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. …
- AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQQ.Assertion (A) : Trichomes help in preventing water loss due to transpiration. Reason (R) : The specialized cells near the guard cells together constituted stomatal apparatus. Identify the correct option from the following (A) (A) and (R) are true. (R) is correct explanation for (A) (B) (A) and (R) are true. But (R) is not correct explanation for (A) (C) (A) is true, but (R) is false (D) (A) is false, but (R) is true
›Reveal solutionSolution
Both statements about trichomes and the stomatal apparatus are individually true, but they describe unrelated structures, so R does not explain A — answer (B).
Concept and Intuition
- Assertion: trichomes (hair-like epidermal outgrowths) on stems and leaves can reduce water loss by trapping a boundary layer of humid, still air near the surface, reducing the rate of transpiration. This is a genuine, well-documented function of trichomes.
- Reason: the stomatal apparatus is indeed defined as the guard cells together with their surrounding specialized subsidiary cells. This is a correct anatomical fact, but it describes the stomatal complex, not the trichomes referenced in the assertion.
- Since the reason is about a completely different epidermal structure (stomatal apparatus) rather than about trichomes, it cannot be the causal explanation for the assertion.
Step-by-Step Solution
- Confirm assertion (A): trichomes reducing transpirational water loss — true.
- Confirm reason (R): guard cells + subsidiary cells = stomatal apparatus — true as an anatomical definition. …
- AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQQ.Choose the correct statements among the following I. Epidermal cells have more cytoplasm and small vacuole. II. Outerwalls of guard cells are thick and inner walls are thin. III. Presence of phloem on either side of xylem is seen in cucurbita vascular bundles. IV. Trichomes of shoot system are multicellular. (A) I and II (B) II and III (C) III and IV (D) I and IV
›Reveal solutionSolution
This question tests recall of four unrelated but classic NCERT anatomy facts; only statements III (bicollateral bundles of Cucurbita) and IV (multicellular shoot trichomes) are correct.
Concept and Intuition
Each statement targets a specific structural detail from plant anatomy:
- Epidermal cells are protective, non-photosynthetic (mostly), thin-walled cells — like most non-meristematic parenchymatous cells they are vacuolated, with a large central vacuole and only a thin peripheral layer of cytoplasm, not the reverse.
- Guard cells regulate the stomatal pore by differential wall thickening: the wall facing the pore (inner/ventral wall) is thick and elastic, while the wall away from the pore (outer/dorsal wall) is thin. This asymmetry is exactly what allows the guard cells to bow outward and open the pore when turgid.
- Cucurbita (bottle gourd family) stems are a textbook example of bicollateral vascular bundles — conjoint, collateral, and open, with phloem present both external AND internal to the xylem (an extra strip of internal phloem, plus a strip of internal conjunctive tissue).
- Trichomes are epidermal appendages; those on the shoot (stem, leaf) are generally multicellular (and may be branched), while root hairs (a different kind of epidermal outgrowth) are unicellular.
Step-by-Step Solution
- Statement I: epidermal cells → large vacuole, little cytoplasm → statement (which says "more cytoplasm and small vacuole") is false. …
- AP EAPCET 2023Set ap-2023-05-22-FN1 markMCQQ.Assertion (A) : Epidermis of hydrophytes is useful for absorption and assimilation Reason (R) : In hydrophytes epidermal cells are devoid of cuticle and contains chloroplasts. (A) Both A and R are correct and R is the correct explanation of A. (B) Both A and R are correct but R is not the correct explanation of A. (C) A is correct but R is incorrect. (D) A is incorrect but R is correct.
›Reveal solutionSolution
Hydrophyte epidermal cells lack a protective cuticle and contain chloroplasts, which is exactly why the epidermis can both absorb dissolved substances directly and carry out photosynthesis (assimilation).
Concept and Intuition
Terrestrial plants have a waxy cuticle on their epidermis mainly to prevent water loss, but hydrophytes are already surrounded by water and don't need this protection, so their epidermis lacks (or has minimal) cuticle. This thin, cuticle-free epidermis allows direct exchange of dissolved gases and nutrients with the surrounding water (absorption). Additionally, in many aquatic plants, the epidermal cells themselves contain chloroplasts (something not typical of terrestrial leaf epidermis), enabling the epidermis to directly participate in photosynthesis (assimilation) — a specialised aquatic adaptation.
Step-by-Step Solution
- Assertion: hydrophyte epidermis is useful for absorption and assimilation — true, since aquatic plants rely on the epidermis for nutrient/gas uptake and, in some tissues, photosynthesis. …
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