Secondary Growth | Vascular Cambium & Cork Cambium | Anatomy of Flowering Plants | Class 11 Biology
Summary
TLDRChapter 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
🌱 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.
🍂 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.
Mindmap
Keywords
💡Primary Growth
💡Secondary Growth
💡Vascular Cambium
💡Cambial Ring
💡Spring Wood
💡Autumn Wood
💡Heartwood
💡Sapwood
💡Cork Cambium
💡Lenticels
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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