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Plant science

The growth of a plant is an everyday phenomenon, but it isnonãtheless remarkable. Without visibly ingesting anything, a plantcan grow larger and more complex. Using only simple inorganicsubstances and sunlight, plants synthesize a huge range of substances

- they are amazing chemical factories, that fuel both themselves and

most food chains in tenestrial ecosystems. Plants retain groups ofstem cells throughout their lives, allowing them to continue to growindefinitely. These groups of cells are called meristems. cells inmeristems are small and go through the cell cycle repeatedly toproduce more cells, by mitosis and cytokinesis' These new cells

ãbsorb nutrients and water and so increase in volume and mass.

primary meristems are found at the tips of stems and roots. They are

called ápical meristems. The root apical meristem is responsible forthe growth of the root. The shoot apical medstem, at the tip of thesteni, is more complex. It throws off the cells that are needed for thegrowth of the stem and also produces the groups of cells that growãnd develop into leaves. Figure I shows the shoot apical meristem ofa dicotyledonous Plant.

The stems, Ieaves and roots of plants can develop in many differentways. There is a huge diversity of plant form, which can be related tothe habitats and adaptations of plants. Hydrophytes, which are plants

that grow in water, à.. u..y different in structure, for example' fromxerophytes (plants that grow in deserts)'

region ofbud stem growth

Figure I Structure of a shoot apicalmeristem

Figure 2 Transverse section of part of youngHelionthus stem

youngestdeveloping

leaf

dome of cellsat centre ofapical meristem

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Working with dato: plon díøgrom of o stem

A plan diagram is used to show the distribution of tissues in a leaf

or other organ. The lines show the edges of each area of tissue'

lndividual cells are not shown'

Figure 2 shows part of a young Helionthus (sunflower) stem in

transverse section.

Look carefully at the cells and count how many different types

there are. Thã size, shape and wall thickness of cells can be used

to distinguish the different cell types.

Draw a plan diagram to show these areas of tissue, including

epidermis, cortex, fibres, phloem, xylem and pith'

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l0 ' Plant science

StemsAs stems grow upwards, they produce new leaves and alsosometimes form branches. The lower parts of the stem therefore haveto support a larger and larger mass. Terrestrial plants supportthemselves in three ways:¡ with turgid cells, which are almost rigid because of their high

pressureo with cells that have thickened cellulose cell wallso with xylem tissue, which has cell walls impregnated with Ìignin,

making it woody and hard.

Flowering plants can be divided into two broad groups -monocotyledons and dicotyledons (see page t23)' The stems ofdicotyledons can grow wider as a plant grows taller, to support theincrease in mass. To do this, a meristem develops in a complete circlearound the stem. This is called a lateral meristem. The lateral meristemdevelops between the xylem and the phloem and the cells it producesdevelop into more of both these tissues. Xylem develops on the innerside of the lateral meristem, gradually adding to the thickness of thewood of the stem. The plants that produce the largest amount ofxylem and can therefore grow tallest and compete for light mosteffectively are called trees. Figure 4 shows a transverse section of astem that has produced extra xylem tissue using its lateral meristem.

Monocotyledons have stems with an apical meristem at the tip of thestem, but the tissues of the stem differentiate in a differentarrangement from those of dicotyledons (see Figure 5).

Figute 4Transversesection of partof a thickenedHelionthusstem

Figure 5 Palm \ree (TrochYcorPusFor-tune)

epidermis

vascularbundle

phloem

xylem

cambium(lateral

' meristem)f bres

cortex

woody

epidermis

cortex

Figure 5 Dicotyledon stem

Data-based questions= comPoring stem structureI Figure 5 is a diagram of a dicotyledon stem b¿sed on the

transverse section shown in Figure 2. Deduce whether the areason the diagram represent single cells or areas of tissue. [2]

2 Outline tr¡,ro simil¿rities in structure between the monocotyledonstem (Figure 6) and the dicotyledon stem. [2]

5 Compare the position of xylem and phloem in the ¡¡,ro types ofstem. [4]

4 Deduce why monocoÇledon stems cannot thicken in the sameway as dicotyledon stems. [2]

Ð

Ð

Ð

Ð

Ð Ðe vascularbundle

phloem

ÐÐ Ð xy em

Figure 6 Monocoty edon stemil8

Leaf structure and adaptationsIn areas that are favourable for plant growth, the vast numbers ofleaves make the Earth appeal gleen from space. Leaves are producedby the apical meristem at the tip of stems. They vary in size andshape considerably, but most have a large upper surface facing thesun and a very narrow distance between the upper and lowersurfaces. The commonest shape is oval (see Figure 7), with a stalk atone endlinking the leaf to the stem and a downwardly curving point at theother end to shed drops of rainwater.

Leaves are organs, because they are composed of groups of differenttissues (see Table l).

Table I Tissues found in plant leaves

upper epidermis with a thick layer of wax on the outside,

called the cuticle

polisode mesophyll-tightly packed cells in the upper half

of the leaf containing many chloroplasts

xylem - mostly xylem vessels, which are long, dead, tubular

structures containing water and dissolved mineral ions

lO o Plant science

Figure 7 Oval leaves of Mirobilis jolopo

Explain which tissue ortissues of the leaf performseach of these functions:

I absorption of light forphotosynthesis [3]

2 gas exchange, including CO2uptake and 02 release [3]

5 support, to ensure that the leaffaces the sun [3]

4 water conservation [3]5 transport of water from the

stem to leaf cells [5]6 transport out ofthe leaf of

sugars and other products ofphotosynthesis. [3]

0

Workíng with datø: plon díøgroms of leovesFigure 8 shows sections of twoleaves of Prunus coroliniono, onethat grew in the sun and one thatgrew in the shade. Draw Plandiagrams of the tissues in e¿ch.

IIT

t

Compare the features of the two leaves,including overall thickness, structure ofpalisade mesophyll and spongy mesophylland the thickness of cuticle. [4]

2 a) Deduce which leaf was grown in the sun,

and which was grown in the shade. [l]b) Discuss why the leaves that grew in

sun and those that grew in shade havedifferences and similarities in theirstructure. [5]

e

a Figure I Micrographs of sun and shade leavesaa a aa aaa aoaaaa aa a a aa a a a aaa aa a a

ll9

tO c Plant science

Translocation in Phloemphloem tissue is found throughout plants, including the stems, rootsand leaves. It links parts of the plant that need a supply of sugars andamino acids to other parts that have a surplus. Table 2 classifies parts

or the plant into sources (areas where sugars and amino acids are

loaded into the phloem) and sinks (where the sugars and amintlacids are unloaded and used).

Photosynthetic tissues :

. mature green leaves

. green stems.

Storage organs that are unloadrng their

stores:. storage tissues in germinating seeds

. tap roots or tubers at the start of thegrowth season.

Table 2

Figure 9 shows the results of a simple experiment in which two ringsol bark were removed from an apple tree. The bark contains thephloem tissue. The ellects on apple growth are clearly visible.

Roots that are growing or absorbing

mineral ions using energy from cell

respiration.

Parts of the plant that are growing or

developing food stores:

. developing fruits

. developing seeds

. growing leaves.Figure 9 Results of apple tree ringingexperiment

State which the sourcesand which the sinks arein this part of the appletree. [2]

Sometimes sinks turn into sources' or vice versa' For this reason the 2 a) compare the sizes of the

tubes in phloem must be able to transpoil biochemicals in either apples' [2]

direction and unlike the blood system of animals, there are no valves b) Explain the conclusions

orcentralpumpinphloem.Howevertherearesimilaritiesbetweenthatcanbedrawnfromthetransport in phloem and blood vessels: in both systems a fluid flows sizes of the apples. [a]

inside tubes because of pressure gradients. Energy is needed to 5 Suggest reasons for thegenerate the pressures, so the flow of blood and the movement of swelling above the upper ring

p¡to"¡¡ sap are both active processes. The movement of substances in of removed bark. [Z]

phloem is called active translocation.r 1n ¡., o ô s n ç t ô ñ * t e d * çd & ¡ I ô È c o ô s å ' t "

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gE rwíë'fa * f¿¡,: corbohydrotes in Cyclamen

Cyclamen persicum plants were dissected and the carbohydrate contentwas measured in parts of the plants. Mean

résults are'shown in Table 3. The * values are standard error figures. There is a 5 per cent probability that the true

mean falls outside the range given by standard error valuesData presentationChoose a suitable presentation formatto display the data in the table,including the standard error values.You can use graphing software or youcan draw graphs, tables, charts ordiagrams by hand.

sucrose glucose fructose starch

stalk, consisting of xylem ¡ 190 t280 tB79' rrrJv conclusionsanc pnloem

Tissue sunounding the 4\1 624 1236 <18 Describe the trends in the data and

vascular bundle tn the leaf t96 t1l4 tl0l5 suSgest reasons for them based on

stalk lvo =t t+ -r-rurJ your knowledge of photosynthesis,

the structure of disaccharides andBuds, roots and tubers 2260 120 310 152 polysaccharides and the transport and(underground storage organs) tg26 t4l !242 !242 storage of carbohydrates in plants.

Table...'¡oô...o..................'o.........o.....o.¡.................

Leaf blade 1312

t212Vascular bundle in the leaf 5751

210

tB8

419

494

1653

r303

62

t25<18

Mean carbohydrate contentfiesh mass t standard error of mean)Plant part (ps

SlnksSources

120

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PhototropismPlants use hormones to control the growth of stems and roots. Boththe rate and the direction of growth are controlled. The direction inwhich stems grow is influenced by two external stimuli: light andgravity. Stems grow towards the source of the brightest light or in theabsence of light they grow upwards, in the opposite direction togravity. These directional growth responses to directional externalstimuli are called tropisms. Growth towards the light is calledphototropism and will be described here as an example of thecontrol of plant growth.

The first stage in phototropism is the absorption of light byphotoreceptors. Proteins called phototropins have this role' Whenthey absorb light of an appropriate wavelength, their conformationchanges. They can then bind to receptors within the cell, whichcontrol the transcription of specific genes. Although much research isstill needed in this field, it seems likely that the genes involved arethose coding for a group of glycoproteins located in the plasmamembrane of cells in the stem that transport the plant hormoneauxin from cell to cell. The position and type of these proteins canbe varied, to transport auxin to where growth is needed. Auxinpromotes the elongation of cells in stems, by causing loosening of theconnections between cellulose microfibrils in cell walls. Auxin issynthesized by the tips of growing stems and is transported down thestem to stimulate growth. If phototropins in the tip detect a greaterintensity of light on one side of the stem than the other, auxin istransported laterally from the side with brighter light to the moreshaded side. Higher concentrations of auxin on the shadier side ofthe stem cause greater growth on this side, so the stem grows in acurve towards the source of the brighter light. The leaves attached tothe stem will therefore receive more light and be able tophotosynthesize at a greater rate.

lO o Plant science

Figure lO Pholoris conoriensis.cotyledons after exposure for 8 h ina box open on one side in front of asouth-west window. Curvature towardsthe light accurately traced. The shorthorizontal lines show the level of theground. From lhe Movements of Plontsby Chorles Dorwin, 1882.

Why did Darwin draw threeseedlings, and not just one? o

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30 60 80 120 ',150 180time in unilateral l¡ght/m¡n

Figure I I Craph showing the rate of a

phototropic response

Rediscovering biology: the act¡on sPectrum for phototroPism

Figure 12 shows apparatus used tomeasure phototropism in the shoots ofseedlings. The syringe is fixed securelyonto a microscope stage and graticulescale in the eyepiece is used tomeasure movements of the shoot tip.Light from one side can be tested,using red, blue and green light emittingdiodes. Figure I I shows resultsobtained using white light-

microscope stagein a vertical position

etpshoot

seed

LICHT

sca egraticule mrcroscoPe

tiP

of view

Figure l2 Apparatus for measuring rate of phototropic response

shoot

bank ofLEDs

moistcottonwool

nozzle of sytingecut off

phototropism, which variables shouldbe on the x and y axes?

I Which colour gives the strongestphototropic response?

2 To drawthe action sPectrum for

microscope slide elastic band

121

lO o Plant science

Root systems and mineral ion absorptionOne of the earliest stages in the development of an embryo plant is the

formation of a root. when seeds germinate, the embryonic root bursts

out of the seed coat and grows downward.s into the soil, anchoring the

therefore increase the capacity for absorption'

water is absorbed into root cells by osmosis. This happens because the

solute concentration inside the root cells is greater than thatin the water in the soil. Most of the solutes in both the loot cells

and the soil are mineral ions. The concentrations of mineral ions in theroot can be lo0 or more times higher than those in the soil. These

, usingare

Mineralionscanonlybeabsorbedbyactivetransportiftheymake

overcome this problem, some plants have developed a relationshipwith a fungus. the fungus grows on the surface of the roots andsometimes even into the cells of the root. The thread-like hyphae of

the iI and absorb mineral ions such as

pho oil particles' These ions are supplied tothe grow successfully in mineral-deficient

Figule l5 Patterns of root branching

Figure l4 Photograph of radish root hairs

I

Data-based questionsz fungol hyphoe ond minerol ion obsorptionFigure 15 show in which seedlings of sitka sPruce,

Piãea sitchensis sterilized soil either with or without

fungi added: C added. The species of fungi added

were:| : Loccorio loccoto; ll : Loccorio omythesteo; lll : TheloPhoro terrestris froma tree nursery; lv : Thelophoro terrestris from a forest; v - Poxillus involutus;

Vl : Pi sol ith u s ti nctori u s'

t a) Discuss the effects of the five species of fungi on the growth of the

roots and shoots of the tree seedlings. [4]

b) Explain rhe effects of the fungi on the growth of tree seedlings. [2]

2 a) State the relationship between root Srowth and shoot growth in the tree

seedlings. [1]b) Suggest a reason for the relationship. Il]c) Using the data in Figure lO.l4, deduce whether the effects of closely

related fungi on tree growth are the same' [2]

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Figure l5 Results of Sitka sPruceexperiment

'/

nocotyledons and dicotYledonsts that Produce seeds enclosed inside lruits are called

ytes. As with many biological terms, this word isved from ancient Greek

Table 4

Number of cotyledons

angeion- a contain er" sperma - a seed; phyton - a plant'

Angiospermophytes are divided into two large and important groups

-Jtoåotyledons and dicotyledons' These names refer to the

number of leaves contained in the embryo, called cotyledons' The

Lo.ro.o,yl.dons include the palms, gingers' lilies' irises' grasses'

,.dg.r, rushes, orchids, bananas, bromeliads and aroids' The

ãi.ãtyt.Ao.ts include most trees and shrubs and many non-woodyptants. These two groups are normally called monocots and dicots'

elthough the differencãs [sted in Table 4 are not found in all

-ono.át, and dicots, the well-informed biologist should be able to

distinguish between them!

attachmentof leaf

to stem

l0 ¡ Plant science

leaf

Trodescontío Pollido

leaf stalkattachedto stem

2

Meristems

Method of formation of new

roots

Vascular tissue in the stem

Apical only - stems cannotgrow wider

Apicaland lateral so stems

can widen

By formation of roots from

the stem

By branching of other roots

Vascular bundles sPread

throughout

Vascular bundles ananged

in a ring

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Íhinking about scíence : clossificotionThe text (ight) is from the second edition ofMethoduèÞlontorum, by John Ray, published in

1703. lt is the first classification to namemonocotyledons and dicotyledons as seParate

grouPS.

I Why did Ray write his classification in Latin

aná why is ihis language still used to publish

descriptions of new sPecies?

2 Ray classified corals and sPonges with p-lants'

To what extent is this classification justified?

' Figure 17

Monocots Dicots

t

123

lO o Plant science

Adaptations of roots, stems and leavesRoots, stems and leaves are the principal plant organs' Plants use

them to build the structures that they need for water and mineralabsorption, support and photosynthesis' Other functions can be

carried out if roots, stems or leaves are modified. For example, roots

usually absorb water and mineral ions, but in some trees they are

modified into props, growing out from the trunk above soil level tohelp support the tree. In trees that grow in mangrove swamps some

,ooì, gi r* upwards above the surface oT the anaerobic mud in the

swamp, acting as breathing roots, or pneumatophores'

Figures I8 and l9 show modified leaves' Tendrils are narroworitgro*ths lrom leaves that rotate through the air until they touch a

soliá support, to which they attach, allowing the plant to climb,pwa.ai. Bulbs consist of swollen leaf bases attached to a short stem.

tirey are used lor food storage' Figures 20 and 2l show swollenunderground. stems and roots that are also used for food storage'

called stem tubers and root tubers'

SinningiosPec,oso

stemruber

IOmm

l0 mm Helionthus

l0mm root

leaf

I0 mml

Figure l9 Bulbs Figure 20 Stem tubers

Bignonioshoot

5mm

leaflet

leaf stalk

l0 mm

Figure l8 Leaves with tendrils

Chlorophytumcomosumroot tubefs

tuber

l0mm Dohlio

tubers

tuberlomm

Figure 2l Root tubers

Littoniomodestoleaf

stem

eaf

garlicbulb

tuberosusstemruberon on stem

root

foot

CRITICAI CONSIDERATIONS: stem and root tubers as food

Thiamine (pg) 81 112 t\ tB 4800

Riboflavin (¡rg) 48 32 34 61 ì200

Niacin (mg) 0.85 0.55 1.07 0'56 80

Pantothenic acid (pg) 107 314 281 800 7000

Vitamin Bu (F8) 88 293 203 209 800

Folate (¡rg) 21 23 18 11 200

VitaminB,,(Fg) o 0 o o l5

Vitamin C (mg) 20.6 17'1 19.7 2'4 40

Table 5 The content of B vitamins and vitamin C per 100 g in four types of tubeç used as a

main food item in Parts of Africa.

b) has the lowest content ofeach vitamin. [3]

2 Suggest reasons whY largeamounts of one food are ofteneaten in an area. [2]

5 Explain, using the data, theadvantages of eating a varietYof different foods. [2]

Sweetpotato

Adult dailyrequirement

Vitamin Cassava PotatoYam

ICT can be used t0 presentthis data in charts, to make analysis easier

eI ldentify which tuber:

a) has the highest contentof each vitamin [3]

124

Stomataplants that live on land, with stems and leaves in contact with the air,

rnust avoid excessive water loss and death by dehydration. Water loss

from the leaves and stems of plants is called transpiration. The waxycuticle that covers the epidermis of leaves reduces transpiration, but italso prevents other small molecules from passing through, includingcarbon dioxide and oxygen. Absorption of carbon dioxide is essential

for photosynthesis, so pores through the epidermis, with its waxy,outillg, are needed. These are called stomata. The physiologicalproblem for plants is that if stomata allow carbon dioxide to be

ãbsorbed, they will usually also let water vapour escape from the leaf,

with the consequent risk of dehydration and death' This is anintractable problem for plants and other organisms: gas exchangewithout water loss is imPossible.

plants minimize water losses through stomata using guard cells.These are the cells that are found in pairs, one on either side of astoma. Guard cells control the aperture of the stoma and can adjustlrom fully open to fully closed. To increase the aperture, the guard cells

absorb water, which raises the plessure and pushes the guard cells

apart. Conversely, movement of water out of guard cells causes themtõ lose pressure and surrounding epidermis cells push them together,closing the stoma. Figure 22 shows open and closed stomata'

The opening and closing of stomata is affected by external stimuli:o light causes stomata in most plants to openo low carbon dioxide concentrations in the air spaces inside the leaf

cause stomata to openo shortage of water, which results in leaf cells becoming deficient in

water, causes stomata to close.

when leaf cells become deficient in water, they synthesize a planthormone called abscisic acid. This hormone causes the closure o1

stomata and it over-rides other external stimuli - the stomata close

even if it is light and carbon dioxide levels inside the leaf are low.Inthis situation it is less important for the plant to continue tophotosynthesize, than to avoid dehydration and death.

lnvestigating biology: distribution and movements of stomata

of varying the carbon dioxide concentration or the abscisic acid concentration can

then be investigated.

l0 o Plant science

Transpiration the loss ofwater vapour from the leavesand stems of planß

closedstoma

guard cellwith lowPressure rnits cytoplasm

oPenstomaguard cellwith highpressure inits cytoplasm

Figure 22 Open and closed stomata

Figure 25 Stomata

I Cut outa sect¡onof leafblade

¡ lniett a nail at thefold to separate themesophyll from thelower epidetmis

2 Fold the

20pn

strip of leaf overneal one end

lowerepidermis

\

*'\ñ\_

Figure 24 Peeling epidermis

lO o Plant science

Abiotic factors affecting transpirationThe rate of transpiration is the amount of water

ä;;;;t u plánt loses from its leaves and stems

p.i ""i, time.ìhe rate depends on several

variables:

o the size of the Planto the thickness of its waxY cuticleo how widelY sPaced its stomata are

o whether the stomata are open or closed'

moist cell walls, the humidity of air in the spaces

i"tia. the leaf is always around 1007o' The

to*", the humidity outside the leaf' the faster

,h. ,u," of diffusion of water out through the

,*u,u and the higher the transpiration rate'

. Win¿ increases thJtranspiration rate' In still air

or light winds, transpiraiion cause.s the humidity

around. the leaf to increase' reducing the rate of

transpiration. ln windy conditions' the rate of

ttunrpi.u,ion becomes maximal for the humidity

of the air.. i.-p.tuture affects the rate at which water

evaporates from the moist surfaces inside the

leaf. ¡.t higher temperatures' evaporation rates

rise. Higher t"-ptiut"es also increase the rate

of diffusion between the air spaces inside the

leaf and the air outside' Increases ini"rnp.ru,rrre also allow the air to hold more

water vapour and so reduce the relativehumidity of air outside the ieaf' The

concentration gradient therefore increases and

water is lost more raPidlY'

These are all factors that the plant can control and

;;th; plant is a living organism' they are biotic

factors. other .*t.t"ãl factors are not controlled by

itr. ptuttt, or any other organism' They are

ifr"åf"t. abiotii lactors' rour abiotic factors have a

mu¡o, effect on the rate of transpiration'

o Light causes stomata to open' increasing the rate

of transpiration' Stomata close in darkness in

-ort ,p!.i.s of plant' as there is no need to

u¡rorU .urUon dioxide' and water can be

conserved without reducing the rate of

photosYnthesis.. 'uumi¿ity is the water vapour content of the air'

It is usually measured as a percentage of the can

" ftoi¿. Because of evaporation of water from

lnvestigatingbiology:theetfectofbioticandabioticfactorsontranspirationThe rate of transpiration is difficultto measure directlY' lnstead, therate of water uPtake is usuallY

measured, using a Potometer'Figure 25 shows one tYPe ofpotometer.To desisn an investigation you will

need tõ discuss the followingquestions.

I How will You measure the rate

of transPiration in Yourinvestigation?

2 What biotic or abiotic factor willyou investigate?

5 How will You vary the level ofthis factor?

4 How manY results do Youneed, at each level of thefactor that r¡ou are varying?

5 How will You keeP otherfactors constant, so that theY

do not affect the rate oftransPiration?

transferred to aPParatusto avoid introducing air

reservoir from whichwater can be let into thecaoillary tube, Pushingthä air bubble back tothe start of the tube

capillary tube

scale calibrated in mml

air bubble moves along tube as

water is absorbed bY shoot

Figure 25 Diagram of a Potometer

fresh shoot, cut under water andunder water

bubbles

taP

air tightseal

-l0 e Plant science

Figure 26 micrograph of primary xylem

thickenings of xylem vesselwall impregnated with lignin

continuous tubular sttucture

Figure 27 Structure of xylem vessels

gfater transPort in xYlemThe structure of xylem vessels allows them to tlansport water inside

plants very efficiently. Xylem vessels are long continuous tubes. Theirwalls are impregnated with lignin, strengthening them, so that they can

withstand very low pressures without collapsing. Xylem vessels aIe

formed from Îiles of cells, arranged end-to-end. The walls betweenadjacent cells in thc file are removed and the plasma membranes andcontents of the cells beak doy¡n. When matule they are therefore non-Ìiving and the flow of water akrng them must be a passive process.

The pressure inside xylem vessels is usually much lower thanatmospheric pressure. The walls of cells inside the leaf genelate this lowpressule. Water is attracted to the cellulose and other substances in thecell walls, which causes adhesion. When water evaporates from thesurface of the wall, adhesion causes water to be drarvn through the cell

wall from the nearest available supply, to replace the water lost byevaporation. The nearest available supply is the xylem vessels in theveins of the leaf. Even if the pressure in the xylem is already low, theforce of adhcsion is strong enough to suck water out of the xylem,further reducing its Pressure.

The low pressure generates a pulling force that is transmitted though thewater in the xylem vessels clown the stem and to the ends of the xylemin the roots. It is callecl the transpiration pull and is strong enough tomove water upwards, against the force of gravity to the top of the tallesttree . For the plant it is a passive process, with all the energy needed for itcoming from the thermal energy that causes transpiration'

The pulling ol water upwarcls in xylem vessels depends on the cohesionthat exists between watcl molecules. Many liquids would be unable toresist the very low pressures in xylem vessels and the column of liquidwould break. This is called cavitation and it does occasionally happeneven with water, but it is unusual. Even though water is a liquid, it cantransmit pulling lorces in the same way as a solid length of rope does'

IIIIIIIIIII¡IIIIIIIIIIIII

Data-based questions: frte Renner exPerimentFigure 28 shows the results of an experiment by the Cermanplãnt physiologist Otto Renner in 1912. A transpiring woodyshoot'was placed in a potometer and the rate of water uptakewas measured. A clamp was attached to the stem to restrict theflow of water uP to the leaves. Later on, the top of the shoot,with all of its leaves, was removed. A vacuum pump was thenattached to the top of the shoo

Ê.20E

_g t0oq+8-7o-Ëbl5

stem clamped

I ,noot removed

432I

3

vacuum PumPI

4

I time (hrs)tI Figure 28 Results of the Renner experiment\---

I Describe the effect of clamPingthe stem on the rate of wateruptake. [5]

2 Explain the effect of cuttrng off the top ofthe shoot on the rate of water uptake. [3]

5 Calculate the difference between therate of water uPtake caused bY thevacuum pump and the rate caused bY

the leaves immediately before the shoottop was cut off. [2]

4 The water in the Potometer was atatmospheric Pressure. The vacuumpump generated a Pressure of zero.Discuss what the results of theexperiment showed about the Pressuresgenerated in the xylem by the leaves ofthe shoot. [2]

127

l0 o Plant science

XerophytesXerophytes are plants adapted to growing in deserts and other dryhabitats.Therearevariousstrategiesthatplantscanusetosurviveinthese habitats, including increasing the rate of water uptake from the

soil and reducing the rate of water loss by transpiration' Some

xerophytes are ephemeral, with a very short life cycle that is.oroit.i"a ln the brief period when water is available after rainfall'rhei then remain dormant as embryos inside seeds until the nextrains, sometimes years later. Other plants are perennial and rely on

storage of water in specialized leaves, stems or roots'

Mostcactiarexerophytes,withleavesthataresoreducedinsizethatthey usually only consist of spines' The stems contain water storage

tissueandbecomeswollenafterrainfall.PleatsallowtheStemtoexpand and contract in volume rapidly' The epidermis of cactus stems

has a thick waxy cuticle and, unlike most plant stems there are

stomata, though they are spaced more widely than in leaves' The

stomata usually op.tt ut night rather than in the day' when it is muchcooler and transpiration occurs more slowly' Carbon dioxide is

absorbedatnightandstoredintheformofafour-carboncompound,malic acid. carbon dioxide is released from the malic acid during the

day, allowing photosynthesis even with the Stomata closed. This is

calied Crassulacean icid metabolism. Plants such as cacti that use this

system are called CAM plants. Cn physiology also helps to reduce

transPiration (see Page 115).

CactiarenativeplantsofNorthandSouthAmerica'Xerophytesinotherparts of the world belong to different plant ramilies. The adaptations inih.r" *"rophytes are often very similar to those of cacti. Some Africanspecies ot Èuphorbiafor example, are difficult to distinguish from cacti

until they Produce flowers.

Gym nocolycium boldionum

reduced

(cactus)viewed fromabove

l0 mm

Euphorbio obeso viewed from above

swollen

5mm

Figure 29 XeroPhYtes

stem

Data-based questioîsi woter Permeonce of woxy cuticle

The graphs in Figure 30 showthe results of investigationsinto the rate at which water is

able to diffuse though thewaxy cuticle of Plants, whichis called the waterpermeance of the cuticle.Figure 30 shows therelationship betweentemperature and waterpermeance of four sPecies ofplant. Figure 30 shows therelationship between thethickness of cuticular wax andwater permeance. The resultsof the experiment show howimportant it is to testhypotheses, even when itmay seem that this is notnecessary.

5

'oE

o,o3ooUcooEqJè

1_ 20E

9lsx0,Ero6o,EOrGJ

6

0

I Using the data in Figure 30, describethe relationship between temPeratureand water permeance. [2] 4

2 Discuss the consequences for plantsof the effect of temPerature oncuticular water Permeance. [S]

5 Using the data in Figure 30, statethe thickness of cuticular wax with:

3

2

I

o12345635 40 45 50 55(a) temperature/oC (b) thickness of cuticularwax/Pm

Figure 50 Factors affecting water Permeance of wary cuticle

a) the highest water Permeanceb) the lowest water Permeance. [2]

Evaluate the hyPothesis that thewater permeance of the cuticle is

positively correlated with itsthickness, using the data in Figure30. [3]

. PyrusI Liriodendron

' Camellia

,,) .iâ r

F^^

aaaaa

Seed structure and disPersal

internal structure of a bean seed (Phaseolus vulgaris\ '

hooks that catch onto the coats of animals'

Some seeds do not immediately germinate, even if given theconditionsnormallyrequired'Thisiscalleddormancyanditallowstime for seeds to be dispersed. It may also help to avoid germinationat an unfavourable time.

testa

l0 o Plant science

micropyle

Figure 5l Structure of bean seed

(Phoseolus vulgoris) ; external structure(above); internal structure (below)

Data-based questio nsi fíre ond seed dormoncy in o plont of the choporrol

Emmenonthe pendulifloro grows in chaparral (shrubland) in california. lt

is rarely ,."n in unburnt chãparral, but appears.after fires, growing to .

about äsomm, flowering, foiming seed and dying in a few months'.Th.e

ãl"J.n micrographs bel"low show the results of an experiment in which

seeds of the piani were treated with smoke for 3 minutes and then

soaked in a solution of lanthanum nitrate hexahydrate'

I The scale bars in the electronmicrographs rePresent 1 Pm.Calculate the thickness of waxy cuticle

between the testa and the embrYoand food stores inside the controlseed. [2]

2 The lanthanum solution aPPears

as dark staining in the electronmicrographs and shows how far waterwas able to Penetrate. Deduce howfar water could Penetrate into thecontrol seeds. [2]

5 a) Compare the staining of the waxy

cuticle in the smoke-treated seedswith the staining of the cuticle inthe control seeds. [2]

b) Suggest a hYPothesis forthe germination of Plants ofEm m e no nth e Pe n d u I if I o ro aÍterfires, based on the differences instaining that You have described'

l2l4 Suggest two advantages to

Emmenonthe Pendulifloro otdormancY ending after fires in thechaparral. [2]

A: controlseed

testa

wa)rycuticle

B: smoketreatedseed testa

wa)rycuticleembryo

A lã':;¡Pç'I

f ' r,¿

Figure 52 Two electron micrographs of Emmenonthe pendulifloro'

(nj Control seed (above); (B) smoke-treated seed (below)' 129

l0 ',' Plant science

GerminationAllseedsneedwaterforgermination.Manyseedsaredryandneedio ,"nya.ute their cells. Sóme seeds contain a hormone that inhibitsgermination and water is needed to wash it out of the seed'

Germination involves growth of the embryo root and shoot and this

also requires water.

The metabolic rate of a dry and dormant seed is close to zero' but

afterabsorptionofwater,metabolicprocessesbeginagain'includingenergy reléase by aerobic cell respiration' Another requirement for

g"rrrilnution is therefore a supply of oxygen' Because g-ermination

involves enzyme-catalysed -tìuUotit reactions' warmth is required

and germination often fails at low temperatures'

Aother metabolic process occurring at the start of germination is

synthesis of gibberellin, a plant hormone' Several genes have to be

ápressed to produce the various enzymes of the metabolic pathway

t.uai.rg to giúberellin. This hormone stimulates mitosis and cell

division in the embryo. In starchy seeds it also stimulates the

froduction of amylase. This enzyme is needed to break down starch

inthefoodreservesintomaltose.otherenzymesconvertthemaltoseintosucroseorglucose.Whereasstarchisinsolubleandimmobile,sucrose and glucose can be transported from the food reserves to

where they are needed in the germinating seed' The embryo rootand shoot need sugars for growth, together with amino acids and

other substances released from the food stores' All parts of the

embryo need glucose for aerobic cell respiration'

lnvestigating biology: factors requ¡red for germination

Most vegetablbeen bred tothey do not u ds

of seed dormgrowers of vegetable croPs

stem

l0 mm

sometimes have difficulty in gettingcrops to germinate after sowing.

Choose one of the Possible causes

of crop failure shown in the mind-map, to investigate.

Design an exPeriment and see

whether you obtain evidence foror against t7our cause.

You will need to decide:

o which seed tYPe to use

o how to vary the factor that Youare investigating

o how to keeP other factorsconstant

o how to collect Your results,including how to assess whethergermination has occurred.

Soilwas too dry and heseeds remained dehYdrated.

Soil tempercture too highor too low

Seeds needed lightforgermination but were sownbelow the soil surface.

Seed too old - not viableany mofe.

Figure 55 Structure of a germinatlng

pea seedling

Seed needed dadrness forgemination búwæ sownon üre soil sudace.

Slugs, snails or ofter gesß

ate úre seedlings or miæate üre seeds.

Seed kept in unsuihbleconditions, e.g. too hot

Seeds sown too dæPlY, so

nn or¡t of food before $ootreadred fte light

Soil waterlogged andanaerobic, so sædlingsdied of ahanol poisoning'

150

Flowering, pollination and feÉilizationWhen a seed germinates, a young plant is formed that grows roots,stems and leaves. These are called vegetative structules, so the plantis in the vegetative phase. This can last for weeks, months oI years,

until a trigger causes the plant to change into the reproductivephase and produce flowers. The change from the vegetative to thereproductive phase happens when meristems in the shoot start toproduce parts of flowers instead of leaves'Figure 34 shows a flower o1 Prunus domestica.In the base of theflower are nectar-secreting glands, which attract insects, especiallybees. The petals are large and white, helping insects to find theflower. The anthers produce pollen, containing the male gametes.The filaments hold the anthers in a position where they are likely tobrush pollen onto visiting insects. If an insect is already carryingpollen from another Prunus domestica flower and brushes it onto thestigma, then the flower has been pollinated.

The next process after pollination is fertilization. From each pollengrain on the stigma a tube grows down the style to the ovary. Thepollen tube carries male gametes to fertilize the ovary. The ovary islocated inside a small rounded structure called an ovule. Thefertilized ovule develops into a seed and the ovaly develops into a

fruit. In this case the fruit is called a plum.

I The data in the Table 6 is difficult to analyse in its current form.Choose suitable presentation formats to display the data clearlyand allow you to identify any significant trends. You can use ICT oryou can draw graphs, tables, charts or diagrams by hand.

2 Describe clearly any trends that you have found in the data. Try toexplain each trend that you describe, using your biological knowledge.

5 ldentify any weaknesses in the data obtained. Suggest how theinvestigation could have been improved.

lO o Plant science

stigma

anther

filament

sepal

ovary

Figure 54 Structure of a plum flower

pollen pollen tubecontaining

gametes

pollen tube grows

Figure 55 Pollen grain germinating ona stigma at the start of the fertilizationprocess

Workíng with dstø: foctors offecting pollen developmentpollen grains sometimes develop when they are placed in drop of fluid on a microscope slide. The composition ofthe fluiã and its temperature affect whether this happens or not. Table 6 show the results of studies of pollen

development in plant species in Hong Kong.

o0.30

0.46

0.60

0.75

0.90

22.5

23,0

13.0

0.0

0.0

Bougoinvilleo globro

Delonix regio

Leucoeno leucocepholo

Bouhinio purpureo

Lilium bulbiferun

Clodiolus gondovensis

44.00

70.30

64.60

71.50

91.60

86.82

41.8

4.9

1|.069,9

l t,r

s0.6

0.75

0.45

0.15

0.45

0.30

0.45

0.0

1,0

2.5

5.0

25.0

33.6

25.1

15.5

r0.B

0.06Table

aaoaa

Sucroseconcentration(mmol dm{)

Percentage ofComellio joponicopollen grains that

developed

Diameter ofpollen gnin {pm)

Mean growth ofpollen tube(rm h") (mmol dm{)

sucfose conc.OptimalPlant species Copper ion

concentration{ppm}

Mean growthof pollentubes of

Bougoinvilleoglobro (Fm h")

t51

l0 ¡ Plant science

Gontrol of floweringObservations of fl hat the trigger for this in some

plants might be a h, but experiments showed that

it is the length of rs, not the length of daylight'

o Long-day plants flower in summer when the nights have become

short enough.o Short-day plants llower in the autumn (fall)' when the nights

have become long enough'

A pigment was discovered in leaves that plants use to measure the

r""itlr of dark periods. It is called phytochrome and is unusual as itcan switch between two fotms, P* and P.o'

o When Po absorbs red light of wavelen g|n 660 nm it is converted

o However, P* is more stable than Pu*, so in darkness Pu* verygraduallY changes into P*'

Further experiments have shown that Puo is the active form of

phytochrome and that receptor proteinJãre present in the cytoplasm to

which Pu* but not Pu binds.

o þlong-day plants, large enough amounts of P.o remain at the end of

short nights to bind to the receptor, which then promotes

plant flowers.

locatedin leaves

Figure 56 lnterconversions ofphytochrome

Data-based questionsi sowíng times for soybeonss and livestock. After germination, soybean plants grow a

es are produced at the nodes' The stem sections are

pods càntaining beans develop from them' When theynd internodes.

Figure 37 shows the mean numbers of nodes of soybean plants sown on different dates in Nebraska'

2220l81614t210I6420

-2

cfEô-oô-tttc)!otro(¡)-ôE=cg(oo)

=

.*I

+2-May+ l7-May+ 30-MaY+ l7-Jun

-.1 I

l r //ar// ãr rr4/z

I,tlJ lzl

-.- i-' Il-May 16-MaY 5l-MaY lsJun 3GJun ls-Jul SoJul l4-Aug 29-Aug

I Compare the growth of the soybean plants

sown on the different dates. [5]2 a) Deduce when the soybeans started to

flower. [2]b) Deduce with reasons, the factor that triggers

flowering in soYbeans. [3]

5 a) Explain the advantage, in terms of soybeanyields, of sowing the crop as early aspossible. [3]

b) Suggest two possible disadvantages of- sowing soybeans earlier than the dates

-ï"il:':::1-?-

PR

Figure 57 Calendat Date

.Es.-!

Chapter l0 questionst a) Phloem sap is nutrient-rich compared with many

other plant products, and the nutrients in it aresmall soluble molecules that do not need to bedigested. Despite this, the only anrmals toconsume it as the main part of their diet areinsects belonging to a grouP called the Hemiptera,including aphids, whitefly, mealybugs and psyllids.The data in this question comes from researchinto aphids.

The sugar content of phloem sap is very high -often greater than I mol dm 3.

¡) Explain how plants increase the sugarconcentration of phloem sap to such highlevels. [1]

Íi) Explain how high sugar concentrations cause ahigh pressure to develop in the phloem, [2]

b) Aphids only ingest a small proportion of the sugarin phloem sap. The remainder Passes out in thefaeces, which is a liquid called honeydew.Because of the high sugar concentrations, phloemsap has a much higher solute concentr¿tion thanaphid cells. Enzymes secreted into the aphid gutreduce the solute concentration of phloem sap byconverting sugars into oligosaccharides. Figure 38shows the relationship between the sucroseconcentrations of phloem sap ingested by aphidsand the oligosaccharide content of the honeydew

0.00 0.25 0.50 0.75 L00Dietary sucrose concentration (mol)

Figure 58

i) Describe the relationship between the sucroseconcentration of phloem sap ingested byaphids and the percentage of oligosaccharidesin the honeydew. [3]

ii) Suggest reasons for aphids secreting enzymesto reduce the solute concentration of the fluidin the gut. [2]

c) Aphids ingest larger volumes of phloem sap thanthey need, to obtain sufficient sugar for cellrespiration. This is because they also need toobtain amino acids and the concentration of aminoacids in phloem sap is low. Figure 39 shows thepercentages of individual amino acids in phloemsap and the percentages in aphid protein. Nine ofthe amino acids cannot be synthesized in aphid

lo Plant science

cells and so are called essential amino acids. Theother amino acids can be synthesized from otheramino acids and so are non-essential.

i|lü

0

c'õo_--lpEèo.cs!oo

.E

E o

oE zo

^o o ,/

Non-essentialamino acidsEssentialamino acids0

I lo0.E

-3 eoo8oogos40

20

0

Figure 59

loAmino acid o/o in

ì00phloem sap

D Evalu¿te phloem saP as a source of aminoacids for aphids. [3]

ii) Suggest reasons for the differences in aminoacid content between phloem sap and aphidprotein. [2]

d) Specialized cells have been discovered in aphidscalled bacteriocytes. These cells contain bacteriacalled Buchnero, which synthesize essential aminoacids from aspartic acid and sucrose. Aspartic acidis a non-essential amino acid that is found inmuch higher concentrations in phloem saP thanany other amino acid. When aphids reproduce,they pass on Buchnero bacteria to their offspring'

i) Explain how antibiotics could be used toobtain evidence for the role of Buchnero inaphids. [2]

ii) Using the data in this question, discuss thereasons for few animals using phloem sap asthe main part of their diet. [3]

2 St¿te one the role of each the following in plants:

a) apical meristems f) Pr*

b) bulbs g) spongY mesoPhYll

c) cotyledons h) tendrils

d) guard cells i) waxY cuticle

e) palisade mesophyll i) xylem.

5 Distinguish between pollination, fertilization and seeddispersal in the reproduction of flowering plants. [6]

4 a) Outline the structure of roots and how it allowswater to be absorbed. [5]

b) Explain how water is transported from the roots tothe top of the tallest trees [8]

c) Suggest reasons why monocotyledonous plants donot grow into large trees, whereas somedicotyledonous plants grow into very large trees. [4]

0

133