Understanding Plant Growth and Girth Increase

Plants exhibit different types of growth. Primary growth leads to an increase in length, while secondary growth leads to an increase in girth or diameter of stems and roots. The question asks which specific tissue is responsible for this increase in girth.

Analyzing the Role of Different Plant Tissues in Girth Increase

Let's examine the functions of the tissues mentioned in the options:

• Parenchyma: Parenchyma cells are living cells that make up the ground tissue. They are involved in various functions like storage, photosynthesis, and secretion. While they contribute to the overall bulk of a plant body, they are not primarily responsible for the significant increase in girth seen during secondary growth.
• Lateral Meristem: Lateral meristems are meristematic tissues located along the sides of stems and roots in vascular plants. They are responsible for secondary growth, which results in an increase in the diameter or girth of the plant organ. The two main types of lateral meristems are the vascular cambium and the cork cambium. The vascular cambium adds secondary xylem (wood) and secondary phloem, while the cork cambium produces cork (part of the bark). Both processes contribute to the thickening of the stem or root.
• Apical Meristem: Apical meristems are found at the tips of shoots (shoot apical meristem) and roots (root apical meristem). They are responsible for primary growth, causing the elongation of stems and roots. They do not contribute to the increase in girth.
• Intercalary Meristem: Intercalary meristems are located at the base of leaves and internodes, particularly in grasses. They are responsible for increasing the length of the internode and are involved in primary growth, allowing the plant to regrow parts removed by grazing. Like apical meristems, they do not cause an increase in girth.

Identifying the Tissue for Stem and Root Thickening

Based on the functions described, the tissue directly responsible for the increase in the girth (diameter) of stems and roots, a process known as secondary growth, is the lateral meristem.

Let's summarize the types of meristems and their roles:

Therefore, the girth of the stem or root increases due to the activity of the lateral meristem.

Conclusion on Girth Increase

The process leading to increased girth in stems and roots is secondary growth, driven by the lateral meristem. The vascular cambium produces new xylem and phloem, adding to the wood, while the cork cambium produces protective cork tissue. These activities collectively result in the thickening of the plant organ.

Revision Table: Plant Meristems and Growth

Additional Information: Secondary Growth Details

Secondary growth is typical in dicotyledonous plants and gymnosperms, but generally absent in monocots. The two key lateral meristems involved are:

• Vascular Cambium: Located between the primary xylem and primary phloem. It divides to produce secondary xylem towards the inside and secondary phloem towards the outside. The accumulation of secondary xylem forms the wood.
• Cork Cambium (Phellogen): Develops usually from the cortex. It divides to produce cork (phellem) towards the outside and secondary cortex (phelloderm) towards the inside. The cork cambium, phellem, and phelloderm together form the periderm, which replaces the epidermis as the protective outer layer in plants undergoing secondary growth. This addition of periderm and secondary vascular tissues significantly increases the girth.