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

Interfascicular cambium is a secondary meristem that develops between the vascular bundles in dicotyledonous stems. 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. These strips of interfascicular cambium connect the fascicular cambium of adjacent vascular bundles, forming a continuous cylinder of cambium around the stem. The interfascicular cambium arises from the dedifferentiation of parenchyma cells and becomes active during secondary growth. It produces secondary xylem on its inner side and secondary phloem on its outer side, contributing to the formation of secondary tissues. The formation of interfascicular cambium is essential for the development of a complete vascular cambium cylinder, which enables continuous secondary growth and increase in girth of the stem.

Stratified cambium and non-stratified cambium are two types of vascular cambium that differ in their structure and the arrangement of their cells. Stratified cambium is found in plants with short fusiform initials that are produced in storied or tiered arrangements, creating a stratified or layered appearance when viewed in cross-section. The cells are arranged in distinct horizontal tiers, and this type of cambium is characteristic of certain woody plants. Non-stratified cambium, on the other hand, is found in plants with long fusiform initials that are produced in non-storied arrangements. These fusiform initials strongly overlap at their ends and do not form distinct tiers, resulting in a non-stratified or random appearance. The overlapping nature of the cells in non-stratified cambium creates a more complex three-dimensional arrangement. Both types of cambium function similarly in producing secondary xylem and phloem, but their structural organization differs, which can be observed in microscopic examination of the cambial region.

• 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 is a protective tissue that replaces the epidermis and primary cortex during secondary growth in dicot stems and roots. It is composed of three distinct layers: the phellem (cork) on the outside, the phellogen (cork cambium) in the middle, and the phelloderm (secondary cortex) on the inside. This entire complex acts as a barrier against water loss, mechanical injury, and pathogen invasion. Polyderm, on the other hand, is a specialized type of protective tissue found in certain plants, particularly in the roots and underground stems of members of the Rosaceae family. It is characterized by an alternating arrangement of miserable suberized layers and multiseriate non-suberized cells within the periderm. This unique structure provides enhanced protection in specific environments, often in underground organs where resistance to decay and physical stress is crucial.

Rhytidome is a technical term used to describe the outer dead bark that forms during successive secondary growth in plants. It consists of periderm along with isolated cortical or phloem tissues that become separated and dead as the plant undergoes repeated cycles of secondary growth. The rhytidome represents the accumulated layers of dead bark that are shed or remain on the outer surface of woody stems and roots. A classic example is seen in Quercus (oak trees), where the characteristic furrowed and deeply grooved bark is composed of rhytidome. This structure serves a protective function, insulating the living tissues beneath from environmental stresses such as temperature fluctuations, mechanical damage, and pathogen invasion. The formation of rhytidome is a natural consequence of the plant's continuous secondary growth, where new periderm layers are formed progressively inward, causing the older outer layers to become isolated from the living tissues and eventually die.

Canada balsam is an organic gum-like substance produced from the resin ducts of the Abies balsamea plant. It is widely used as a permanent mounting medium for microscopic slide preparation in botanical and biological laboratories. Canada balsam serves several important functions in microscopy: it has a refractive index very close to that of glass, which allows for clear visualization of mounted specimens without distortion or loss of detail. Its transparency and stability make it ideal for long-term preservation of delicate biological specimens. Once applied to a microscopic slide, Canada balsam hardens and forms a permanent seal that protects the specimen from environmental contamination, moisture loss, and microbial degradation. This permanence is demonstrated by the fact that specimens mounted in Canada balsam decades ago remain in excellent condition, such as the example of a 60-year-old holotype specimen of a flatworm that is still well-preserved. The medium is particularly valuable for preserving the structural details of thin sections of plant tissues, making it an indispensable tool in botanical research and education.

Primary growth and secondary growth are two distinct processes of plant development that differ in their origin, direction, and function. Primary growth is the increase in length and width of plant organs that originates from the apical meristems located at the tips of roots and shoots. During primary growth, cells produced by the apical meristems undergo elongation and expansion, resulting in longitudinal or lengthwise growth of the plant body. This process is responsible for the initial development of roots and stems and occurs throughout the life of the plant. Primary growth is also called longitudinal growth because it increases the length of the plant organs. Secondary growth, in contrast, is the increase in thickness or diameter of stems and roots that occurs after primary growth has been established. Secondary growth is characteristic of gymnosperms and most dicotyledonous angiosperms, though some monocots also exhibit this type of growth. This growth is achieved through the activity of lateral meristems, particularly the vascular cambium and cork cambium, which produce secondary xylem and secondary phloem tissues. Secondary growth is also called latitudinal growth because it increases the width or thickness of plant organs. While primary growth is responsible for the overall shape and reach of the plant, secondary growth provides structural support and increased capacity for transport of water and nutrients, making the plant more robust and long-lived.

• 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

False annual rings, also called pseudo-annual rings, are additional growth rings that develop within a single year rather than the typical one ring per year. These rings form when the plant experiences adverse environmental conditions or natural calamities during the middle of a growing season that temporarily interrupt normal growth. Such interruptions can be caused by various factors including drought, which reduces water availability and causes the plant to cease growth temporarily; frost, which damages actively growing tissues; defoliation caused by insect damage or disease, which reduces the plant's photosynthetic capacity; flooding, which affects root function and nutrient uptake; mechanical injury to the plant body; and biotic factors such as pest infestations or pathogenic infections. When growth resumes after these adverse conditions pass, a new ring begins to form within the same year, resulting in the formation of more than one annual ring. The presence of false annual rings can complicate dendrochronological studies and age determination of trees, as counting rings alone may overestimate the actual age of the tree. These rings are typically narrower and less distinct than true annual rings, and their presence indicates that the tree experienced significant stress during its growth period.

Diffuse porous wood is a type of wood where the vessels, which are responsible for water transport, are more or less uniform in diameter and are evenly distributed throughout the annual ring. This type of wood is typically formed in plants that grow in regions with uniform climatic conditions, meaning there are no significant seasonal variations in temperature or water availability. As a result, the growth rate of the plant is relatively consistent throughout the growing season, leading to a homogeneous distribution of vessels. Ring porous wood, in contrast, is characterized by a distinct difference in vessel size within an annual ring. The vessels formed early in the growing season (earlywood) are typically much wider and more numerous, forming a prominent ring, while the vessels formed later in the season (latewood) are narrower and less abundant. This pattern is common in plants growing in climates with distinct seasonal changes, where rapid water transport is required during the spring growth flush, followed by reduced transport during drier or colder periods. Consequently, the vessels are not uniformly distributed throughout the wood, with larger vessels concentrated in the earlywood.

Porous wood, also commonly referred to as hardwood, is characteristic of angiosperms. Its porous nature stems from the presence of vessels, which are specialized conducting elements that form continuous tubes for efficient water transport throughout the plant. These vessels are typically wider than tracheids and are a defining feature of angiosperm wood, contributing to its generally higher density and strength. An example of a tree producing porous wood is Morus (mulberry). Non-porous wood, also known as softwood, is characteristic of gymnosperms. Unlike porous wood, it lacks vessels. Instead, water transport in non-porous wood is primarily carried out by tracheids, which are elongated, spindle-shaped cells with pitted walls. While tracheids are less efficient at water conduction than vessels, they provide structural support. The absence of vessels gives non-porous wood a more uniform appearance. Pinus (pine) is a classic example of a tree that produces non-porous wood.

Sapwood and heartwood are two distinct regions of secondary xylem that differ in their physiological state, location, appearance, and functional properties. Sapwood, also called alburnum, is the living part of the wood located on the outer side of the secondary xylem. It is lighter in color and relatively soft in nature compared to heartwood. The cells of sapwood remain living and actively participate in the transport of water and minerals from the roots to the leaves through the xylem vessels. Sapwood contains no tyloses, which are balloon-like outgrowths of parenchyma cells that block the xylem vessels. Because the cells are living and the vessels are open, sapwood is not durable and is susceptible to attack by microorganisms, insects, and termites. Heartwood, also called duramen, is the dead part of the secondary xylem located in the central region of the wood. It is dark in color and hard in nature due to the deposition of various substances such as tannins, resins, and other organic compounds in the cell walls and lumens. The cells of heartwood are no longer living, and the xylem vessels are often blocked by tyloses, which prevent the movement of water through these vessels. The presence of these deposited substances and the blockage by tyloses make heartwood more durable and resistant to decay caused by microorganisms, insects, and termites. Although heartwood no longer functions in water transport, it provides structural support to the plant and is more valuable for timber and construction purposes due to its durability and resistance to degradation.

Phellem and phelloderm are two distinct tissues produced by the phellogen during secondary growth, differing in their position, structure, function, and composition. Phellem, commonly called cork, is formed on the outer side of the phellogen. Its cells are compactly arranged in regular tiers and rows without any intercellular spaces, creating a dense, impermeable layer. The cells of phellem are non-living at maturity and possess suberized walls, meaning they are impregnated with suberin, a waxy, water-resistant substance. This structure makes phellem primarily protective in function, serving as a barrier against water loss, mechanical damage, and pathogen invasion. Lenticels, which are raised openings or pores that allow gas exchange, are present in phellem. Phelloderm, also called secondary cortex, is formed on the inner side of the phellogen. Its cells are loosely arranged with intercellular spaces between them, creating a more open structure. The cells of phelloderm are living, parenchymatous in nature, and do not have suberized walls. Because these cells contain chloroplasts, phelloderm is capable of photosynthesis and serves to synthesize and store food materials. Lenticels are absent in phelloderm. Together, phellem and phelloderm, along with the phellogen that produces them, form the periderm, which replaces the epidermis as the plant undergoes secondary growth.

A lenticel is a raised opening or pore that develops on the epidermis or bark of stems and roots during secondary growth. Lenticels are formed in regions where the phellogen is particularly active and produces a mass of loosely arranged, thin-walled parenchyma cells instead of the typical compact cork cells. This loosely arranged tissue is called complementary tissue or filling tissue and creates a pathway through the otherwise impermeable cork layer. The structure of a lenticel allows for the exchange of gases between the internal tissues of the plant and the external atmosphere, which is essential for respiration of the living cells beneath the bark. Additionally, lenticels facilitate transpiration, the loss of water vapor from the plant, through a process called lenticular transpiration. This is particularly important in woody plants where the waxy, suberized cork layer would otherwise prevent gas exchange and water loss. Lenticels are visible as small, raised dots or lines on the bark of many trees and shrubs, and their presence and distribution can be characteristic features used in plant identification. The number and arrangement of lenticels can vary among different plant species and may increase in size and prominence as the plant ages and the bark thickens.

Bark serves multiple crucial functions for a tree's survival and well-being. Primarily, it acts as a robust protective layer, shielding the delicate inner tissues from a wide array of external threats. This includes protection against parasitic fungi and insects, which can cause significant damage or disease. The bark also plays a vital role in preventing excessive water loss from the plant through evaporation, especially during dry periods, due to its often suberized and impermeable nature. Furthermore, it insulates the tree, guarding against extreme variations in external temperature, thus protecting the living cells within. Beyond these protective roles, bark can possess insect-repellent properties, deterring pests, and can be decay-proof, contributing to the longevity of the tree. Some barks are even fire-resistant to a certain extent. Economically, bark is a valuable resource, used in obtaining various drugs or spices, such as quinine from Cinchona bark or cinnamon from Cinnamomum bark. Physiologically, the phloem cells located within the inner bark are actively involved in the conduction of food (sugars) produced during photosynthesis from the leaves to other parts of the plant, while secondary cortical cells within the bark are often involved in the storage of food reserves.

• 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.

Dendrochronology is the scientific method of dating tree rings to the exact year they were formed. It is based on the principle that each annual ring of a tree corresponds to one year's growth. By carefully counting these rings, scientists can accurately determine the age of the plant. This technique provides a precise chronological record. The significance of studying growth rings extends beyond simply determining age. It allows for the calculation of the age of the wood itself, which can be further verified by methods like radioactive carbon dating for older samples. In forensic investigations, dendrochronology can provide crucial evidence, for instance, in dating wooden artifacts or determining the time of an event involving trees. Moreover, the width and characteristics of annual rings reflect past environmental conditions, such as rainfall, temperature, and drought, making dendrochronology an invaluable tool for studying climate change and ecological history over centuries.

In a cross-section of a stem showing vascular cambial activity, the parts can be labeled as follows with reference to the vascular cambium designated as A. The vascular cambium, labeled as A, is the lateral meristem responsible for secondary growth. Moving outward from the cambium, B represents the first formed phloem, which is the primary phloem produced during primary growth before the cambium became active. Further outward, C represents the first formed xylem, which is the primary xylem also produced during primary growth. Moving inward from the cambium, D represents the second formed phloem, which is the secondary phloem produced by the outward activity of the vascular cambium during secondary growth. Finally, E represents the second formed xylem, which is the secondary xylem produced by the inward activity of the vascular cambium during secondary growth. The secondary xylem (E) accumulates in much larger quantities than secondary phloem (D) because the cambium is more active on its inner side, producing more xylem cells than phloem cells. This differential activity results in the characteristic structure of woody stems where the secondary xylem forms the bulk of the wood, while the secondary phloem remains a relatively thin layer.

Phellem, also known as cork, is formed on the outer side of the phellogen (cork cambium). Its cells are typically compactly arranged in regular tiers and rows, with little to no intercellular spaces, forming a dense and protective layer. Phellem consists of non-living cells at maturity, and their walls are heavily suberized, meaning they are impregnated with suberin, a waxy substance that makes them impermeable to water and gases. This impermeability is crucial for its primary function of protection against desiccation, mechanical injury, and pathogen invasion. Lenticels, which are specialized pores for gaseous exchange, are present within the phellem. Phelloderm, also known as the secondary cortex, is formed on the inner side of the phellogen. Its cells are generally more loosely arranged compared to phellem cells, often exhibiting intercellular spaces. Phelloderm consists of living cells that are parenchymatous in nature and do not have suberin in their walls. These cells may contain chloroplasts, especially in younger phelloderm, allowing them to synthesize and store food, thus contributing to the plant's metabolic activities. Unlike phellem, lenticels are typically absent in the phelloderm layer.

Cork and bark, while both protective tissues, differ significantly in their composition and extent. Cork, specifically, refers to the phellem layer of the bark. It is primarily composed of suberin, a hydrophobic substance that makes it impermeable to water and gases. This unique chemical composition gives cork its characteristic properties: it is impermeable, buoyant, elastic, and fire-retardant. Due to these properties, cork has various commercial applications, most notably in making bottle stoppers, such as those obtained from the bark of Quercus suber, the cork oak. Bark, on the other hand, is a broader term that encompasses all tissues located outside the vascular cambium. This includes the periderm (which itself consists of phellem, phellogen, and phelloderm), the primary cortex, and both primary and secondary phloem. The complex structure of bark provides a comprehensive protective barrier for the tree. It possesses insect-repellent, decay-proof, and fire-proof properties, safeguarding the underlying living tissues. Beyond protection, bark is a source of various useful substances. For instance, the bark of Cinchona yields quinine, an important anti-malarial drug, while the bark of Cinnamomum is widely used as a spice. The phloem within the bark is also crucial for the transport of sugars throughout the plant.

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