13.10 Limiting water loss in plants

Learning outcomesStudents should be able to understand the

following:How terrestrial plants are adapted to

efficiently exchange gases and conserve waterHow Xerophytic plants are adapted to live

where water loss may exceed their water uptake

How Xerophytes are adapted to limit water loss through transpiration

Learning outcomesCandidates should be able to:Explain structural and functional

compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects and xerophytic plants

Balancing gas exchange with water lossInsects and plants compared

• Both have waxy / waterproof coverings over their gas exchange surfaces to limit water loss by evaporation• Both can close the openings to their gas exchange systems (spiracles / stomata) to regulate water loss

o In plants stomata are predominantly on the underside of leaves to limit water loss by evaporationo Insects have evolved small SA : Volume ratios to limit water loss from the body surfaceo But the leaves of plants require a large surface area in order to absorb light and exchange gases for photosynthesis

Adaptations of plantsPlants in different habitats are adapted to cope

with different problems of water availability:

Mesophytes - plants adapted to a habitat with adequate water

Xerophytes - plants adapted to a dry habitat

Halophytes - plants adapted to a salty habitat

Hydrophytes - plants adapted to a freshwater habitat

Adaptation How it works Examplesmall leaf surface

arealess area for evaporation

conifer needles, cactus spines

low stomata density

fewer gaps in leaves

stomata on lower surface of leaf

only

more humid air on lower surface, so less evaporation

most dicots

sunken stomata maintains humid air around stomata

marram grass, pine

stomatal hairs maintains humid air around stomata

marram grass, couch grass, heather

Some adaptations of xerophytes are:

Adaptation How it works Examplethick cuticle stops uncontrolled

evaporation through leaf cells

Most dicots

Evergreens

e.g. Holly

shedding leaves in dry/cold season

reduce water loss at certain times of

year

deciduous plants

folded leaves maintains humid air around stomata

marram grass,

succulent leaves and stem

stores water cacti

extensive roots maximise water uptake

cacti

Some adaptations of xerophytes are:

All Cacti are xerophytes

Left and right Epidermis of the cactus Rhipsalis dissimilis. Left: View of the epidermis surface. The crater-shaped depressions with a guard cell each at their base can be seen.Right: X-section through the epidermis & underlying tissues. The guard cells are countersunk, the cuticle is thickened. These are classic xerophyte adaptations.

Transverse Section Through Leaf of Xerophytic Plant

Marram grass

Marram grass

Written tasks 1. AQA AS Biology textbook pg 201

Summary questions 1-4

2. Read ‘Not only desert plants have problems obtaining water’ on page 201 then answer application questions 1-4

3. Complete the exam style question about Xerophytic plants

Mark scheme - exam style questionAnswers – Xerophytes question  1.(a) E. superba;

Largest proportion of shallow roots; 2 (b) Roots go to greatest depths;

Able to get water when surface soil dried out;orGreatest root mass;Able to store more water; 2

(c) Curled leaves;

Thick cuticle;Sunken stomata;Hairs; max 2

[6] 

Homework Referring to the AQA AS Biology textbook and

other (internet) sources:

Use pictures and brief explanations to show how a variety of Xerophytic plants are adapted conserve water in dry habitats

Learning outcomesStudents should be able to understand the

following:How terrestrial plants are adapted to

efficiently exchange gases and conserve waterHow Xerophytic plants are adapted to live

where water loss may exceed their water uptake

How Xerophytes are adapted to limit water loss through transpiration