Samacheer Class 11 Bio Botany - Secondary Growth

The secondary growth in dicots and gymnosperms is brought about by two lateral meristems.
* Vascular cambium and
* Cork cambium
1. Vascular cambium:
The vascular cambium is the lateral meristem that produces the secondary vascular tissues, i.e.. secondary xylem and secondary phloem.
Origin and Formation of Vascular Cambium:
* A strip of vascular cambium originate from the procambium is present between xylem and phloem of the vascular bundle. This cambial strip is known as intrafascicular or fascicular cambium.
* In between the vascular bundles, a few parenchymatous cells of the medullary rays that are in line with the fascicular cambium become meristematic and form strips of vascular cambium. It is called interfascicular cambium.
A. Organization of Vascular cambium:
* The active vascular cambium possesses cells with large central vacuole (or vacuoles) surrounded by a thin, layers of dense cytoplasm.
* The most important character of the vascular cambium is the presence of two kinds of initials, namely fusiform initials and ray initials.
Fusiform Initials:
* These are vertically elongated cells. They give rise to the longitudinal or axial system of the secondary xylem (tracheary elements, fibres, and Axia? parenchyma) and pholem (sieve, elements, fibres, and axial parenchyma).
* Based on the arrangement of the fusiform initials two types of vascular cambium are recognized.
Stoned (Stratified cambium) and Non – storied (Non – stratified cambium)
* If the fusiform initials are arranged in horizontal tiers, with the end of the cells of one tier appearing at approximately the same level, as seen in tangential longitudinal section (TLS) it is called storied (stratified) cambium. It is the characteristic of the plants with short fùsiform initials.
* In plants with long fusiform initials, they strongly overlap at the ends, and this type of cambium is called non – storied (non stratified) cambium.
Ray Initials:
These are horizontally elongated cells. They give rise to the ray cells and form the elements of the radial system of secondary xylem and pholem.
Activity of Vascular Cambium:
* The vascular cambial ring, when active, cuts off new cells both towards the inner and outer side. The cells which are produced outward form secondary phloem and inward secondary xylem.
* Due to the continued formation of secondary xylem and phloem through vascular cambial activity, both the primary xylem and phloem get gradually crushed.
B. Phellogen (Cork Cambium)
* It is a secondary lateral meristem. It comprises homogenous meristematic cells unlike vascular cambium. It arises from epidermis, cortex, pholem or pericycle (extrastelar in origin). Its cells divide periclinally and produce radially arranged files of cells.
* The cells towards the outer side differentiate into phellem (cork) and those towards the inside as phelloderm (secondary cortex).

• In wood, the older it is, the stronger it becomes.
• Log A – Which was 50 years old is stronger and it will last longer.
• In a tree the central part of the wood will be darker in colour, dead in nature known as Heartwood or Duramen, and the outer sad wood is lighter in colour, living and conducting water.
• In the central Heartwood the conduction is blocked by the formation of tyloses from the nearby parenchyma cells, and dead.
• In the fully developed tyloses, starch crystals, resins, gums, oils tannins and coloured substances are found and it becomes very hard and durable.
• It is more resistant to the attack of microbes and insects like termites.
• Older woods have more heartwood than sapwood.
• Here log ‘A’ is older, has more heartwood and it is stronger and will last longer.

Growth (or) Annual Rings:
1. In the spring season cambium is very active and produces large number of xylary elements called Earlywood or Springwood. In the Winter season – cambium is less active and form few xylary elements – Latewood or Autumn Wood.
2. The springwood is lighter in color and has a lower density whereas the autumn wood is darker and has a higher density. The annual ring denotes the combination of earlywood and latewood and the ring becomes evident to our eye due to the high density of latewood. Sometimes annual rings are called growth rings
3. Pseudo – Annual Rings:
Additional growth rings are developed within a year due to adverse natural calamities like drought, frost defoliation, flood, mechanical, injury and biotic factors. Such rings arc called pseudo – or false – annual rings
4. Dendrochronology:
Each annual ring corresponds to one year’s growth and on the basis of these rings, the age of a particular plant can easily be calculated. The determination of the age of a tree by counting the annual rings is called dendrochronology.
Part – II.
11th Bio Botany Guide Secondary Growth Additional Important Questions and Answers
I. Choose The Correct Answer.

Column I
Column II
A. Springwood or earlywood
B. Autumn wood or Latewood
1. Lighter in colour
2. Density high
3. Density low
4. Darker in colour
5. Larger number of xylem elements
6. Vessels with wider cavity
7. Lesser number of xylem elements
8. Vessels with a small cavity
Which of the following combination is correct?
a) A – 2, 4, 7, 8 B- 1,3, 5,6
b) A- 1,2, 7,8 B-3,4, 5, 6
c) A – 1,3, 5,6 B – 2,4, 7, 8
d) A- 1,3,7, 8 B – 2, 4, 5, 6
b) A – 1, 2, 7,8 B-3,4,5, 6
IV. Assertion And Reason

Primary growth
* The roots and stem grow in length with the help of Apical meristem
* It is known as longitudinal growth
* Eg. Angiosperms & Gymnosperms
Secondary growth
* The roots and stem show an increase in thickness or width with the help of Lateral meristem
* It is also known as latitudinal growth or growth in girth
* Eg. Most Angiosperms, including some Monocots and Gymnosperms

In between the vascular bundles, a few parenchymatous cells of the medullary rays that are in line with the fascicular cambium become meristematic and form strips of the vascular cambium. It is called interfascicular cambium.

Stratified cambium: Plants with short fusiform initials, produced, storied, cambium in horizontal tiers known as
stratified cambium
Non-stratified cambium: Plants with long fusiform initials produced non-storied cambium, strongly overlap at the ends, known as Non-stratified cambium

• Porous wood is wood with xylem vessels which appear as a pore in cross-section.
• When a tree stem become old, most of its vessels are blocked by tyloses with deposition of gum, resin, tannin, oils, etc. (Heartwood)
• So porous wood is harder and commercially important.

• Turpentine is a resin obtained from the bark of conifers Eg. pinus
• It is also used as a thinner for oil-based paints.
• It is also used as an organic solvent.
• It is also used as a balm to relieve muscular pain.

Periderm:
* The secondary growth replaces the epidermis and primary cortex and forms the Periderm
* It consists of
* Phellem
* Phellogen
* Phelloderm
* Eg. Dicot stem & roots
Polyderm:
* It is a special type of protective tissue consisting of a miserable suberized layer, alternating with multiseriate nonsuberized cells in periderm
* Eg. Roots and underground stems of Rosaceae plants

Rhytidome is a technical term used for the outer dead bark which consists of periderm and isolated cortical or phloem tissues ? formed during successive secondary growth, eg: Quercus.

From the resin ducts, the Abies balsamea plant produces an organic gum-like substance, used as a permanent mounting medium for microscopic slide preparation.
Eg. A slide of 60 years old holotype specimen of a flatworm is permanently mounted in Canada balsam.
VI. 3 Mark Questions

1. Primary growth: The plant organs originating from the apical meristems pass through a period of expansion in length and width. The roots and stems grow in length with the help of apical meristems. This is tailed primary growth or longitudinal growth.
2. Secondary growth: The gymnosperms and most angiosperms, including some monocots, show an increase in the thickness of stems and roots by means of secondary growth or latitudinal growth.

• The aerating pores are seen as raised opening on the surface of bark as scars on old stems and roots.
• It is fonned during secondary growth in stems.
• In this portion phellogen activity is more than elsewhere, a filling tissue known as complementary tissue (loosely arranged parenchyma) is formed.
• Lenticel is helpful in the exchange of gases and also facilitate the little amount of transpiration

Additional growth rings are developed within a year due to adverse natural calamities like drought, frost, defoliation, flood, mechanical injury and biotic factors during the middle of a growing season, which results in the formation of more than one annual ring. Such rings are called pseudo – or false – annual rings.

Diffuse porous wood:
* This type of wood is formed where the climatic conditions are uniform
* The vessels are more or less equal in diameter in any annual ring
* The vessels are uniformly distributed throughout the wood
Ring porous wood:
* This type of wood is formed where the climatic conditions are not uniform
* The vessels are wide and narrow within an annual ring
* The vessels are not uniformly distributed throughout the wood

Porous wood or Hardwood, Ex: Morus:
* Common in Angiosperms
* Porous because it contains vessels
Non-porous wood or softwood Ex: Pinus
* Common in Gymnosperms
* Non – porous because it does not contain vessels

Sapwood (Alburnum)
Heartwood (Duramen)
1. Living part of the wood
1. Dead part of the wood
2. It is situated on the outer side of wood
2. It is situated in the centre part of wood
3. It is lighter in colour
3. It is dark coloured
4. Very soft in nature
4. Hard in nature
5. Tyloses are absent
5. Tyloses are present
6. It is not durable and not resistant to microorganisms
6. It is more durable and resists microorganisms insects and termites

Phellem (Cork)
Phelloderm (secondary cortex)
1. It is formed on the outer side of phellogen1
1. It is formed on the inner side of the phellogen
2. Cells are compactly arranged in regular tires and rows without intercellular spaces.
2. Cells are loosely arranged with intercellular spaces.
3. Protective in function.
3. As it contains chloroplasts, it synthesizes and stores food
4. Consists of non-living cells with suberized walls
4. Consists of living cells, parenchymatous in nature and does not have suberin
5. Lenticels are present
5. Lenticels are absent

Lenticel is raised opening or pore on the epidermis or bark of stems and roots. It is formed during secondary growth in stems. When phellogen is more active in the region of lenticels, a mass of loosely arranged thin-walled parenchyma cells is formed. It is called complementary tissue or filling tissue. Lenticel is helpful in the exchange of gases and transpiration called lenticular transpiration.

Bark protects the plant from parasitic fungi and insects, prevents water loss by evaporation, and guards against variations of external temperature. It is insect repellent, decay proof, fireproof, and is used in obtaining drugs or spices. The phloem cells of the bark are involved in the conduction of food while secondary cortical cells involved in storage.

• In the desert, as well seashore regions the climatic condition remain the same throughout the year.
• Secondary growth in plants is influenced by seasonal changes since in these areas seasonal changes are not significant enough to bring in distinct Annual rings with early and latewood formation alternatively.

• Sapwood is a living part of the wood, perform water conduction, that’s why it is known as Sapwood.
• Heartwood is a dead part of the wood, do not perform water conduction so if destroyed, no vital function of the plant is affected.
• If sapwood is damaged, or exposed conduction of water will be blocked, water loss is rapid leading to decay and decomposition of tissues and leads to death of the plant.

* The annual ring of a tree corresponds to one year’s growth.
* If we count the rings we can determine the very age of the plant.
* This method of calculating the age of a tree by counting the annual ring is known as dendrochronology.
Significance of studying growth rings:
* Age of wood – calculated
* Age verified by Radioactive carbon dating
* Provides evidence in Forensic investigation.

A – Vascular cambium
B – First formed phloem – (Primary phloem)
C – First formed xylem – (Primary xylem)
D – Second formed phloem – (Secondary phloem)
E – Second formed xylem – (Secondary xylem)

Phellem (Cork):
* It is formed on the outer side of phellogen.
* Cells are compactly arranged in regular tires and rows without intercellular spaces.
* Protective in function.
* Consists of nonliving cells with suberized walls.
* Lenticels are present.
Phelloderm (Secondary cortex):
* It is formed on the inner side of phellogen.
* Cells are loosely arranged with intercellular spaces.
* As it contains chloroplast, it synthesises and stores food.
* Consists of living cells, parenchymatous in nature and does not have suberin.
* Lenticels are absent.

Cork:
* It includes only phellem layer of bark
* It is composed of suberin a hydrophobic substance
* It has impermeable buoyant, elastic and fibre retartant properties
* Used in making bottle stoppers Eg. Bark of Quercus suber
Bark:
* It includes all tissues outside vascular cambium (Periderm, Cortex, Primary and secondary phloem)
* It has insect repellent, decay proof, fire proof properties.
* Used as Drugs or spices.
* Eg. Bark of Chichona – (AntimalariaJ drug), Bark of Cinnamomum (Used as spice)

Fusiform initials:
* Vertically elongated cells
* Give rise to axial system of secondary tissues, xylem and phloem
* Secondary xylem includes tracheary elements, fibres and axial parenchyma
* Secondary phloem includes sieve elements
* Based on arrangement of fusiform initials 2 types of vascular cambium recognised
a – stratified cambium
b – Nonstratified cambium
Ray initials
* Horizontally elongated cells
* Give rise to radial system of secondary xylem and phloem
* Radial system consists of rows of parenchymatous cells oriented at right angles to the longitudinal axis of xylem elements
* Secondary phloem include phloem rays fibres and axial parenchyma