Q.What are the cells that make the leaves curl in plants during water stress?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Epidermal Tissue System
The Skin of the Plant
Think about your own skin. It's the outermost barrier — it protects you from germs, prevents water loss, senses the environment, and sometimes even breathes. A plant has the same need for a protective outer layer. That layer is the epidermal tissue system.
The intuition is simple: every living thing that faces the outside world needs a boundary. For plants, that boundary is a single layer of tightly packed cells covering the entire body — roots, stems, leaves, flowers, and fruits. It's the plant's first line of defence.
The Precise Statement
The epidermal tissue system is the outermost covering of the plant body. It consists of:
- Epidermis — the living, one-cell-thick layer of cells
- Cuticle — a waxy, waterproof coating secreted on the outer surface of the epidermis
- Stomata — specialised pores for gas exchange and transpiration
- Trichomes — epidermal outgrowths (hairs, scales, or glands)
The entire plant body — root, stem, leaf — is covered by the epidermis. But the structure of the epidermis changes depending on where it is. The epidermis of a root is different from that of a leaf, because their jobs are different.
Breaking It Down
1. The Epidermis Itself
The epidermis is a single layer of living cells, tightly joined with no intercellular spaces. These cells are usually flat and rectangular in cross-section. They are transparent — which matters because light needs to reach the photosynthetic cells below.
On the root, the epidermis is called the rhizodermis or epiblema. Here, the cells are not covered by a cuticle (roots need to absorb water, not keep it out). Some cells extend outward as root hairs, increasing surface area for absorption.
On the stem and leaves, the epidermis is covered by a cuticle and contains stomata.
A common mistake: thinking the epidermis is dead. It is living in most plants. The cuticle is dead and non-living, but the epidermal cells beneath it are alive.
2. The Cuticle
The cuticle is a waxy layer (cutin — a fatty substance) secreted by the epidermal cells onto their outer surface. It is impermeable to water.
Why does a plant need this? A plant that loses too much water wilts and dies. The cuticle is the plant's raincoat — it prevents uncontrolled evaporation from the surface. The thicker the cuticle, the better the plant can survive in dry conditions (think desert plants like cacti).
The cuticle is absent on root epidermis (roots need to absorb water) and is thin or absent on young stems and underwater leaves of aquatic plants.
3. Stomata
Stomata (singular: stoma) are pores in the epidermis, mostly on leaves. Each stoma is surrounded by two guard cells — specialised epidermal cells that are kidney-shaped (in dicots) or dumbbell-shaped (in monocots).
The guard cells are the only epidermal cells that contain chloroplasts. They control the opening and closing of the pore.
Why stomata matter:
- Gas exchange: Carbon dioxide enters for photosynthesis; oxygen exits.
- Transpiration: Water vapour exits, creating the pull that draws water up from the roots.
The number and distribution of stomata vary. Most plants have more stomata on the lower surface of the leaf than the upper. This reduces water loss because the lower surface is cooler and less exposed to wind and sun.
4. Trichomes
Trichomes are hair-like outgrowths from epidermal cells. They can be:
- Unicellular (single cell) or multicellular (many cells)
- Branched or unbranched
- Living or dead
- Glandular (secreting substances) or non-glandular
Functions of trichomes:
- Reduce water loss by trapping a layer of moist air near the leaf surface
- Reflect excess sunlight (white or silvery hairs)
- Deter herbivores (stinging hairs in nettles, sticky glandular hairs) …
In grass leaves, certain large, empty, colourless epidermal cells called bulliform cells are found along the veins on the upper (adaxial) surface. When these cells lose water and become flaccid under water stress, they cause the leaf to curl inward, which reduces the exposed surface and he …
Bulliform cells, large empty epidermal cells found along the veins of grass leaves, cause the leaf to curl inward when they lose turgor during water stress.
Grass (monocot, isobilateral) leaves possess a specialised group of epidermal cells, on the upper surface along the veins, that are noticeably larger, empty and colourless compared to the surrounding epidermal cells — these are the bulliform cells.
- When water is plentiful, bulliform cells stay turgid (swollen with water), and their turgidity keeps the leaf blade spread flat and fully exposed. …
Method: Reasoning From Hydraulics, Not From the Name Alone
Rather than simply naming "bulliform cells," derive their role from basic turgor-pressure hydraulics — the same principle that explains how a wilting plant droops, applied specifically to why the collapse of certain cells would curl a leaf rather than just make it limp.
State the hydraulic principle: a plant cell's shape and firmness depend on how much water is inside it (turgor). A fully turgid cell is firm and pushes outward on its neighbours; a flaccid (water-deficient) cell collapses inward and loses that outward push.
Apply the principle to a flat structure like a leaf blade: if certain LARGE cells sit specifically along one surface (say, the upper surface, running along the veins), and those large cells lose turgor while the smaller ordinary cells around them are relatively less affected, the leaf blade will not shrink uniformly — it will bend AWAY from the side that lost its supporting turgor pressure, because that side has effectively "deflated" more than the opposite side. A flat structure that loses volume unevenly on one face curls toward the deflated face. …
- CBSE 2026Set ANNUAL1 markMCQQ.The epiblema in young root plays an important role mainly in:(a) Storing food(b) Water absorption through root hair(c) Providing mechanical strength(d) Secondary growth
›Reveal solutionSolution
The epiblema is the piliferous (hair-bearing) layer of the young root; its root hairs are the main structures through which the root absorbs water and dissolved minerals from the soil.
In a transverse section of a dicot root, the outermost protective layer is called the epiblema (not "epidermis", since it lacks a cuticle and stomata, unlike a stem or leaf epidermis). Its distinguishing feature is that many of its cells develop thin-walled, tubular outgrowths called root hairs.
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- CBSE 2022Set ANNUAL1 markMCQQ.Pick out the correct statement from the following:(a) Root hairs are multi-cellular.(b) Trichomes helps in dispersal of fruits and seeds.(c) Dumb-bell shaped guard cells are seen in Dicots.(d) Stomata are seen more in the upper epidermis than the lower epidermis.
›Reveal solutionSolution
Among the four statements, only 'Trichomes helps in dispersal of fruits and seeds' is correct; the other three describe plant anatomy incorrectly.
Let us examine each statement:
- Root hairs are multi-cellular — INCORRECT. Root hairs are simple, tubular, unicellular (single-celled) extensions of the epidermal (piliferous layer) cells of the root, which increase the surface area for water and mineral absorption.
- Trichomes helps in dispersal of fruits and seeds — CORRECT. Trichomes are epidermal hairs found on the aerial parts of the plant. Besides their common role in reducing water loss and providing protection, hair-like trichomes on certain fruits and seeds (such as the silky hairs on cotton seeds or the plumose pappus of composite fruits) aid in their dispersal by wind or other agents. …
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