Applying Inquiry Skills

5. A microscopic view of a plant section reveals a greater than usualnumber of xylem cells. What might this suggest about the plant’sability to conduct and store water? In what kind of environmentwould a large number of xylem cells be beneficial to survival?Explain why.

6. Identifying and learning the names of plant tissues can be chal-lenging, even for experienced plant biologists. Create a tablewhich lists the three major tissue types, the specific cell types ineach, and their special structures including the name, a descrip-tion, and the function of each structure. Add a section to describethe location and importance of meristematic tissue.

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13.3

13.3 Leaves Green leaves are the major sites of photosynthesis. They contain chlorophyll, thegreen pigment necessary to capture light energy. They must also be able to obtaincarbon dioxide from the air and water to use as the building blocks for sugarsand starches, the products of photosynthesis. If maximizing photosynthesis werethe only objective, we would expect leaves to be very wide to maximize theirexposure to the light, and to have systems to readily obtain carbon dioxide andwater. However, there are other equally important considerations. Leaves mustnot dry out—a difficult problem when faced with bright sunlight, hot, dry air,and high winds. There is also the problem of hungry herbivores. As a result,leaves occur in a great variety of shapes, sizes, and textures. Leaves also have avariety of internal structures. These characteristics have evolved very gradually.They are the features which allowed the plants to survive the biotic and abiotic

factors of their habitats. Those plants with characteristics which did not promotesurvival simply died.

The blade is the flattened main body of the leaf. Leaves are positioned alongthe stem at points called nodes. The distance between successive nodes is calledan internode (Figure 1). Typically, leaves have a network of veins or vascular bun-dles of conducting and supporting tissue. In many plants, each leaf is connected

biotic: describes anything related to livingthings. Biotic factors are all living things inan area and include interactions within andbetween species, such as competition andpredation.

abiotic: describes anything related to non-living things. Abiotic factors include tempera-ture, humidity, light availability, and soilconditions such as water content, texture,and mineral composition.

nodes: the locations where leaves areattached to the stem

internode: the space between two succes-sive nodes on the same stem

petiole

blade

internode

node

nodesheath

node

bladeaxillary

bud

(a) (b)Figure 1

Typical leaf forms of (a) dicots and (b) monocots

502 Chapter 13

Figure 2

(a) A small part of a dicot leaf (from a silvermaple tree) showing net venation

(b) The monocot leaves of a plant called rosetwisted stalk, showing parallel venation (a) (b)

Figure 3

Examples of (a) simple leaves and (b) com-pound leaves

poplar(Populus)

oak(Quercus)

maple(Acer)

red buckeye(Aesculus)

black locust(Robinia)

honey locust(Gleditsia)

(a)

(b)

to the stem by a leaf stalk called a petiole. The vascular tissue in the stem usuallysends out one branch to the leaf through the petiole. Once in the leaf, the vasculartissue branches out. If these new veins branch and rebranch throughout the wholeleaf, the leaf is said to have net venation. This is the normal pattern for dicots(Figure 2(a)). If these new leaf veins tend to run from the petiole to the leaf tipwithout joining one another, the leaf is said to have parallel venation. This is thenormal pattern for monocots (Figure 2(b)). If the leaf has a single, undividedblade, it is called a simple leaf. A compound leaf has a blade divided into two ormore leaflets (Figure 3).

simple leaf: a leaf that is not divided intoleaflets

compound leaf: a leaf that is dividedinto two or more leaflets

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Stomata

During photosynthesis, the leaf must acquire a constant supply of carbon dioxideand be able to release the oxygen produced. The exchange of these gases is regu-lated by tiny pores called stomata (singular: stoma). Stomata are found in the epi-dermis of leaves or stems but are mostly found in the lower epidermis of leaves.As well as regulating carbon dioxide and oxygen diffusion, stomata also allowwater vapour to escape from the leaf. The loss of water vapour in plants is calledtranspiration. The water diffuses and evaporates into the air spaces of the leavesand out to the atmosphere through the stomata.

When the stomata are open, the plant can obtain needed carbon dioxide;however, the plant also loses water, which can pose significant problems for theplant. When the stomata are closed, water is conserved, but carbon dioxidecannot be obtained. The opening and closing of each stoma is regulated by a pairof sausage-shaped guard cells (Figure 5).

transpiration: the loss of water throughthe surfaces of the plants. Most transpirationoccurs through leaf stomata.

guard cells: the cells that occur in pairsaround each stoma in the epidermis of a leafor a stem. They regulate the opening andclosing of the stoma.

xylem

air spaces

phloemvein

cuticle

upper epidermis

palisade mesophyll

spongy mesophyll

lower epidermis

cuticle

2 pairs of guard cells

Figure 4

A three-dimensional drawing of a typical leafshowing internal and surface structures

Most epidermal cells do not contain chloroplasts. However, the guard cellsaround the stomata do contain many chloroplasts. When a pair of guard cells con-tains low levels of water, they are somewhat limp and rest against each other,closing the stoma. As water builds up in the leaf tissues, as it does most nights, theguard cells tend to swell. However, the portion of the cell wall of each guard cellthat faces the other is thickened. As the cells enlarge, they swell less where theirwalls are thickened. The pairs of swollen guard cells look similar to kidney beansand the stomata are now open. At sunrise, photosynthesis begins in the chloro-plasts. Carbon dioxide levels drop and oxygen levels increase in the leaves relativeto the concentration of these gases in air. Because the stomata are open, gaseousexchange occurs by simple diffusion. Water vapour is also lost.

Throughout the day, as the water concentration in the guard cells drops, thecells begin to shrink. Gradually, as the pairs of guard cells become limp and col-lapse, the stomata close. The opening and closing of the stomata are also relatedto the concentration of carbon dioxide in the guard cells. This mechanism tendsto allow the stomata to be open in the daytime for gaseous exchange and closedat night to conserve water.

During the hottest part of the day, plants may lose excessive water. If this lossoccurs, the guard cells, along with all the other cells, lose water and become limp.The stomata close and as a result, gaseous exchange of carbon dioxide andoxygen is prevented and photosynthesis is slowed down or temporarily stopped.In addition, light levels, temperature, and abscisic acid concentrations play a keyrole in the opening and closing of the stomata.

Figure 5

The closing (a) and opening (b) of stomataare regulated by the levels of water andcarbon dioxide in the guard cells.

stoma closed

thickenedguard cellwalls

guard cells

stoma open

epidermalcells

(a)

(b)

H20

CO2

CO2

H20

504 Chapter 13

Counting Stomata• Obtain a prepared slide of the epidermis of any leaf. Alternatively,

ask your teacher how to make your own wet mount, or imprint.• Observe the specimen under medium power and note the appear-

ance and pattern of stomata among the epidermal cells. Note theshape of the guard cells.

• Count the number of stomata in a single field of view. If there aretoo many stomata, count the number in a one-quarter “pie section”of the field of view and then multiply your result by four.

• Your teacher will provide you with the field of view diameter foryour microscope from which you can determine the radius.Determine the area (in square millimetres) of your field of viewusing this formula: area = pr2. Show your calculations with yourrecorded result.

• Use your results to estimate the total number of stomata on a leaf.Explain how you obtained this number.

Try ThisActivity

mesophyll: the region of photosyntheticcells between the epidermal layers of leaves

palisade mesophyll: one or two layersof brick-shaped cells, rich in chloroplasts andfound tightly packed beneath the upper epi-dermis of most leaves

spongy mesophyll: a layer of irregularlyshaped cells containing chloroplasts betweenthe palisade mesophyll and the lower epi-dermis of most leaves. Many air spaces arerandomly distributed within this layer.

Mesophyll

Between the upper and lower surfaces of a leaf is a photosynthetic region calledmesophyll (Figure 4). The mesophyll consists of parenchyma cells containinglots of chloroplasts. In most plants, the mesophyll has two different areas basedon the orientation and shape of the cells. The palisade mesophyll occurs underthe upper epidermis. Here, the cells are shaped like bricks and are tightly packedtogether in one or two layers. The longer sides of the cells are at right angles tothe upper epidermis. These palisade cells contain many chloroplasts and are theprimary site for photosynthesis. The spongy mesophyll lies between the palisademesophyll and the lower epidermis. These cells have fewer chloroplasts, are irreg-ular in shape, and are randomly arranged with large air spaces scattered amongthem. These air spaces promote the rapid diffusion of carbon dioxide into cellsand oxygen gas out of them. In the leaves of some plants, the mesophyll does notform two distinct areas.

As a result of photosynthesis, carbon dioxide levels drop in the mesophyllcells. Since carbon dioxide levels are lower within the cells than in the sur-rounding air spaces, carbon dioxide diffuses into the mesophyll cells, providingmore reactants for photosynthesis. Similarly, as oxygen gas is produced withinthe cells during photosynthesis, the concentration rises, resulting in the diffusionof oxygen gas out of the cells into the surrounding air spaces. If the stomata areopen, gaseous exchange occurs between the air spaces and the atmosphere. If thestomata are closed, the process of photosynthesis quickly consumes the availablecarbon dioxide within the air spaces in the mesophyll and further photosynthesiseffectively stops.

Leaf Adaptations to Abiotic Factors

The extreme conditions of some terrestrial environments have made it difficultfor many plants to survive. Diversity of species has resulted in the survival of somespecies in those locations but not others. In addition, diversity within a species hasalso allowed those individuals of a species which could somehow cope with theextreme conditions to survive, while others died. Plants with very broad leaves totrap low light energy will survive in shaded areas but die if they germinate inopen, sunny fields. Plants whose leaves appear in the very early spring, before

The single most abundant protein on Earth,RUBISCO (acronym for ribulose 1,5-bisphos-phate carboxylase/oxygenase), is the enzymeresponsible for creating organic moleculescontaining carbon from the inorganic carbondioxide in the air.

DID YOU KNOW ?

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leaves of the surrounding trees emerge and create shaded conditions, will survivein a deciduous forest, but plants whose leaves appear late will not survive.

Most conifers are evergreen, which means they keep their leaves throughoutthe winter. This characteristic is especially beneficial in regions with a shortgrowing season. These trees avoid expending the large amounts of time, energy,and nutrients required to grow a complete set of new leaves each year. The leavesof most conifers, such as pine and spruce, are modified as thin, long needles. Theneedles have a small surface area and a thick, waxy cuticle. Although these fea-tures make the leaves inefficient for photosynthesis, they greatly reduce waterloss. This prevention is advantageous since, during the winter, lost water cannotbe replaced by roots buried in frozen ground.

Plants that can survive in areas of low precipitation or high salt content in thesoil usually have leaves with thick layers of water storage tissue or are covered withan extra thick, waxy cuticle to prevent water loss. These plants also tend to havefewer than the usual number of stomata. The spines of cacti are the remnants ofleaves. Water loss is reduced because they have very few stomata and extremelysmall surface areas. Photosynthesis takes place in the fleshy, green stems. Cactithrive in sunny desserts but would not survive in shaded forests (Figure 6).

Examining Water LossIn this activity, you will examine water loss in leaves.

Materials: large fresh green leaf, scissors, water, two small binder clipsor clothes pins, paper towel, balance, incubator or small fan

Procedure

• Trace the leaf on the paper towel. Cut out the leaf shape.• Wet the paper leaf until it is saturated but not dripping.• Determine and record the masses of the leaf and the wet paper

cutout. • If possible, place the leaf and the cutout in a drying oven or incu-

bator overnight. Otherwise, hang them up to dry. You may wish touse a small fan to shorten the drying time.

• When the paper cutout looks noticeably drier, determine the finalmasses of both the leaf and the cutout.

• Calculate the percent loss of mass for both the leaf and the papercutout.

• Comment on the effectiveness of the leaf cuticle in preventingdrying out.

Try ThisActivity

Figure 6

(a) Conifer needles are modified leaveswhich reduce water loss but still performphotosynthesis.

(b) Cactus spines are more radically modifiedleaves. Most photosynthesis occurs in thefleshy stems. (a) (b)

506 Chapter 13

Leaf Adaptations to Biotic Factors

Leaves are extremely vulnerable to herbivores. Herbivores are attracted to tenderleaves with mild flavours. Any plants that happen to have tough, hairy, prickly, orbitter leaves are more likely to survive herbivore appetites (Figure 7). However,there is always a tradeoff. The very characteristics which help the plants surviveherbivores reduce their photosynthetic efficiency. Also, diversity among the her-bivores has allowed certain herbivores to cope with the plant features. Some her-bivores have tough mouth tissues, efficient teeth, or special digestive enzymes;others have a poor sense of taste.

Diversity also means that while some plants continue to supply nutritiousfood for herbivores, other plants produce toxic chemicals in their tissues whichactually control herbivore populations. The nicotine in tobacco leaves is an insec-ticide. An even more convoluted situation involves the common milkweed plantand the monarch butterfly. While milkweeds produce a toxin which has a horribletaste and is highly toxic to almost all insects and large herbivores, monarch but-terfly caterpillars are immune to this poison. In fact, the milkweed toxins accu-mulate in the fatty tissues of these caterpillars. In this way, the caterpillars andadult monarchs are themselves toxic and unpalatable to their enemies.

Figure 7

Some leaf adaptations discourage hungryherbivores. Plants such as (a) woolly lamb’sears, which have hairy leaves, stems, andflower heads, and (b) common milkweed,which have a horrible taste, are avoided byherbivores. Look closely and you can see amonarch butterfly caterpillar among theleaves and seed pods of the milkweed. (a) (b)

Other Leaf Adaptations

In addition to performing photosynthesis, some plants use leaves to accomplisha number of other functions (Figure 8). In some plants, all the leaves are modi-fied, but in others, regular leaves exist along with modified ones. Onion bulbs aremodified leaves that are specialized for storage of water and nutrients. Someplants develop specialized leaves called tendrils for attachment to surfaces orobjects for support. A variety of plants, such as cacti, produce sharp spines,which are actually modified leaves. Plants with such specialized leaves survivebetter than those without them when dealing with hungry herbivores. Even thepetals of flowers are modified leaves which attract pollinators for reproductivepurposes. The Venus fly-trap, the pitcher plant, and sundews are all examples ofcarnivorous plants, although the term carnivorous is misleading because they donot require animals as food; they all can photosynthesize. However, studies haveshown that they thrive when animal protein is available. There are also some

toxin: a poison produced in the body of aliving organism. It is not harmful to theorganism itself but to other organisms.

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plants which have lost not only their leaves but also their ability to photosynthe-size. For example, Indian pipe leaves are reduced to tiny bits of tissue. There areno chloroplasts in any part of this plant. Thus, these plants are heterotrophic.They are saprophytes and obtain their nutrients from decaying organic materialin the soil. Indian pipe has vascular tissue and complete flowers.

Practice

Understanding Concepts

1. State the primary function of leaves.

2. What two functions are served by vascular tissue within leaves?

3. Why are guard cells essential?

4. What two substances control the swelling or collapse of guard cells?

5. Describe four plant features which help protect them from hungryherbivores.

Activity 13.3.1

Leaf Adaptations

Leaves are the site of photosynthesis and exhibit many adaptations to maximizetheir photosynthetic efficiency. Leaves are also the site of water loss through tran-spiration and evaporation. Up to 90% of the water taken in by roots is lost to theair through leaves. Plants called xerophytes can survive, or even thrive, in water-deficient environments and have evolved numerous features to conserve water.In contrast, plants which live in water are called hydrophytes and exhibit features

(a) (b) (c)

(d) (e) (f)

Figure 8

Leaf adaptations.(a) An onion bulb consists of modified leaves.

Note the dried real leaves at one end of thebulb and dried up roots at the other end.

(b) A very prickly plant growing along aroadside in southern Spain

(c) Colourful petals of common spring gardenflowers

(d) Leaves of the pitcher plant attract, trap,and digest insects in a pool of water.

(e) Leaves of sundew with sticky projectionsthat bend down to trap insects that landon them

(f) Indian pipe has no chlorophyll and is aheterotrophic plant.

xerophytes: plants that survive or thrivein areas with very little moisture

hydrophytes: plants living on or in water

508 Chapter 13

uniquely suited to their environment. Most plants thrive in environments with amoderate water supply and are referred to as mesophytes.

Questions

What are the features of a typical mesophytic leaf?What leaf adaptations are found in xerophytes?What special features do the leaves of hydrophytic plants have?

Materials

prepared slides of Syringa, Yucca, Zea (corn), Potamogeton, Pinus (pine),Verbascum, and Oleanderwhole samples of Aloe, rubber plant leaf, and fig leafmicroscopeany potted mesophyte, such as Geranium or Coleusany potted xerophyte, such as jadecobalt chloride paperpaper clip or adhesive tapeplastic wrap

Procedure

Part 1: Observing Leaf AdaptationsThis lab will consist of a series of numbered lab stations. Each station will haveone or two specimens for you to examine. Take approximately 10 min to com-plete and record your observations at each station. Your teacher will instruct youon how and when to change stations.

1. Copy Table 1 into your notebook but leave large spaces for your answers,about 15 lines for each row of the table. Your whole table may occupy 3notebook pages. Record your answers as you complete the steps. At eachlab station, use the information and questions provided here as guides toyour observations. List and describe the most significant features of eachleaf. Add drawings where appropriate.

mesophytes: plants that thrive withmoderate moisture

Table 1

Station Specimen(s) Observed feature(s) Adaptive significance

1 Syringa ? ?

2 Yucca ? ?

Aloe ? ?

3 Pinus ? ?

Zea ? ?

4 Verbascum ? ?

Oleander ? ?

5 Potamogeton ? ?

6 Philodendron ? ?

Station 1: Syringa—A Mesophyte2. The prepared Syringa slide has a cross section of a leaf blade. View the leaf

under low power. This leaf represents a typical mesophyte dicot leaf.Observe and record the shape of the whole cross section of this leaf.

Plants: Form and Function 509

13.3

3. View the slide under medium and high powers. Look for the palisade mes-ophyll. Describe and draw what you see.

4. Look for the dark pairs of guard cells in the epidermal layers which openand close the stomata. Compare the number of guard cells seen in theupper and lower epidermal tissue.

Station 2: Yucca and Aloe—Succulent Xerophytes5. View the prepared Yucca slide under medium power. Look for large num-

bers of vascular bundles and a thick cuticle.

6. Break off a small piece of one Aloe leaf. Observe and describe its interior.

Station 3: Zea and Pinus—Monocot and GymnospermXerophytes

7. View the prepared slides of a Zea leaf and a Pinus needle using mediumand high powers. Corn and pine are well adapted for conserving waterduring hot summer days. Note that most stomata are located on the upperepidermis of the corn leaf. The leaf also possesses unusually large epi-dermal cells. These cells collapse in hot, dry conditions, causing the leaf tocurl up and inwards. Describe the position of the pine guard cells relativeto the needle surface.

Station 4: Verbascum and Oleander—Xerophytes with SpecializedSurface Features

8. View the prepared slides of Verbascum and Oleander under medium andhigh powers. Note the hairs extending from the surface of the Verbascum leaf.

9. Carefully note the location of stomata within the large pits on the lowersurface of the Oleander leaf. These pits also contain tiny hairs. Look for athick cuticle and multilayered epidermis in the Oleander.

Station 5: Potamogeton—A Hydrophyte10. View the prepared slide of a leaf cross section under medium and high

powers. Note that the palisade layer is near the upper leaf surface. Note thelocation of the stomata and the size of the spongy mesophyll layer. Whateffect would this specialized tissue have on leaves that grow in water?

Station 6: Philodendron—A Tropical Rain Forest Vine11. Philodendrons are vines that live in tropical rain forests. They begin life on

the ground in deep shade and grow up into the canopy along tree trunks.Examine the leaves, stems, and aerial roots of this plant. Note the “drip tip”on the leaf.

Part 2: Testing Transpiration in Leaves12. Dry cobalt chloride paper is blue. In the presence of water, it turns pink.

Select a healthy, large leaf on a potted mesophyte. Do not remove this leaf.While being careful not to damage the leaf or plant stem, fold a piece ofcobalt chloride paper over the edge of the leaf, covering part of the upperand lower leaf surface. Cover the cobalt chloride paper with a slightly largerpiece of plastic wrap. Fasten the paper and plastic to the leaf with a paperclip or tape, being careful not to damage the leaf (Figure 9).

13. After 15 min, remove and examine the indicator paper. Record yourobservations.

14. Repeat steps 12 and 13 using a xerophyte plant. Figure 9

Without removing or damaging the leaf,apply the cobalt chloride paper as shown.

510 Chapter 13

Analysis

(a) In the Syringa leaf, how does the location of the palisade layer enhancephotosynthesis? Where are most of the stomata located?

(b) Compare the Yucca and Aloe leaves with the leaves of other commonplants. How do the special features of these leaves contribute to theirfunction?

(c) What is the function of the thick cuticle of the Yucca leaf?(d) Comment on the size and shape of the Pinus needles. How does their

overall size influence water loss?(e) How does Zea’s ability to curl its leaves in hot weather influence

water loss?(f) How might the hairs on the Verbascum leaf influence surface air flow?

How might the hairs influence water loss?(g) How do the stomatal pits on the Oleander leaf influence water loss?(h) How is the location of the stomata on the Potamogeton leaf well suited

to a floating leaf?(i) Discuss whether the large air spaces in the Potamogeton leaf are wasted

space or if they serve a useful purpose.(j) Describe the leaf surfaces of the Philodendron as smooth and waxy or

rough and dull. How would the surface influence what happens torainwater that falls on these leaves?

(k) How might the “drip tip” on Philodendron leaves influence what happensto rainwater that falls on these leaves?

(l) What is the advantage of having leaves that shed surface water quickly?(m) How is water able to escape from the surfaces of leaves? What struc-

tures are involved?(n) Compare the transpiration rates from top and bottom leaf surfaces.

Account for any differences.(o) Compare the overall transpiration rates from the mesophytes and

xerophytes. Account for any differences.(p) What advantages are there to having stomata in the lower leaf surface

rather than the upper leaf surface?(q) What features or structures of a xerophytic plant allow it to survive in

hot, dry environments?(r) If the leaves of xerophytes have far fewer leaf stomata than those of

mesophytes, what can you infer about the potential rates of photosyn-thesis and growth in these plants? Explain your answer.

Leaves

1. Leaves are the site of photosynthesis.

2. Transpiration and evaporation of water and gaseous exchange of carbondioxide and oxygen occur through the stomata..

3. When there is plenty of water in the guard cells, they swell in a way thatopens the stomata, but when the water content decreases, the guard cellscollapse, closing the stomata.

4. The mesophyll is a photosynthetic layer consisting of palisade and spongymesophyll.

• The palisade mesophyll cells, found just under the upper epidermisof leaves, are brick shaped, tightly packed together, and have manychloroplasts.

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13.3

Understanding Concepts

1. What is the advantage of having air spaces within leaves?

2. List the features of xerophyte leaves that reduce water loss.

3. Describe a special adaptation found in each of the followingleaves:(a) Potamogeton (b) Pinus (c) Verbascum(d) Aloe (e) Zea

4. Photosynthesis requires water and carbon dioxide and producessugars and oxygen. Using proper terminology, describe the path-ways taken by these reactants and products as they enter and/orexit the leaf.

5. Describe the key problem that exists for plants that have manybroad, thin leaves. How does the cuticle help overcome thisproblem?

6. Tropical rain forest plants have no shortage of water. (a) How have the leaves of rain forest plants evolved to cope

with rainy conditions?(b) How might rain forest plants be harmed by having continu-

ously damp or wet leaves?

7. During the winter, the ground is frozen. The days can be coldand sunny while the air is very dry. Given these conditions, sug-gest reasons why pine trees have evolved to retain their leavesduring the winter. Also, suggest the benefits of their verynarrow, waxy leaves.

Applying Inquiry Skills

8. A number of leaves are obtained from a rare plant collection andcarefully examined. Based on the following information, classifyeach leaf type as a mesophyte, xerophyte, or hydrophyte.• Leaf A: broad and thin with a double palisade layer and many

stomata on the lower leaf surface • Leaf B: no stomata on the lower surface but does have stomata

on the upper surface; vascular bundles almost completelylacking in xylem

• Leaf C: very fleshy with many vascular bundles but very fewstomata

9. Examine a variety of houseplants. Name each one and describeany features it has which might allow it to survive dry, humid,cold, and hot climates. Look for and mention any plants with “driptips.” Speculate on the natural habitat of each plant described.

(continued)

Section 13.3 Questions

• The irregularly shaped, spongy mesophyll cells with interspersed large airspaces lie between the palisade mesophyll and the lower epidermis ofleaves and have fewer chloroplasts.

5. Leaves have adaptations (e.g., cactus spines, evergreen needles, tough fibres,hairy leaves, thorns, bright colours, and toxic compounds) to various abi-otic and biotic factors.

6. A xerophyte is a plant that survives or thrives in areas with very littlemoisture.

7. A hydrophyte is a plant living on or in water.

8. A mesophyte is a plant that thrives with moderate moisture.