In plants, which one of the following meristematic tissues is responsible for the increase in girth (thickness) of the stem and root during secondary growth?

Introduction / Context:
Plant stems and roots can increase in thickness through a process called secondary growth. This growth involves the activity of specific meristematic tissues that add new layers of vascular tissue and protective tissue. Understanding which meristem is responsible for increase in girth is a key concept in plant anatomy and is frequently tested. This question asks you to identify the tissue responsible for this increase in thickness.

Given Data / Assumptions:
• The question focuses on increase of girth, that is, secondary growth in stem or root.• The options list tracheid, pericycle, intercalary meristem, lateral meristem, and apical meristem.• We assume typical dicot plants where secondary growth is prominent.• We focus on meristematic tissues rather than mature conducting cells.

Concept / Approach:
Lateral meristems are cylinders of actively dividing cells arranged parallel to the long axis of stem or root. The two main lateral meristems are vascular cambium and cork cambium (phellogen). Vascular cambium produces secondary xylem and secondary phloem, increasing the diameter of stems and roots. Cork cambium forms periderm, replacing the epidermis in older stems. Intercalary meristems, by contrast, are responsible for lengthening at nodes in grasses, while apical meristems produce primary growth in length. Tracheids and pericycle are not meristems responsible for girth increase. Therefore lateral meristem is the correct answer.

Step-by-Step Solution:
Step 1: Recognise that the increase of girth refers to secondary growth, not primary elongation.Step 2: Recall that secondary growth in dicots is due to activity of lateral meristems, especially vascular cambium.Step 3: Vascular cambium adds layers of secondary xylem toward the inside and secondary phloem toward the outside, thickening the stem.Step 4: Cork cambium, another lateral meristem, produces cork cells outward, contributing to bark formation.Step 5: Understand that apical meristems at root and shoot tips cause increase in length, not girth.Step 6: Conclude that lateral meristem is the tissue responsible for increase in girth and select that option.

Verification / Alternative check:
Cross sections of old dicot stems show annual rings of secondary xylem produced by the vascular cambium. The thickness of these stems is clearly due to repeated seasonal activity of this lateral meristem. When lateral meristems are absent or not active, as in many monocots, there is little secondary thickening. Textbooks consistently describe lateral meristem as the origin of secondary growth, whereas apical and intercalary meristems are associated with elongation. This evidence confirms that lateral meristem is the correct choice.

Why Other Options Are Wrong:
Tracheid is an individual conducting cell of xylem that helps in water transport, but it is not a meristem and does not generate new tissue. Pericycle is a cell layer that can give rise to lateral roots and, in some cases, contributes to secondary growth indirectly, but it is not the main meristem responsible for girth increase in stems. Intercalary meristem, found at nodes and bases of leaves in grasses, primarily contributes to elongation after grazing or cutting. Apical meristem is located at tips of roots and shoots and causes primary growth in length. None of these alone explains the thickening of stems and roots like the lateral meristems do.

Common Pitfalls:
Students often focus on familiar terms like apical meristem and forget that secondary growth is a distinct process from primary growth. Another pitfall is confusion about pericycle, which does have meristematic potential but is not the principal secondary growth meristem in dicot stems. To avoid confusion, remember that lateral refers to side and that lateral meristems form cylinders that add layers to the sides of stems and roots, increasing girth, while apical meristems are at tips and intercalary meristems are in between nodes.

Final Answer:
The tissue responsible for increase of girth in stems and roots during secondary growth is the Lateral meristem, such as vascular cambium and cork cambium.