7.2.47 · Transport in Plants

Xerophytic Plant Leaf Adaptations — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 11 of 15 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

7.2.4 Xerophytic Plant Leaf Adaptations — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe and explain structural and physiological adaptations of xerophytes that reduce water loss in dry habitats.

Leaf section
Outer surface
Inner surface
Microclimate
Hinge cells
CAM photosynthesis

Part 1 of 6What a xerophyte is

Xerophyte

Xerophyte a plant adapted to dry and arid conditions, with structural (xeromorphic) and physiological adaptations that conserve water.

Habitats

Habitats deserts, sand dunes, salt marshes and exposed windy places.
- (physiologically dry because salty water has a very low water potential)

Strategy

Strategy stomata must still open for photosynthesis, so adaptations reduce transpiration, store water or tolerate drought.

Examples

Examples marram grass, cacti and oleander (Nerium oleander).
- (Ammophila arenaria)

Part 2 of 6Adaptations that reduce water loss

Adaptation	How it works	Example
Thick waxy cuticle	Waterproof layer increases the distance and reduces evaporation from the epidermis	Pine, prickly pear
Sunken stomata in pits or grooves	Traps humid air, reducing the water potential gradient	Pine, Nerium
Hairs (trichomes) on the leaf surface	Trap moist air and reduce air movement over stomata	Marram grass
Rolled leaves with hinge cells	Hinge cells shrink when flaccid, rolling the leaf and putting stomata in a humid still-air chamber with the thick cuticle exposed	Marram grass
Reduced leaf surface area (needles, spines, scales)	Smaller surface for evaporation	Cactus (Opuntia)
Fewer stomata, or stomata closed by day and open at night	Reduces diffusion of water vapour, and CO₂ is fixed at night	Nerium; pineapple, yucca
Fleshy leaves or stems for water storage (succulents)	Reserves of water in large parenchyma cells	Bryophyllum, cacti
Deep or widespread roots	Reach deep water or collect from a large area	Desert shrubs

Part 3 of 6Marram grass

Habitat

Habitat sand dunes, where soil drains quickly, salt spray is common and wind is strong.

Leaf section

Leaf section the leaf is rolled with stomata on the inner surface, enclosed in a space.

Outer surface

Outer surface it has a thick cuticle.

Inner surface

Inner surface it is covered by hairs, and the stomata are sunken in pits.

Microclimate

Microclimate together these create high humidity, which lowers the water potential gradient and slows diffusion of water vapour.

Hinge cells

Hinge cells when the leaf is very dry they lose turgor and roll the leaf more tightly.

Part 4 of 6Other xerophytes

Cacti

Cacti fleshy stems store water and photosynthesise, leaves are reduced to spines that cut surface area and deter herbivores, and the cuticle is thick.

CAM photosynthesis

CAM photosynthesis stomata open at night to take in carbon dioxide,
- which is stored as an organic acid and used in the light period when stomata are shut.

Leaf shedding

Leaf shedding plants such as the creosote bush shed leaves in drought.

Drought escapers

Drought escapers many desert annuals complete their life cycle quickly after rain and survive the dry period as seeds.

Trade-off

Trade-off many adaptations also reduce carbon dioxide uptake, so xerophytes often grow slowly.

Part 5 of 6Xerophytes compared with other plant types

Type	Conditions	Features
Xerophyte	Dry	Thick cuticle, smaller leaf area, lower stomatal density, stomata hidden
Mesophyte	Moderate water supply	Thin cuticle, stomata at the surface, flat blade; wilts when water is short
Hydrophyte	Water or waterlogged	Large air spaces (aerenchyma), stomata on the upper surface of floating leaves, thin cuticle, reduced xylem
Halophyte	Salty	Lose salt through glands or store it in vacuoles so cells keep a low water potential and can take up water from saline soil

Roots
- long tap roots reaching groundwater or shallow widespread roots capturing rare rain are also xerophytic adaptations.

Part 6 of 6More from the specification

Phloem Translocation of Plant Assimilates

Photosynthetic assimilates, primarily sucrose molecules, are actively loaded into the sieve tube elements of phloem tissue at source regions by companion cells utilizing proton-sucrose symporters.

Apoplast and Symplast Loading Pathways

Sucrose travels from photosynthetic mesophyll cells toward phloem sieve tubes through either the symplast pathway or the apoplast pathway (diffusing through cellulose cell walls and intercellular spaces).
- (passing through cytoplasm interconnected by plasmodesmata)

Hydrostatic Pressure Gradient and Mass Flow

Active loading of sucrose reduces water potential within phloem sieve tubes, drawing in water by osmosis;
- this generates high hydrostatic pressure at the source that drives the mass flow of phloem sap toward sink tissue along hydrostatic pressure gradients.

Phloem Translocation of Plant Assimilates

Photosynthetic assimilates, primarily sucrose molecules, are actively loaded into the sieve tube elements of phloem tissue at source regions by companion cells utilizing proton-sucrose symporters.

Apoplast and Symplast Loading Pathways

Sucrose travels from photosynthetic mesophyll cells toward phloem sieve tubes through either the symplast pathway or the apoplast pathway (diffusing through cellulose cell walls and intercellular spaces).
- (passing through cytoplasm interconnected by plasmodesmata)

Hydrostatic Pressure Gradient and Mass Flow

Active loading of sucrose reduces water potential within phloem sieve tubes, drawing in water by osmosis;
- this generates high hydrostatic pressure at the source that drives the mass flow of phloem sap toward sink tissue along hydrostatic pressure gradients.

Companion cell

A cell that supplies energy to a sieve tube.

Concentration gradient

The difference in concentration of a substance between two regions.

Osmosis

The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Vessel

A long tube formed from dead xylem cells.

Solute

A substance dissolved in a solvent.

Movement of sucrose into the sieve tubes at a source.

Xylem vessel

A tube of dead lignified cells carrying water upwards.

Transpiration stream

Continuous movement of water up the xylem from roots to leaves.

Hydrostatic pressure

The pressure a fluid exerts on the walls of its container.

Sieve tube

A living tube in phloem that carries sap.

Sucrose

The sugar mainly transported in phloem.

Phloem

Tissue carrying sucrose and amino acids.

Tissue

A group of cells of similar structure that perform the same function.

Assimilates

Organic substances such as sucrose and amino acids transported in the phloem.

Apoplast

Pathway through cell walls.

Symplast

Pathway through cytoplasm and plasmodesmata.

Source

A part of a plant that releases sucrose or amino acids.

Mineral

A nutrient such as calcium or iron needed in small amounts.

Key terms

Term	Meaning
Xerophyte	A plant adapted to survive in dry conditions.
Cuticle	A waxy waterproof layer on the outside of the epidermis.
Sunken stomata	Stomata set in pits, trapping humid air.
Succulent	A plant that stores water in thick fleshy tissues.
CAM plant	A plant that opens stomata at night and fixes carbon dioxide into organic acids.
Mesophyte	A plant adapted to moderate water conditions.
Trichome	A hair on the surface of a leaf or stem.

Avoid these mistakes

Saying xerophytes have no stomata — they have stomata, often fewer or hidden.

Saying hairs absorb water — their function is to trap humid air.

Mixing up xerophytes and hydrophytes.

Saying a thick cuticle reduces gas exchange through stomata — it reduces loss across the epidermis.

Saying an adaptation stops water loss — it reduces it, and each should be linked to the diffusion gradient or distance.

Last updated: 15 October 2026