_______ causes the stem or root to enlarge in diameter. 

The correct answer is Cambium.

Key Points

• Cambium is a layer of actively dividing cells located between the xylem and phloem tissues in plants.
• This layer is responsible for secondary growth, which increases the girth of stems and roots.
• The cells of the cambium divide and differentiate to form secondary xylem (wood) towards the inside and secondary phloem towards the outside.
• Cambium activity is seasonal in temperate regions, with growth rings forming as a result of variations in growth rate during different seasons.

Additional Information

• Primary Growth: This occurs at the tips of roots and shoots due to the activity of apical meristems, leading to an increase in length.
• Apical Meristem: These are the regions at the tips of roots and shoots where cells actively divide, contributing to the primary growth of the plant.
• Intercalary Meristem: Found at the base of leaves or internodes, especially in grasses, contributing to lengthening of these plant parts.
• Secondary Growth: This involves the thickening of stems and roots, occurring due to the activity of the vascular cambium and cork cambium.