Question
The micrograph displays xylem vessels in a longitudinal section of a plant stem.
How do the structures labeled X assist in the movement of water upward through the stem?
A. They make xylem walls more permeable to water.
B. They develop cohesive forces with water molecules.
C. They allow xylem vessels to withstand low pressures.
D. They increase adhesion between neighboring xylem vessels.
▶️Answer/Explanation
Answer: C. They allow xylem vessels to withstand low pressures.
Explanation:
How does xylem structure help move water upward in a plant?
Water moves upward in the xylem through transpiration pull, supported by cohesion (water molecules sticking to each other) and adhesion (water sticking to xylem walls). Xylem vessels have thick lignified walls, which help them resist collapsing under the negative pressure caused by transpiration.
Now, let’s evaluate each option:
Option A. Incorrect – They make xylem walls more permeable to water.
Xylem walls are not meant to be highly permeable. Their main role is conduction, not water absorption. The walls are often lignified, making them less permeable, not more.
Option B. Incorrect – They develop cohesive forces with water molecules.
Cohesion occurs between water molecules, not between the xylem wall structures and water. Xylem structures don’t “develop” cohesion — it’s a property of water itself.
Option C. Correct – They allow xylem vessels to withstand low pressures.
The structures labeled X are likely lignin rings or thickenings in the xylem walls. These provide strength and rigidity, allowing the vessels to withstand the low (negative) pressures generated during transpiration without collapsing.
Option D. Incorrect – They increase adhesion between neighboring xylem vessels.
Adhesion refers to water sticking to the vessel walls, not to neighboring xylem vessels sticking to each other. This isn’t a function of the labeled structures.
Question
The micrograph shows the cross-section of the vascular tissue in a dicotyledonous root. Which letter identifies phloem sieve tubes?
▶️Answer/Explanation
Answer: C
Explanation:
Structure Identification:
A – Xylem
- Large, thick-walled, centrally located
- Main job = Transport of water & minerals from root to shoot
B – Phloem (Sieve Tubes)
- Found between the arms of xylem
- Main job = Transport of organic nutrients (like glucose/sucrose)
C – Phloem Companion Cells or associated phloem tissue
- Supports sieve tube elements
- Helps with loading and unloading sugars
D – Endodermis
- Surrounds vascular bundle
- Has Casparian strip that controls water & nutrient entry into xylem
- Like the root’s bouncer.
Question
The image shows part of a section through the stem of a non-woody plant. Where does transport of sucrose occur?
▶️Answer/Explanation
Answer: D
Explanation:
This is a cross-section of a non-woody dicot stem, and it’s labeled with vascular bundles arranged in a ring. Each vascular bundle contains:
- Xylem (usually on the inner side)
- Phloem (on the outer side)
- Sometimes cambium between them
Let’s reassess the labels:
- Label D points to the outer part of the vascular bundle – this is where phloem is located.
- Label A points to the inner part of the vascular bundle – this is where xylem is located.
- Label B points to the cortex (parenchyma cells).
- Label C points to the pith in the center of the stem.