The epidermal tissue system
Run a finger over a leaf: its outer skin is smooth, sometimes waxy or hairy. That skin protects the leaf, controls water loss and lets air in and out through tiny pores.
Plants have three tissue systems: epidermal, ground and vascular. The epidermal tissue system is the outermost covering of the plant body. It is made of epidermal cells, stomata and epidermal appendages (root hairs and trichomes).
In the figure : Parts of the epidermal tissue system.
Rule 1: Epidermal cells and cuticle — Epidermis is usually a single layer of elongated, compactly arranged parenchymatous cells. The outer surface of the aerial parts is covered by a waxy cuticle that prevents water loss. Roots have no cuticle.
- Easy : A waxy sheen on a leaf is the cuticle.
- Medium : A potted plant kept in the sun loses less water because of its cuticle.
- Exam-type : Q: Is a cuticle found on roots? A: No, it is absent in roots.
Rule 2: Stomata and hairs — Stomata regulate transpiration and gas exchange. Each has two guard cells that enclose a pore: bean-shaped in dicots and dumb-bell shaped in grasses. The inner walls of guard cells are thick and the outer walls thin; they have chloroplasts. Sometimes neighbouring cells are subsidiary cells. The pore, guard cells and subsidiary cells together form the stomatal apparatus. Root hairs are unicellular elongations of the epidermis that absorb water and minerals; trichomes on stems are multicellular and cut water loss.
- Easy : Bean-shaped guard cells are seen in a bean leaf.
- Medium : A grass leaf has dumb-bell shaped guard cells.
- Exam-type : Q: What forms the stomatal apparatus? A: The stomatal pore, the guard cells and the surrounding subsidiary cells.
Root hairs are unicellular and absorb water. Trichomes of the shoot are usually multicellular and reduce water loss.
Thinking root hairs are multicellular. They are single-celled outgrowths of the epidermal cell.
Memory clue: ESH — Epidermis has Stomata and Hairs.
Spot the difference
| Point | Root hair | Trichome |
|---|---|---|
| Cells | Unicellular | Usually multicellular |
| Position | Root epidermis | Shoot epidermis |
| Function | Absorb water and minerals | Reduce water loss; may be secretory |
Practice questions
- 1Name the three tissue systems of a plant.
- 2What is the function of stomata?
- 3What is the cuticle made of and what does it do?
- 4What is the shape of guard cells in grasses?
- 5Which cells make up the stomatal apparatus?
- 6What are trichomes?
- 7Why do the guard cells have thick inner walls and thin outer walls?
- 8Why is a cuticle absent in roots?
Show answers (try them yourself first)
- 1Epidermal, ground and vascular tissue systems.
- 2They regulate transpiration and gas exchange.
- 3It is a waxy layer that reduces water loss from the aerial parts.
- 4Dumb-bell shaped.
- 5The pore, the guard cells and the subsidiary cells.
- 6Hairs that grow on the epidermis of the shoot; they are usually multicellular.
- 7The uneven thickness lets them bend and open or close the pore as they gain or lose water.
- 8Roots have to absorb water from the soil, so a waterproof layer would block their function.
The ground tissue system
The fleshy part of a potato, the pith in a sugarcane stem and the green layer of a leaf are the 'filling' of the plant between the skin and the pipes. This filling is the ground tissue.
All tissues other than the epidermis and the vascular bundles form the ground tissue system. It consists of simple tissues: parenchyma, collenchyma and sclerenchyma. In leaves it is the chlorophyll-containing mesophyll.
In the figure : Ground tissue of a stem or root is divided into cortex, pericycle and pith.
Rule 1: Zones of ground tissue — In stems and roots, it has the cortex, pericycle and pith. Parenchyma is usually found in the cortex, pericycle, pith and medullary rays of primary stems and roots. In leaves, the thin-walled cells with chloroplasts are called mesophyll.
- Easy : The soft pith of a young stem is parenchyma.
- Medium : The green tissue of a leaf between the two epidermal layers is mesophyll.
- Exam-type : Q: What is the ground tissue of leaves called? A: Mesophyll.
Rule 2: Simple tissues — Parenchyma: thin-walled, living cells for food storage and photosynthesis. Collenchyma: living cells with extra thickening at corners, giving flexible support (hypodermis of dicot stems). Sclerenchyma: dead cells with thick lignified walls, giving rigidity (pericycle patches, bundle sheaths).
- Easy : The strings of a celery stalk are collenchyma and sclerenchyma.
- Medium : Pericycle of a dicot stem has semi-lunar patches of sclerenchyma above the phloem.
- Exam-type : Q: Which ground tissue gives flexible support to the young stem? A: Collenchyma.
Ground tissue = everything except epidermis and vascular bundles.
Calling the epidermis part of the ground tissue. It is a separate system.
Memory clue: P-C-S — Parenchyma stores, Collenchyma bends, Sclerenchyma is stiff.
Spot the difference
| Tissue | Cell wall | Function |
|---|---|---|
| Parenchyma | Thin | Storage, photosynthesis |
| Collenchyma | Thick at corners | Flexible support |
| Sclerenchyma | Thick, lignified; dead | Strength |
Practice questions
- 1What does ground tissue consist of?
- 2Name the three zones of ground tissue in stems and roots.
- 3What is mesophyll?
- 4Which of the three simple tissues is dead at maturity?
- 5Where is collenchyma found in a dicot stem?
- 6What are medullary rays?
- 7Why does a young stem need collenchyma rather than sclerenchyma?
- 8Which tissues are not part of the ground tissue system?
Show answers (try them yourself first)
- 1Parenchyma, collenchyma and sclerenchyma.
- 2Cortex, pericycle and pith.
- 3The ground tissue of the leaf, with chloroplasts, that does photosynthesis.
- 4Sclerenchyma.
- 5In the hypodermis, just below the epidermis.
- 6Radially placed parenchyma cells between the vascular bundles.
- 7It must support the stem while still growing, and collenchyma gives support and flexibility.
- 8The epidermis and the vascular bundles.
The vascular tissue system and types of vascular bundles
Pipes in a building carry water in one direction and wires carry power in another. In a plant, xylem carries water up and phloem carries food around. They sit side by side in bundles.
The vascular tissue system has complex tissues: xylem (water and minerals) and phloem (food). Together they form vascular bundles. Based on cambium, bundles are open or closed; based on arrangement, they are radial or conjoint.
In the figure : Types of vascular bundles.
Rule 1: Open and closed bundles — In dicot stems, cambium lies between the xylem and the phloem. Such bundles can form secondary xylem and phloem and are called open. In monocots there is no cambium, so the bundles are closed and do not form secondary tissues.
- Easy : A sunflower stem has open bundles.
- Medium : A maize stem has closed bundles.
- Exam-type : Q: Why can a dicot stem grow thicker? A: Its bundles are open and have cambium.
Rule 2: Radial and conjoint bundles — In a radial bundle xylem and phloem lie on different radii in alternate patches, as in roots. In a conjoint bundle they lie on the same radius, as in stems and leaves, usually with the phloem on the outer side of the xylem.
- Easy : Roots have a radial arrangement.
- Medium : Stems and leaves have conjoint bundles.
- Exam-type : Q: In which organ are bundles radial? A: In roots.
Roots: radial. Stems and leaves: conjoint. Dicots: open. Monocots: closed.
Mixing open with radial. Open or closed is about cambium, while radial or conjoint is about position.
Memory clue: R-R, C-S/L — Radial = Root; Conjoint = Stem and Leaf.
Spot the difference
| Point | Open bundle | Closed bundle |
|---|---|---|
| Cambium | Present | Absent |
| Secondary growth | Yes | No |
| Found in | Dicot stem | Monocot stem |
Practice questions
- 1Name the two tissues of the vascular system.
- 2What does xylem carry?
- 3What is an open vascular bundle?
- 4In which organ is a radial arrangement found?
- 5What is a conjoint bundle?
- 6Why do monocots not show secondary growth?
- 7Where is the phloem placed in a conjoint bundle?
- 8Give one difference between radial and conjoint bundles.
Show answers (try them yourself first)
- 1Xylem and phloem.
- 2Water and minerals.
- 3A bundle with cambium between the xylem and phloem.
- 4In roots.
- 5A bundle in which xylem and phloem lie on the same radius.
- 6Their bundles are closed, with no cambium.
- 7Usually on the outer side of the xylem.
- 8In radial bundles xylem and phloem lie on different radii; in conjoint bundles they lie together on the same radius.
Anatomy of dicot and monocot roots
If you cut a thin slice across a radish root and look at it with a lens, you see rings of different tissues. The same slice of a maize root shows a larger centre and many more xylem patches.
A transverse section (T.S.) of a root shows, from outside to inside: the epiblema, cortex, endodermis, pericycle, vascular bundles and pith. All the tissues inside the endodermis together form the stele.
In the figure : From outside to inside in a T.S. of a root.
Rule 1: Dicot root (sunflower) — Epiblema bears unicellular root hairs. The cortex has thin-walled parenchyma with intercellular spaces. The endodermis has barrel-shaped cells with Casparian strips of suberin, a waxy water-proof material, on its walls. Pericycle gives rise to lateral roots and the vascular cambium. Xylem has two to four patches (diarch to tetrarch), the pith is small or absent, and conjunctive tissue lies between xylem and phloem.
- Easy : A sunflower root has few xylem patches.
- Medium : Lateral roots arise from the pericycle.
- Exam-type : Q: What is the role of the Casparian strip? A: It blocks water movement through the cell walls, so water has to pass through the cells.
Rule 2: Monocot root — It has the same layers but more than six xylem bundles (polyarch), a large well-developed pith, and no secondary growth.
- Easy : A maize root has many xylem bundles.
- Medium : A monocot root cannot grow thicker with time.
- Exam-type : Q: Is the pith large in a monocot root? A: Yes, it is large and well developed.
Dicot root: 2–4 xylem patches, small pith. Monocot root: more than 6, large pith.
Writing that the Casparian strip is made of cellulose. It is made of suberin.
Memory clue: E-C-E-P-V-P — Epiblema, Cortex, Endodermis, Pericycle, Vascular, Pith.
Spot the difference
| Point | Dicot root | Monocot root |
|---|---|---|
| Xylem bundles | 2 to 4 | More than 6 (polyarch) |
| Pith | Small or absent | Large |
| Secondary growth | Present | Absent |
Practice questions
- 1What is the outermost layer of a root called?
- 2What is the stele?
- 3What forms the Casparian strips?
- 4Which layer gives rise to lateral roots?
- 5How many xylem bundles does a monocot root usually have?
- 6What does the endodermis look like?
- 7Write two differences between dicot and monocot roots.
- 8Why does a monocot root not thicken with time?
Show answers (try them yourself first)
- 1Epiblema.
- 2All the tissues inside the endodermis: pericycle, vascular bundles and pith.
- 3Suberin, a waxy water-proof substance.
- 4The pericycle.
- 5More than six.
- 6A single layer of barrel-shaped cells without intercellular spaces.
- 7A dicot root has fewer xylem bundles and a small pith; a monocot root has more than six and a large pith.
- 8It does not undergo secondary growth because it has no cambium.
Anatomy of dicot and monocot stems
A slice across a sunflower stem shows neat dots in a ring, while a slice of maize stem shows dots scattered all over like seeds on a biscuit. This pattern tells the two kinds of stems apart.
A dicot stem has bundles arranged in a ring; a monocot stem has scattered bundles. The layers of a dicot stem are epidermis, cortex (hypodermis, parenchyma, endodermis), pericycle, medullary rays, bundles and pith.
In the figure : How vascular bundles are arranged in dicot and monocot stems.
Rule 1: Dicot stem — The epidermis has a thin cuticle, and may have trichomes and a few stomata. The hypodermis is collenchymatous, below it is rounded parenchyma, and the innermost cortex layer is the endodermis (starch sheath). Pericycle is in semi-lunar sclerenchyma patches above the phloem. A ring of bundles, each conjoint, open and with endarch protoxylem, surrounds a large parenchymatous pith.
- Easy : The ring of bundles is the main clue for a dicot stem.
- Medium : Starch grains in the endodermis give it the name starch sheath.
- Exam-type : Q: What is the type of vascular bundle in a dicot stem? A: Conjoint and open.
Rule 2: Monocot stem — The hypodermis is sclerenchymatous, many vascular bundles are scattered in a large parenchymatous ground tissue, and each bundle has a sclerenchymatous sheath. The bundles are conjoint and closed; peripheral bundles are smaller than central ones. Phloem parenchyma is absent and water-containing cavities are present in the bundles.
- Easy : Maize stem shows scattered bundles.
- Medium : Central bundles are larger than peripheral ones.
- Exam-type : Q: Name two features of a monocot stem. A: Scattered closed bundles with sclerenchymatous sheath, and no phloem parenchyma.
Dicot stem: ring, open, collenchymatous hypodermis. Monocot stem: scattered, closed, sclerenchymatous hypodermis.
Believing monocot bundles form a ring. They are scattered.
Memory clue: D-ring / M-mess — Dicot bundles in a Ring; Monocot bundles in a Mess (scattered).
Spot the difference
| Point | Dicot stem | Monocot stem |
|---|---|---|
| Bundles | In a ring; open | Scattered; closed |
| Hypodermis | Collenchymatous | Sclerenchymatous |
| Bundle sheath | Absent | Present |
| Pith | Large, parenchymatous | Ground tissue throughout |
Practice questions
- 1How are the vascular bundles arranged in a dicot stem?
- 2What is the hypodermis of a monocot stem made of?
- 3What are the medullary rays?
- 4What is the starch sheath?
- 5What type of vascular bundles does a monocot stem have?
- 6Which features are absent in monocot vascular bundles?
- 7How can you identify a stem from its T.S. as monocot?
- 8Why can a dicot stem undergo secondary growth, but a monocot stem cannot?
Show answers (try them yourself first)
- 1In a ring.
- 2Sclerenchyma.
- 3Radial rows of parenchyma between bundles in dicot stems.
- 4The endodermis of a dicot stem, rich in starch grains.
- 5Conjoint and closed.
- 6Cambium and phloem parenchyma.
- 7By the scattered bundles, each with a sclerenchymatous sheath, and by the absence of a ring and pith.
- 8Dicot bundles are open with cambium; monocot bundles are closed with no cambium.
Anatomy of leaves: dorsiventral and isobilateral
The upper surface of a mango leaf is dark and shiny, and the lower surface is paler. A blade of grass looks the same on both sides. These differences come from the way their inner tissues are arranged.
A dorsiventral leaf (dicot) has different upper (adaxial) and lower (abaxial) surfaces. An isobilateral leaf (monocot) looks similar on both sides. A leaf has epidermis, mesophyll and vascular tissue.
In the figure : Two kinds of leaves in transverse section.
Rule 1: Dorsiventral leaf — Both epidermal layers have a cuticle, and the abaxial epidermis generally has more stomata. The mesophyll has the adaxial palisade parenchyma (vertically placed, elongated cells) and the spongy parenchyma (round, loosely arranged cells with air spaces). Veins have bundles surrounded by thick-walled bundle sheath cells.
- Easy : Palisade cells are packed under the upper epidermis to catch light.
- Medium : Air spaces in the spongy tissue help in gas exchange.
- Exam-type : Q: Which surface of a dicot leaf has more stomata? A: The lower (abaxial) surface.
Rule 2: Isobilateral leaf — Stomata are present on both surfaces and the mesophyll is not divided into palisade and spongy parenchyma. In grasses, some adaxial epidermal cells along the veins form large colourless bulliform cells. When turgid they expose the leaf surface; when flaccid in water stress they make the leaf curl inwards to reduce water loss. Vascular bundles are of nearly equal size, in parallel venation.
- Easy : A grass leaf rolls up in a dry spell because of the bulliform cells.
- Medium : Both surfaces of a maize leaf have stomata.
- Exam-type : Q: What is the role of bulliform cells? A: They help the leaf roll inward to cut water loss.
Dicot leaf = palisade + spongy, more stomata below. Monocot leaf = undivided mesophyll, stomata on both sides.
Saying palisade cells are in the lower part of the mesophyll. They lie just below the upper epidermis.
Memory clue: P-up, S-down — Palisade is up, Spongy is down.
Spot the difference
| Point | Dorsiventral | Isobilateral |
|---|---|---|
| Plant | Dicot | Monocot |
| Mesophyll | Palisade + spongy | Not differentiated |
| Stomata | More on the lower surface | On both surfaces |
| Bulliform cells | Absent | Present in grasses |
Practice questions
- 1What is the adaxial surface of a leaf?
- 2Name the two types of cells in the mesophyll of a dicot leaf.
- 3What are bulliform cells?
- 4Which kind of leaf has stomata on both surfaces?
- 5Where is palisade parenchyma located?
- 6What surrounds the vascular bundles of a leaf?
- 7Why does a grass leaf curl in dry weather?
- 8State three differences between a dicot and a monocot leaf.
Show answers (try them yourself first)
- 1The upper surface.
- 2Palisade parenchyma and spongy parenchyma.
- 3Large, empty, colourless cells on the upper epidermis of grass leaves.
- 4The isobilateral (monocot) leaf.
- 5Below the adaxial epidermis.
- 6A layer of thick-walled bundle sheath cells.
- 7The bulliform cells lose water and become flaccid, so the leaf rolls inward and loses less water.
- 8A dicot leaf has palisade and spongy tissues, more stomata below and no bulliform cells; a monocot leaf has undifferentiated mesophyll, stomata on both sides and bulliform cells.