Question

Increase in girth (diameter) of plant as a result of the activities of lateral meristems is called (a) primary growth (b) secondary growth (c) open form of growth (d) diffuse growth.

Short answer

The increase in girth (diameter) of a plant as a result of the activities of lateral meristems is called secondary growth.

Step-by-step solution

Understanding the Concepts of Growth in Plants

In plants, primary growth is the result of the activities of the apical meristem resulting in the increase in length, while secondary growth is due to the activity of lateral meristems which results in an increase in girth or thickness of the plant.

Identifying Types of Plant Growth

Lateral meristems, which include the vascular cambium and cork cambium, are responsible for the secondary growth in plants. This process increases the diameter of stems and roots as the plant grows older.

Choosing the Correct Option

Since the increase in girth (diameter) is due to lateral meristems, the correct term for this process is 'secondary growth'.

Key concepts

Understanding Lateral Meristems

Plants have various ways of growing, and when it comes to increasing in thickness or girth, it's the lateral meristems that play a critical role. These growth regions are found parallel to the long axis of roots and stems, where they enable the plant to grow wider as opposed to taller. More importantly, lateral meristems are made up of actively dividing cells that contribute to the plant’s secondary growth – a key concept when studying plant biology.

Lateral meristems consist primarily of the vascular cambium and cork cambium. They're located inside of the plant's stem and root. Their main job is to add layers of cells to these parts of the plant, allowing them to grow in girth. This type of growth is characteristic and significant in woody plants, especially trees, which get thicker every year.

The Role of Vascular Cambium

The vascular cambium is a type of lateral meristem that is found between the wood (xylem) and the bark (phloem) inside the plant. This cylindrical layer of tissue is responsible for adding layers of vascular tissue called secondary xylem (wood) and secondary phloem.

As part of secondary growth, the vascular cambium forms a ring that extends throughout the length of roots and stems, and with each growing season, it divides to produce new tissue, thus adding 'rings' to a tree - something we often think of when counting the age of a tree cut in cross-section. Understanding the role of vascular cambium is crucial to grasp how these important tissues provide support and nutrients transportation throughout the plant.

Cork Cambium Functionality

While the vascular cambium is busy adding to the bulk of the plant, the cork cambium (or phellogen) is at work forming the protective outer layer of the stems and roots. It's this layer that generates the cork (phellem), which most of us know as the outer bark of a tree.

The cork cambium also produces phelloderm, a layer of cells just interior to the cork. Together, these layers form what's called the periderm, contributing to a plant's protection from physical damage and water loss. Over time, as more and more layers are added, the older, outermost layers of cork are sloughed off in many tree species, which is why mature trees have thick, sometimes rugged bark.