus wrapped around the

particles of the mineral skeleton. Humus is a substance derived from dead plant and animal

remains, together with animal faeces, that have been decomposed by the actions of micro-organisms. Until the soil is fully formed, it retains little water, even when water is freely

available.

The growth and death of the first plants to invade (typically quick-growing weed species –known as pioneer plants) add humus, and so some soil water is retained. Nutrients are added to

the soil when all organisms die – including the ‘minibeasts’ that feed on pioneer plants. Nutrients available to plants increase steadily.

A wider range of plants now grow, and start to shade out the pioneers. The conditions in thesoil become increasingly favourable to micro-organisms and soil animals, which invade thehabitat from the surroundings. Herbaceous (non-woody) plants are followed by shrubs and small

trees, all growing from seeds that are carried in by wind, water or the activities of animals.

Perhaps in thirty or more years a small wood will have been formed. So succession can be seen asa directional change in a community with time.

Primary successions also develop in aquatic habitats, such as in a pond formed and fed by a

spring. Here the sequence of pioneer plants differs from a dry land primary succession, but theresult may well be a woodland climax community, too.

Secondary successions

Sometimes an established community is suddenly disrupted and destroyed. This occurs, forexample, when fire destroys a large area of vegetation, or may occur as a result of human

activities. In these habitats, soil is already formed, but the existing biota (living things) has been

largely or totally destroyed. The successions that result, starting from existing soil, are known assecondary successions. Secondary successions normally happen quite quickly, since the

necessary soil for plant life is already present. Plants are established as their seeds are blown in,or as they grow in from the surrounding, unharmed climax communities.



extension 4.1 - 4.4

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