CDS Biology · Plant Biology
Tissues and Water Transport
Where a plant grows (the meristems), the simple and complex permanent tissues, what xylem and phloem carry, and how water rises, escapes and is saved in dry and salty places.
Why this matters
Thirteen CDS questions from 2018 (II) to 2024 (II), four of them in the 2024 (II) paper. Five are about xylem and phloem, and two of those ask exactly what xylem carries (water and the minerals dissolved in it). The intercalary meristem has been asked twice.
Concept 1 of 4: Meristems: where a plant grows
A plant does not grow all over its body. It grows only where it keeps a patch of young, dividing cells, called a meristem. Patches at the tips make the root and shoot longer. A patch at the base of a grass leaf lets the leaf grow back after a cow bites off the top. Rings of dividing cells along the sides make a tree trunk thicker each year.
Definition
The three meristems (NCERT Class 9, Tissues):
- Apical meristem: at the tips of the stem and root; increases length.
- Intercalary meristem: at the base of the leaves or internodes (on either side of the node); found in grasses, which is why they grow back after grazing or mowing.
- Lateral meristem (cambium): along the sides of stem and root; increases girth. The vascular cambium and the cork cambium are lateral meristems, and the growth in girth they make is secondary growth.
- Meristem cells are small, thin-walled, packed with dense cytoplasm, and keep dividing. Once they stop and take up a job, they form permanent tissue.
| Meristem | Where | What it adds |
|---|---|---|
| Apical | Tips of root and shoot | Length |
| Intercalary | Base of the leaves or internodes | Length of leaf and internode; regrowth of grassQ Asked twice: CDS 2021 (II) and 2023 (I). The answer is the base of the leaves (or internodes), never the tip. |
| Lateral (vascular and cork cambium) | Sides of stem and root | Girth (secondary growth) |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2019 · CDS (I) 2019 — General Knowledge · Q65Moderate
Example 1 · Plant Biology · Tissues and Water Transport
Which one of the following tissues is responsible for increase of girth in the stem of a plant?
Intercalary is at the base, not the tip
The intercalary meristem sits at the base of the leaves or internodes. The tips belong to the apical meristem and the sides to the lateral meristem.
Girth comes from the lateral meristem only
The apical and intercalary meristems add length. Only the lateral meristem (vascular and cork cambium) adds girth. Pericycle and tracheids are not the answer.
Concept 2 of 4: Simple permanent tissues
A simple tissue is made of one kind of cell. Parenchyma is the soft filling of a plant: living cells with thin walls that store food. Collenchyma is a little tougher, so a leaf stalk can bend without snapping. Sclerenchyma is the hard part: its cells build thick woody walls and then die, leaving a strong, stiff frame.
Definition
The three simple permanent tissues (NCERT Class 9):
- Parenchyma: living cells with thin cellulose walls and spaces between them; stores food. With chlorophyll it is chlorenchyma; with large air spaces in water plants it is aerenchyma, which helps them float.
- Collenchyma: living cells, walls thickened at the corners; gives flexibility, as in leaf stalks and young stems.
- Sclerenchyma: dead cells with thick walls hardened by lignin; gives strength and hardness, as in the husk of coconut and fibres such as jute.
- Suberin is the waxy substance in the walls of cork cells, not of sclerenchyma.
| Tissue | Living or dead | Wall | Job |
|---|---|---|---|
| Parenchyma | Living | Thin, cellulose | Storage; photosynthesis as chlorenchyma |
| Collenchyma | Living | Thickened at the corners | Flexibility |
| Sclerenchyma | Dead | Thick, lignified | StrengthQ CDS 2020 (II): mature sclerenchyma cells have lignified walls and are dead. |
| Cork | Dead | Suberin | Protection; keeps water and gases out |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (II) 2020 — General Knowledge · Q108Moderate
Example 2 · Plant Biology · Tissues and Water Transport
Mature sclerenchyma cells have
Lignin with sclerenchyma, suberin with cork
Sclerenchyma walls are hardened with lignin. Suberin is the waxy substance of cork cells. Both kinds of cell are dead, so match the wall, not just 'dead'.
Two simple tissues are living
Parenchyma and collenchyma are living; only sclerenchyma is dead at maturity.
Concept 3 of 4: Xylem and phloem
Xylem and phloem are the plant's two sets of pipes, and each is a complex tissue made of several kinds of cell. Xylem carries water and the minerals dissolved in it up from the roots; most of its cells are dead, empty tubes. Phloem carries the food made in the leaves to every part of the plant, and most of its cells are alive.
Definition
The complex tissues (NCERT Class 9):
- Xylem carries water and dissolved minerals upward from the root. Its elements: tracheids, vessels, xylem fibres (all dead) and xylem parenchyma (the only living part, which stores food).
- Phloem carries the food made by photosynthesis (mainly sugar) both up and down. Its elements: sieve tubes, companion cells, phloem parenchyma (living) and phloem fibres (the only dead part).
- A vascular bundle is made of xylem, phloem and, in the open bundles of dicot stems, the cambium between them.
- The endodermis is the innermost layer of the cortex, outside the bundles. It is not part of a vascular bundle.
| Element | Tissue | Living or dead |
|---|---|---|
| Tracheids and vessels | Xylem | Dead |
| Xylem parenchyma | Xylem | LivingQ CDS 2023 (I): of tracheid, vessel, xylem parenchyma and xylem fibre, only xylem parenchyma is living. |
| Xylem fibres | Xylem | Dead |
| Sieve tubes and companion cells | Phloem | Living |
| Phloem parenchyma | Phloem | Living |
| Phloem fibres | Phloem | Dead |
Two more questions ask what xylem carries: CDS 2018 (II) (a blocked xylem stops water AND solutes) and CDS 2024 (II) (water and dissolved minerals). CDS 2024 (II) also asks which is not part of a vascular bundle: the endodermis.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (II) 2020 — General Knowledge · Q107Easy
Example 3 · Plant Biology · Tissues and Water Transport
Which one of the following statements about phloem is correct?
Xylem carries water AND minerals
Xylem does not carry water alone: the minerals absorbed by the roots are dissolved in that water and rise with it. So blocking xylem stops both.
Phloem is complex, and it carries food
Phloem is a complex tissue that carries the products of photosynthesis. Water, minerals and mechanical support belong to the xylem.
Endodermis sits outside the bundle
A vascular bundle is xylem, phloem and cambium. The endodermis is a layer of the cortex that rings the vascular tissue from outside.
Concept 4 of 4: How water rises and escapes
As water evaporates from the leaves, it pulls on the water behind it in the xylem, like sucking on a straw. The water does not break, because its molecules cling to each other, and it does not slip back, because it also clings to the walls of the narrow tubes. Plants of dry places hold on to water by losing as little as they can, and plants of salty soil have their own ways to cope with salt.
Definition
Water in the plant (NCERT Class 11, Transport in Plants):
- Transpiration: loss of water as vapour, mostly through the stomata.
- Ascent of sap in tall trees is by transpiration pull, with the water column held by cohesion (water to water) and adhesion (water to the xylem walls).
- Root pressure pushes water up a little from below; it is not enough for a tall tree.
- Guttation: loss of liquid water as drops, through hydathodes at the leaf edges, seen on grass on cool, humid mornings. It is caused by root pressure.
- Xerophytes (dry places, such as cactus and Opuntia): small or spiny leaves, a thick waxy cuticle, few stomata sunk in pits, deep roots.
- Halophytes grow in saline (salty) soil, such as the mangroves, many of which have breathing roots (pneumatophores).
- Hydrophytes grow in water, such as the water lily and Hydrilla.
| Term | Meaning |
|---|---|
| Transpiration | Loss of water as vapour through stomata |
| Guttation | Loss of water as liquid drops through hydathodesQ CDS 2018 (II): water escaping as droplets is guttation, not transpiration. |
| Cohesion | Water molecules holding on to each other |
| Adhesion | Water molecules holding on to the xylem walls |
| Xerophyte | Plant of dry places: small leaves, waxy cuticle, few sunken stomataQ CDS 2024 (II): a large number of stomata is NOT a feature of xerophytes. |
| Halophyte | Plant of saline soil, such as mangrovesQ CDS 2024 (II): halophytes grow on saline soil. |
| Hydrophyte | Plant of water, such as the water lily |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (II) 2023 — General Knowledge · Q46Moderate
Example 4 · Plant Biology · Tissues and Water Transport
Water goes up through xylem vessels in tall trees. The process in plants and properties of water that help in upward movement are :
Respiration does not lift water
Water rises in a tall tree by transpiration pull held by cohesion and adhesion. Root pressure helps only a little and respiration not at all, so an option without transpiration pull is wrong.
Drops mean guttation
Transpiration loses water as vapour; guttation loses it as liquid drops. Secretion and excretion are not the names for either.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (4)
Meristems: where a plant grows3 rows
| Meristem | Where | What it adds |
|---|---|---|
| Apical | Tips of root and shoot | Length |
| Intercalary | Base of the leaves or internodes | Length of leaf and internode; regrowth of grassQ Asked twice: CDS 2021 (II) and 2023 (I). The answer is the base of the leaves (or internodes), never the tip. |
| Lateral (vascular and cork cambium) | Sides of stem and root | Girth (secondary growth) |
Simple permanent tissues4 rows
| Tissue | Living or dead | Wall | Job |
|---|---|---|---|
| Parenchyma | Living | Thin, cellulose | Storage; photosynthesis as chlorenchyma |
| Collenchyma | Living | Thickened at the corners | Flexibility |
| Sclerenchyma | Dead | Thick, lignified | StrengthQ CDS 2020 (II): mature sclerenchyma cells have lignified walls and are dead. |
| Cork | Dead | Suberin | Protection; keeps water and gases out |
Xylem and phloem6 rows
| Element | Tissue | Living or dead |
|---|---|---|
| Tracheids and vessels | Xylem | Dead |
| Xylem parenchyma | Xylem | LivingQ CDS 2023 (I): of tracheid, vessel, xylem parenchyma and xylem fibre, only xylem parenchyma is living. |
| Xylem fibres | Xylem | Dead |
| Sieve tubes and companion cells | Phloem | Living |
| Phloem parenchyma | Phloem | Living |
| Phloem fibres | Phloem | Dead |
Two more questions ask what xylem carries: CDS 2018 (II) (a blocked xylem stops water AND solutes) and CDS 2024 (II) (water and dissolved minerals). CDS 2024 (II) also asks which is not part of a vascular bundle: the endodermis.
How water rises and escapes7 rows
| Term | Meaning |
|---|---|
| Transpiration | Loss of water as vapour through stomata |
| Guttation | Loss of water as liquid drops through hydathodesQ CDS 2018 (II): water escaping as droplets is guttation, not transpiration. |
| Cohesion | Water molecules holding on to each other |
| Adhesion | Water molecules holding on to the xylem walls |
| Xerophyte | Plant of dry places: small leaves, waxy cuticle, few sunken stomataQ CDS 2024 (II): a large number of stomata is NOT a feature of xerophytes. |
| Halophyte | Plant of saline soil, such as mangrovesQ CDS 2024 (II): halophytes grow on saline soil. |
| Hydrophyte | Plant of water, such as the water lily |
Watch out for (9)
- Intercalary is at the base, not the tip→ Meristems: where a plant grows
- Girth comes from the lateral meristem only→ Meristems: where a plant grows
- Lignin with sclerenchyma, suberin with cork→ Simple permanent tissues
- Two simple tissues are living→ Simple permanent tissues
- Xylem carries water AND minerals→ Xylem and phloem
- Phloem is complex, and it carries food→ Xylem and phloem
- Endodermis sits outside the bundle→ Xylem and phloem
- Respiration does not lift water→ How water rises and escapes
- Drops mean guttation→ How water rises and escapes
Test yourself on Plant Biology
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.