Stomata must open to let carbon dioxide in, and water vapour escapes through the same pores. Transpiration drives the flow of water up the plant, and xerophytes such as marram grass are built to slow it down.
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly the leaves. Water evaporates from the wet surfaces of mesophyll cell walls into the air spaces inside the leaf, and the water vapour then diffuses out through open stomata, down a water potential gradient, to the drier air outside. Low humidity, high temperature, wind and the light that opens stomata all increase the rate. The water lost is replaced from the xylem, which is what pulls the water column up the plant. Xerophytes are plants adapted to dry places. Marram grass reduces transpiration with rolled leaves (rolled by hinge cells), sunken stomata in grooves on the inner surface, hairs that trap moist air and a thick waxy cuticle on the exposed outer surface. Each adaptation either keeps humid air next to the stomata, so the water potential gradient is shallow, or makes the surface less permeable.
Fig. 1: an annotated drawing of a transverse section of a rolled marram grass leaf, a xerophyte
Section 01
Evaporate, then diffuse.
Watch where the water vapour particles start and how they leave the leaf. Then move the Humidity, Temperature and Wind sliders one at a time and read the transpiration rate each time.
Evaporation: water moves out of mesophyll cells into their cell walls, and evaporates from the wet wall surfaces into the air spaces, which become saturated with water vapour.
Diffusion: when the stomata are open, water vapour diffuses from the air spaces (high water potential) to the air outside (lower water potential).
Humidity: humid outside air makes the gradient shallow, so the rate falls. In the lab the rate reaches zero when the outside air is fully saturated.
Temperature: water molecules have more kinetic energy, so evaporation and diffusion are faster; warm air also holds more water vapour.
Wind: moving air removes the layer of humid air next to the stomata, keeping the gradient steep.
Light: stomata open in the light for gas exchange, so transpiration is faster by day than at night.
Transpiration is an unavoidable consequence of gas exchange: stomata open to let carbon dioxide in for photosynthesis, and water vapour escapes through the same pores. It also helps by cooling the leaf and by pulling water and mineral ions up the xylem.
Examiner trap
Say that water evaporates from the internal surfaces (the mesophyll cell walls) and then water vapour diffuses out through the stomata. Writing that "water evaporates out of the stomata" or that "water diffuses out of the leaf" merges two steps and loses marks.
Section 02
Built to hold on to water.
The lab opens on a marram grass leaf. Click each adaptation in turn, then switch to the ordinary leaf and compare the cuticle, the stomata and the shape of the surface.
Adaptation
Example
How it reduces water loss
rolled leaf
marram grass
traps a pocket of humid air inside, so the water potential gradient from the air spaces to the air around the stomata is shallow
hinge cells
marram grass
large thin-walled cells that lose water and shrink in dry conditions, rolling the leaf up
sunken stomata in grooves
marram grass, pine needles
still, humid air collects in the pits above the stomata
hairs
marram grass
trap a layer of still, moist air next to the leaf surface
thick waxy cuticle
marram grass, cacti
waterproof layer reduces evaporation through the epidermis; in marram it is on the exposed outer surface
small surface area to volume ratio
cacti (leaves reduced to spines), pine needles
less surface for water to evaporate from
few stomata, often open only at night
cacti
fewer pores and cooler, more humid air when they open
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the transpiration tab, where does liquid water evaporate from?
Quick check+10 XP
In the lab, you drag the Humidity slider fully to the right. What happens to the transpiration rate, and why?
Quick check+10 XP
In the marram grass leaf, which cells lose water in dry conditions and make the leaf roll up?
Quick check+10 XP
Where are the stomata of the marram grass leaf, compared with the ordinary leaf in the lab?
Section 03
Making an annotated drawing.
An annotation is more than a label: it names the feature and explains how it reduces water loss by transpiration.
Plan first: draw the outline of the rolled leaf and the ridged inner surface as a plan diagram, without individual cells.
Label, then explain: for example, "sunken stomata: humid air trapped in the groove reduces the water potential gradient, so less water vapour diffuses out".
Correct side: the thick cuticle is on the outer surface; the stomata, hairs and hinge cells are on the inner (upper) surface, inside the roll.
Link to the gradient: nearly every adaptation works either by making the surface less permeable or by keeping humid air near the stomata.
Examiner trap
Xerophyte adaptations reduce water loss; they do not stop it. The stomata still have to open for carbon dioxide, so some transpiration always happens.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which statement describes transpiration correctly?
Exam style+20 XP
Why does wind increase the rate of transpiration?
Exam style+20 XP
How do the hairs on the inner surface of a marram grass leaf reduce water loss?
Exam style+20 XP
Why can a xerophyte not avoid transpiration completely by keeping its stomata closed all the time?
Skill unlocked
Transpiration and xerophytes, mastered.
This skill is now recorded on your profile. Keep the chain going.