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.

Outlines

00:00

🌱 Secondary Growth in Flowering Plants

This paragraph introduces the concept of secondary growth in dicot plants, in addition to the primary growth caused by apical meristems. It defines secondary growth as the increase in girth due to lateral meristems: the vascular cambium and the cork cambium. The vascular cambium is described as responsible for producing vascular tissues (xylem and phloem), initially forming in patches between the primary xylem and phloem. Over time, it forms a complete cambial ring. The paragraph also discusses how the cambial ring cuts off new cells inwardly to form secondary xylem and outwardly to form secondary phloem.

05:02

🍂 Annual Rings and Heartwood vs. Sapwood

This section elaborates on how cambium activity changes with the seasons, resulting in the formation of spring wood (lighter and less dense) and autumn wood (darker and denser). These alternating layers form annual rings, which can be used to estimate the tree's age. It also explains the distinction between heartwood (dead, lignified tissue that provides mechanical support but doesn't conduct water) and sapwood (lighter, living tissue involved in water conduction). Heartwood is resistant to decay due to the presence of substances like tannins, oils, and resins.

Keywords

💡Primary Growth

Primary growth refers to the increase in the length of roots and stems through the activity of the apical meristem. This type of growth allows plants to extend vertically and is essential for the elongation of new shoots and roots. In the video, it is distinguished from secondary growth, which focuses on the increase in girth.

💡Secondary Growth

Secondary growth is the increase in the thickness or girth of roots and stems, primarily in dicotyledonous plants. This process involves the lateral meristems, such as the vascular cambium and cork cambium. Secondary growth enhances the structural support of plants as they mature, allowing them to withstand environmental stresses.

💡Vascular Cambium

The vascular cambium is a meristematic layer responsible for producing new vascular tissues (xylem and phloem). It is located between the xylem and phloem in the stem and forms a ring during secondary growth. This cambium actively divides, producing secondary xylem (wood) inward and secondary phloem outward.

💡Cambial Ring

The cambial ring is a continuous layer formed by the joining of intrafascicular cambium and interfascicular cambium during secondary growth. It encircles the stem and plays a crucial role in the production of secondary vascular tissues, contributing to the thickening of the plant.

💡Spring Wood

Spring wood, also known as early wood, is the lighter, less dense wood formed during the spring season when cambial activity is high. This wood contains larger vessels that facilitate efficient water transport, reflecting the favorable growth conditions during spring. It contrasts with autumn wood in terms of color and density.

💡Autumn Wood

Autumn wood, or late wood, is formed during the autumn season when the cambium is less active. It has narrower vessels and is darker and denser than spring wood. The alternating bands of spring and autumn wood create annual rings, which can be used to estimate a tree's age.

💡Heartwood

Heartwood is the innermost, older part of a tree's secondary xylem. It is dark in color due to the accumulation of compounds like resins, tannins, and oils. While heartwood no longer conducts water, it provides structural support and is resistant to decay due to its dense, lignified walls.

💡Sapwood

Sapwood is the lighter, outer portion of the secondary xylem that actively conducts water and minerals from the roots to the leaves. Unlike heartwood, sapwood remains functional in water transport and plays a vital role in the overall health of the plant.

💡Cork Cambium

Cork cambium, also known as phellogen, is a lateral meristem that produces protective tissues like cork (phellem) and secondary cortex (phelloderm). It replaces the epidermis in stems that undergo secondary growth, providing protection and helping prevent water loss as the plant expands.

💡Lenticels

Lenticels are lens-shaped openings in the bark of woody trees, formed by the cork cambium. They allow for gas exchange between the internal tissues and the atmosphere, compensating for the impervious nature of the cork layer. Lenticels are essential for respiration in stems and roots.

Highlights

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

Secondary growth occurs in dicot plants and is responsible for the increase in girth, involving lateral meristems like vascular cambium and cork cambium.

Vascular cambium is responsible for forming vascular tissues such as xylem and phloem.

In dicot stems, the cambial ring is formed by intrafascicular cambium and interfascicular cambium, which originate from medullary rays.

The cambial ring becomes active and produces secondary xylem towards the inner side and secondary phloem towards the outer side.

Secondary xylem is produced in larger quantities compared to secondary phloem, leading to the formation of a compact mass.

Spring wood is lighter with wider vessels, while autumn wood is darker and denser, forming concentric rings known as annual rings, which indicate a tree's age.

Heartwood is the central, dark-colored part of the secondary xylem, rich in organic compounds, providing mechanical support but not water conduction.

Sapwood is the peripheral, lighter-colored part of the secondary xylem, actively involved in conducting water and minerals.

Cork cambium replaces the broken outer cortical and epidermis layers with protective layers like cork (phellem) and secondary cortex (phelloderm).

Cork is impermeable to water due to suberin deposition, and together with phellem and phelloderm, they form the periderm.

Bark refers to all tissues external to the vascular cambium, including periderm and secondary phloem, with early and late bark forming in different seasons.

Lenticels are lens-shaped openings formed in the cork for gas exchange between the outer atmosphere and internal tissues.

In dicot roots, secondary growth originates from tissues below the phloem bundles, forming a wavy ring that later becomes circular.

Secondary growth also occurs in gymnosperms but does not occur in monocots.

Transcripts

chapter six anatomy of flowering plants

6.4 secondary growth the growth of the

roots and Stems in length with the help

of apical meristem is called the primary

growth apart from primary growth most

diotti inous plants exhibit an increase

in grth this increase is called the

secondary growth the tissues involved in

secondary growth are the two lateral

meristems vascular cambium and cor

cambium let's begin with vascular

cambium the meristematic layer that is

responsible for cutting off vascular

tissues cylum and fum is called vascular

cambium in the young stem it is present

in patches as a single layer between the

cylum and flu later it forms a complete

ring let's see formation of cambial ring

in D stems the cells of cambium present

between primary cylum and primary flu is

the intrafascicular cambium the cells of

medular rays adjoining these intasc

cambium become meristematic and form the

interfascicular cambium thus a

continuous ring of cambium is formed Now

activity of the cambial Ring The cambial

Ring becomes active and begins to cut

off new cells both towards the inner and

the outer sides the cells cut off

towards pit mature andri syum and the

cells cut off towards periphery mature

into secondary flu the cambium is

generally more active on the inner side

than on the outer as a result the amount

of secondary cylum produced is more than

secondary flum and soon forms a compact

Mass the primary and secondary flums get

gradually crushed due to the continued

formation and accumulation of secondary

cylum the primary cylum however remains

more or less intact in or around the

center at some places the cambium forms

an arrow band of p and Kimmer which

passes through the secondary cylum and

the secondary flum in the radial

directions these are the secondary

magary Rays compare spring wood and

Autumn Wood the activity of cambium is

under the control of many physiological

and environmental factors in temperate

regions the climatic conditions are not

uniform through the year in the spring

season cambium is very active and produc

produces a large number of cyer elements

having vessels with wider cavities the

wood formed during this season is called

Spring wood or early wood in Winter the

cambium is less active and forms fewer

cyer elements that have narrow vessels

and this wood is called Autumn Wood or

late wood the spring wood is lighter in

color and has a lower density whereas

the Autumn Wood is darker and has a

higher density the two kinds of woods

that appear as alternate concentric

Rings constitute an annual ring annual

rings seen in a cut stem given estimate

of the age of the tree next compare

Heartwood and sapwood in Old trees the

greater part of secondary cylum is dark

brown due to deposition of organic

compounds like tannins resins oils gums

aromatic substances and essential oils

in the central or inmost layers of the

stem these substances make it hard

durable and resistant to the attacks of

microorganisms and insects

this region comprises dead elements with

highly lignified walls and is called

Heartwood the Heartwood does not conduct

water but it gives mechanical support to

the stem the peripheral region of the

secondary cylum is lighter in color and

is known as the sap wood it is involved

in the conduction of water and minerals

from root to Leaf next is cor cambium as

the stem continues to increase in girth

due to the activity of vascular cambium

the outer cortical and epidermis layers

get broken and need to be replaced to

provide new protective cell layers hence

sooner or later another meristematic

tissue called cambium or felen

develops usually in the cortex region

felen is a couple of layers thick it is

made of narrow thin walled and nearly

rectangular cells felen cuts off cells

on both sides the outer cells

differentiate into of fum while the

inner cells differentiate into secondary

cortex or fadum the cor is impervious to

water due to subaran deposition in the

cell wall the cells of secondary cortex

area animatus felen felim and fadum are

collectively known as parum due to

activity of the cor cambium pressure

builds up on the remaining layers

peripheral to felen and ultimately these

layers die and slow off bur is a

non-technical term that refers to all

tissues exterior to the vascular cambium

therefore incl cluding secondary flu bar

refers to a number of tissue types viz

parid and secondary flu bark that is

formed early in the season is called

early or soft bark towards the end of

the season late or hard bark is formed

name the various kinds of cell layers

which constitute the bark at certain

regions the felen cuts off closely

arranged enim cells on the outer side

instead of cells these enim cells

soon rupture the dermis forming a

lens-shaped openings called lenticels

lenil permit the exchange of gases

between the outer atmosphere and the

internal tissue of the stem these occur

in most Woody trees lastly secondary

growth in Roots in the daar root the

vascular cambium is completely secondary

in origin it originates from the tissue

located just below the flam bundles a

portion of pericycle tissue above the

protoy forming a complete and continuous

wavy ring which later becomes circular

further events are similar to those

already described above for a dcot Leen

stem secondary growth also occurs in

stems and roots of

gymnosperms however secondary growth

does not occur in mon

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