Secondary Growth | Vascular Cambium & Cork Cambium | Anatomy of Flowering Plants | Class 11 Biology

Summary

Chapter six of 'Anatomy of Flowering Plants' explores secondary growth, focusing on the increase in the girth of roots and stems. It details the roles of lateral meristems like vascular cambium and cork cambium. Key concepts include the formation of the cambial ring, differences between spring and autumn wood, the functions of heartwood and sapwood, and the process of bark formation. The text also highlights secondary growth in roots, explaining the development and significance of these processes in dicot plants and gymnosperms, while noting the absence of secondary growth in monocots.

Takeaways

• 🌱 The primary growth of roots and stems occurs through apical meristem, while secondary growth involves lateral meristems like vascular cambium and cork cambium.
• 🌿 Vascular cambium is responsible for producing new vascular tissues, forming a continuous ring between the xylem and phloem in dicot stems.
• 🌀 The cambial ring actively produces secondary xylem on the inner side and secondary phloem on the outer side, resulting in more xylem than phloem over time.
• 🌳 Spring wood has wider vessels and is lighter in color, while autumn wood has narrower vessels and is darker, forming distinct annual rings that indicate the age of the tree.
• 🌲 Heartwood is dark, hard, and non-conductive, providing mechanical support, while sapwood is lighter and responsible for conducting water and minerals.
• 🛡️ Cork cambium forms as the stem increases in girth, producing protective layers like cork on the outside and secondary cortex on the inside.
• 💧 Cork is impermeable due to suberin deposition, helping protect the stem as the outer layers are replaced and shed off over time.
• 🍂 Bark consists of all tissues outside the vascular cambium, including cork, cork cambium, and secondary phloem, and it develops differently throughout the growing season.
• 🍃 Lenticels are small openings formed in woody stems to allow gas exchange between the atmosphere and internal tissues.
• 🌾 In roots, secondary growth occurs similarly to stems, with vascular cambium originating from tissues below the phloem bundles.

Q & A

What is primary growth in plants?

Primary growth is the increase in length of roots and stems due to the activity of apical meristems.

What is secondary growth, and which plants exhibit it?

Secondary growth is the increase in girth or thickness of plants, commonly exhibited by dicotyledonous plants. It involves the activity of two lateral meristems: vascular cambium and cork cambium.

What role does the vascular cambium play in secondary growth?

The vascular cambium is responsible for producing vascular tissues (xylem and phloem) during secondary growth. It forms a continuous ring in the stem and actively cuts off new cells both towards the inner and outer sides.

How is the cambial ring formed in dicot stems?

In dicot stems, the cambial ring is formed by the intrafascicular cambium located between the primary xylem and phloem, and the interfascicular cambium formed from medullary rays, eventually creating a continuous ring of cambium.

What is the difference between spring wood and autumn wood?

Spring wood (early wood) is formed during the active growth season and has large vessels with wider cavities, making it lighter in color and lower in density. Autumn wood (late wood) forms during the less active growth period, has narrow vessels, and is darker with a higher density.

How can the age of a tree be determined from its stem?

The age of a tree can be estimated by counting the annual rings, which consist of alternating spring wood and autumn wood that form concentric rings.

What is heartwood, and what is its function?

Heartwood is the darker, central region of the secondary xylem, composed of dead cells with lignified walls. It does not conduct water but provides mechanical support to the stem.

What is sapwood, and how does it differ from heartwood?

Sapwood is the lighter-colored peripheral region of the secondary xylem that actively conducts water and minerals from roots to leaves, unlike heartwood which no longer performs this function.

What is the cork cambium, and what role does it play in secondary growth?

The cork cambium (phellogen) is a meristematic tissue that forms in the cortex during secondary growth. It produces cork (phellem) on the outer side and secondary cortex (phelloderm) on the inner side, replacing broken outer layers.

What are lenticels, and why are they important in woody plants?

Lenticels are lens-shaped openings in the bark formed by the cork cambium, allowing the exchange of gases between the internal tissues and the external environment, which is essential for respiration in woody trees.