Bulliform Cells: Why Grass Leaves Curl in Dry Weather
Biology · Anatomy of Flowering Plants · NEET
Bulliform cells are large, empty, colourless cells on the upper (adaxial) epidermis of grass (monocot) leaves, found along the veins. When they are turgid (full of water) the leaf stays flat and open. When water is short, they become flaccid and pull the leaf so it curls or rolls inwards. This reduces the exposed surface and lowers water loss. Memory hook: "Bulliform = Balloon". Full balloon (turgid) = flat leaf; deflated balloon (flaccid) = rolled leaf.
Bulliform (balloon-like) cells on the upper epidermis of a grass leaf. When turgid the leaf stays flat; when flaccid under water stress they lose turgor and the leaf curls inwards to cut water loss.
Your doubts, answered
What exactly are bulliform cells?
They are large, empty, colourless cells on the upper (adaxial) epidermis of grass leaves. NCERT (Class XI, Ch 6) says: in grasses, certain adaxial epidermal cells along the veins modify into large, empty, colourless cells called bulliform cells. They have no chloroplast (colourless) and mainly hold water, so they act like water sacs.
Why do grass leaves curl inwards during dry weather?
Because the bulliform cells become flaccid (lose water) during water stress. When full of water they are turgid and keep the leaf surface flat and exposed. When they lose water and go flaccid, they can no longer hold the leaf open, so the leaf curls or rolls inwards. This is the NEET 2019 answer: flaccidity of bulliform cells.
Turgid or flaccid: which state opens the leaf and which curls it?
Turgid (full of water) = leaf surface exposed and flat. Flaccid (water stress) = leaf curls inwards. Students often flip these. Remember: water present = leaf open (plant is comfortable); water absent = leaf rolls (plant protects itself).
Are bulliform cells the same as subsidiary cells?
No. This is a common trap. Bulliform cells are large empty water-storing cells in grass leaves that cause leaf curling. Subsidiary cells are the specialised epidermal cells around the guard cells that form part of the stomatal apparatus. NEET 2016 asked which cells surround guard cells, and the answer was subsidiary cells, not bulliform cells.
What is the main function of bulliform cells?
To minimise water loss during water stress. By curling the leaf inwards, they reduce the surface area exposed to air, so less transpiration happens. NEET has asked this directly (2024 and 2026). The curling itself is the mechanism; water conservation is the purpose.
Do bulliform cells help in photosynthesis?
No. They are colourless, meaning they lack chloroplasts, so they cannot do photosynthesis. NEET 2024 and 2026 both used photosynthesis and sugar storage as wrong options. Their job is water storage and leaf movement, not making food.
⚠️ The NEET trap ✗ Grass leaves curl inwards because of closure of stomata during dry weather. ✓ Grass leaves curl inwards because of flaccidity of bulliform cells. Stomatal closure conserves water too, but it does not cause the mechanical inward curling of the leaf. 🧠 NEET 2019 listed stomatal closure as a tempting distractor. Both help save water, but only the bulliform cells physically roll the leaf. Match the mechanism to the movement.
Real NEET questions
2019
Grass leaves curl inwards during very dry weather. Select the most appropriate reason from the following:
A · Closure of stomata
B · Flaccidity of bulliform cells ✓
C · Shrinkage of air spaces in spongy mesophyll
D · Tyloses in vessels
Solution: In grass (isobilateral, monocot) leaves, large empty colourless adaxial epidermal cells along the veins are the bulliform cells. When turgid the leaf stays exposed; under water stress they become flaccid and pull the leaf so it rolls inwards, reducing exposed surface and cutting water loss. Stomatal closure also saves water but does not cause the mechanical curling.
2024
Bulliform cells are responsible for
A · Protecting the plant from salt stress.
B · Increased photosynthesis in monocots.
C · Providing large spaces for storage of sugars.
D · Inward curling of leaves in monocots. ✓
Solution: Bulliform cells are large, empty, colourless cells along the veins in grasses. When flaccid under water stress they lose turgor and make the leaf curl inwards, reducing exposed surface area and water loss. They are colourless, so they do not do photosynthesis, and they store water, not sugars.
2026
The main function of bulliform cells in grasses is :
A · to make the leaf impermeable to fungal spores.
B · to transport water.
C · to perform photosynthesis.
D · to minimize water loss during water stress. ✓
Solution: When turgid the leaf surface is exposed; under water stress bulliform cells become flaccid and curl the leaf inwards, reducing exposed surface area and minimising transpirational water loss. They do not transport water (that is xylem) and are colourless so cannot photosynthesise.
Solved Anatomy of Flowering Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
On which side of the leaf are bulliform cells found?
On the adaxial side, which is the upper surface of the leaf. They lie in the epidermis along the veins in grass (monocot) leaves.
Are bulliform cells living or dead?
They are living cells. They are large, thin-walled, colourless and mostly filled with water, and they change shape based on how much water they hold (turgid or flaccid).
In which type of leaf are bulliform cells present?
In isobilateral (monocotyledonous) leaves, such as grasses. They are a key feature of monocot leaf anatomy and are not typically found in dorsiventral dicot leaves.
Why are bulliform cells called colourless?
Because they have no chloroplasts. Without chloroplasts they cannot carry out photosynthesis, so NEET options claiming they do photosynthesis are always wrong.
What is the difference between bulliform cells and motor cells?
They are the same thing. Bulliform cells are also called motor cells because they drive the movement (rolling and unrolling) of the grass leaf by changing their turgor.
