In plant growth, the increase in thickness or girth of the stem or root (secondary growth) in woody plants is mainly due to the activity of which type of meristematic tissue?

Introduction / Context:
Plants grow not only in length but also in thickness. The increase in length of roots and shoots is called primary growth, while the increase in girth or thickness of stems and roots, especially in woody plants, is called secondary growth. This thickening is important for providing mechanical support and for forming wood and bark. Different types of meristematic tissues are responsible for primary and secondary growth. This question asks which meristem mainly causes the increase in girth of stems and roots.

Given Data / Assumptions:

• Meristems are regions of actively dividing cells in plants.
• Apical meristems are located at tips of roots and shoots.
• Lateral meristems include vascular cambium and cork cambium.
• Intercalary meristems occur at internodes or the base of leaves in some plants.
• The question refers specifically to increase in thickness or girth (secondary growth).

Concept / Approach:
Secondary growth in woody dicots and gymnosperms is primarily due to the activity of lateral meristems. The vascular cambium produces secondary xylem (wood) to the inside and secondary phloem to the outside, increasing the diameter of the stem or root. The cork cambium (phellogen) produces cork cells and part of the bark, contributing to the outer protective layers. Apical meristems, located at the tips of roots and shoots, are responsible for primary growth in length, not thickness. Intercalary meristems help in elongation of internodes, especially in grasses. The term "extra meristem" is not a standard classification in basic botany textbooks.

Step-by-Step Solution:
Step 1: Identify that the question is about increase in girth or thickness, which is secondary growth. Step 2: Recall that secondary growth is associated with lateral meristems such as vascular cambium and cork cambium. Step 3: Recognise that vascular cambium adds new layers of xylem and phloem, increasing stem and root diameter. Step 4: Understand that cork cambium produces protective cork cells, contributing to the outer layers of bark. Step 5: Note that apical meristems at root and shoot tips mainly increase length, not girth. Step 6: Observe that intercalary meristems primarily help in elongation of internodes in some monocots, not in thickening woody stems. Step 7: Conclude that lateral meristems are responsible for the increase in girth of stems and roots.

Verification / Alternative check:
Standard plant anatomy diagrams show a ring of vascular cambium inside the stem of a dicot plant. Over time, this ring produces successive layers of secondary xylem, forming annual rings in trees. Textbooks explain that secondary growth is absent or minimal in most monocots, which lack typical lateral meristems. They also emphasise that apical meristems drive elongation, while lateral meristems are associated with radial or lateral expansion. No credible source describes "extra meristem" as a distinct category. This evidence confirms that lateral meristem is the correct answer.

Why Other Options Are Wrong:
Apical meristem at the tips of roots and shoots is incorrect because it causes primary growth in length, not the thickening associated with secondary growth in woody stems and roots.
Intercalary meristem located at internodes or leaf bases is wrong because it mainly contributes to elongation of segments in plants like grasses, not to overall increase in girth.
Extra meristem, a non-standard term in basic botany, is not recognised as a category responsible for secondary growth in textbooks.

Common Pitfalls:
Students often remember only that meristems cause growth and may not differentiate between apical, lateral, and intercalary types. This can lead to confusion about which meristem does what. Another common mistake is to assume that "apical" means all growth, including thickness. To avoid this, remember "apical = tip = length" and "lateral = side = thickness." Linking these simple associations helps you correctly identify that lateral meristems, especially vascular cambium and cork cambium, are responsible for increase in girth.

Final Answer:
The increase in girth of stems and roots is mainly due to Lateral meristem, such as vascular cambium and cork cambium.