SECONDARY GROWTH

Increase in girth or thickness or diameter of the axis occurs due to formation of new tissues as a result of joint activity of vascular cambium and cork cambium in steler and extrasteler region respectively.

It occurs in the root and stem of gymnosperms and dicots.

Secondary growth in dicot stem completes in following steps :

A. Formation of vascular cambium ring :

(i) Intrafascicular cambium : It is primary in origin and is present between primary phloem and primary xylem.

(ii) Interfascicular cambium : It is true secondary meristem. It originates from parenchyma cells of medullary rays region. It lies in between the vascular bundles.

(iii) Vascular cambium ring: Both intrafascicular and interfascicular cambia join together and form a cambium ring.

Cells of cambium are of two types:

(a) Fusiform initials : They form tracheids, vessels, fibres and axial parenchyma in secondary xylem and sieve tubes, companion cells, fibres and axial parenchyma in secondary phloem.

(b) Ray initials : These are isodiametric and form ray parenchyma (vascular rays).

(iv) Periclinal division in cells of vascular cambium ring.

(v) Formation of secondary phloem (outside the vascular cambium) and secondary xylem (inner to vascular cambium). The amount of secondary xylem produced is 8-10 times greater than secondary phloem.

(vi) Due to formation of secondary phloem primary phloem is crushed to death, known as obliteration. Primary xylem being dead and lignified, persists in the pith region by replacing the pith cells.

(vii) At some places, the cambium forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These are the secondary medullary rays.

(viii) Formation of secondary structures i.e. annual rings, sapwood and heart wood, hardwood and softwood etc.

Different stages of secondary growth in a typical dicot stem

(a) Annual rings:

These are formed by the seasonal activity of vascular cambium.

Cambium is not uniformly active throughout the year.

In spring or summer cambium is more active and form large sized xylem elements (vessels) which constitute spring or early wood.

In Autumn or winter, cambium is less active and cuts off small sized xylem elements (vessels) and constitute autumn wood or late wood.

Both autumn and spring wood constitute a growth or annual ring.

In one year only one growth ring is formed.

In successive years numerous growth rings are formed.

Thus by counting the number of annual rings in the main stem at the base we can determine the age of a tree.

This branch of science is known as dendrochronology.

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Growth rings are distinct or sharply demarcated in the plants of temperate climate e.g., Shimla, Nainital, Mussourie due to presence of contrasting seasonal variations.

Growth rings are not distinct or sharply demarcated in the trees of tropical climate (near equator) e.g., Calcutta, Bombay, Madras due to absence of contrasting seasonal variations.

(b) Heart wood and sap wood:

The young elements of secondary xylem in the peripheral region constitute sap wood or alburnum.

It is light in colour and physiologically active.

The water conduction takes place through sap wood.

Sap wood is converted into heart wood or duramen in the central region.

It is dark in colour due to deposition of tannins, gums, resins and is physiologically inactive (almost dead) and provides only mechanical support.

During the conversion of sap wood into heartwood, the most important change is development of tyloses in the heart wood.

Tyloses are balloon like structures in lumen of vessels, developing from xylem parenchyma.

These tyloses block the passage of xylem vessels and are also called tracheal plug.

The heart wood is commercially used as wood.

When the plant is made hollow, it will not die because the water conduction takes place through sap wood.

The heart wood is well developed in Morus alba (Mulberry).

The heart wood is absent in Populus and Salix plant.

The wood of Tectona grandis is termite resistant.

As a tree grows older thickness of heartwood increases and sap wood remains same.

Heart wood is much more durable and resistant to microorganisms, insects and pests etc. than sap wood.

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Wood of dicot trees is called porous or hard wood because it consists of vessels (pores).

The wood of gymnosperms does not contain vessels (pores) and is known as soft or non porous wood.

Such wood consists of 90 to 95% tracheids and 5 to 10% of ray cells.

Sap wood will decay faster if exposed freely to the air.

On the basis of distribution and size of vessels, porous wood is of two types:

(a) Diffuse porous wood (primitive) : Vessels of same size are uniformly distributed throughout the growth, e.g., Pyrus, Betula.

(b) Ring porous wood (advanced) : Large vessels are formed in early wood when the need of water is great and small vessels are formed in late wood, e.g., Quercus, Morus.

B. Formation of cork cambium:

Cork cambium or phellogen develops from outer layer of cortex.

It produces secondary cortex or phelloderm on innerside and cork or phellem on outer side.

The cells of phellem are dead, suberized and impervious to water.

Cork cells are airtight and used as bottle stopper or cork.

The bottle cork is prepared from the cork of Quercus suber (Oak tree).

Cells of phelloderm are thin walled, living and store food. Phellem, phellogen and phelloderm are collectively called periderm.

Periderm is secondary protective tissue.

Due to pressure of secondary tissues, epidermis ruptures and cortex is largely lost after two or three years of secondary growth.

In the cork layer the lenticels are present which are meant for gaseous exchange.

In cork, lenticels have loosely arranged cells called complementary cells with intercellular spaces.

For bottle corks the cork, is processed in such manner, so that lenticels come in vertical direction.

Structure of (a) Lenticel and (b) Bark

Bark is a non-technical term for all the dead and living tissues outside the vascular cambium. Bark formed early in the season is called soft or early bark. Towards the end of the season late or hard bark is formed.

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(i) Scaly bark: Develops in strips e.g., Acacia, Psidium.

(ii) Ring bark: Develops in the form of sheet or ring, e.g., Betula (Bhojpatra).

The outermost layer of bark is dead and called as rhytidome.

The bark of Betula was being used as substitute of paper in ancient time to write manuscript.

Secondary Growth in Dicot Root

Vascular bundles in dicot root are radial, exarch and mostly triarch.

Vascular cambium is formed secondarily from conjuctive parenchyma cells lying just below each phloem strand.

Thus, the number of cambium strips formed equals the number of phloem strands.

The cells of pericycle lying outside the protoxylem also become meristematic to form part of strips of cambium.

These cambial strips join the first formed cambium strips to form complete, but wavy ring of vascular cambium.

This cambium ring produces secondary xylem on inner side and secondary phloem on outer side.

In roots, the growth rings are not distinct because there is no seasonal variation under the soil.

From the outer layers of pericycle arises the phellogen which cuts phellem (cork) on the outer side and secondary cortex or phelloderm toward the inner side.

Diagrams showing secondary growth in dicot root

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(i) Dicots with scattered vascular bundles in stem, e.g., Podophyllum, Peperomia, Piper, Papaver.

(ii) Cortical vascular bundles, e.g., Nyctanthus, Kalanchoe, Casuarina.

(iii) Medullary bundles, e.g.; Mirabilis, Bougainvillea, Amaranthus, Achyranthus.

(iv) Polystelic condition, e.g., Primula, Dianthera

(v) Anomalous or abnormal secondary growth occurs in Bougainvillea, Boerhaavia, Chenopodium, Aristolochia.

(vi) Some monocots show abnormal secondary growth by meristematic tissue which develops around vascular bundles, e,g., Dracaena, Yucca, Agave .

(vii) Virgin cork is first formed periderm.

(viii) Wound cork: It is secondary meristem; formed below injured area. It forms cork on outer side and callus below; which heals the wound.

(ix) Abnormal secondary growth in dicot root occurs in Beet root (Beta vulgaris) and Sweet potato (Ipomoea batatas) by the formation of numerous accessory rings of cambium which cut more storage parenchyma in secondary phloem and less secondary xylem.

(x) Homoxylous wood-wood of vesselless dicots, e.g., Ranales (Winteraceae, Tetracentraceae, Trochodendraceae) .

(xi) Heteroxylous wood is wood of vessel bearing dicots.

(xii) Polyderm is a special type of protective tissue occurs in roots and underground stems of members of Rosaceae and Myrtaceae. Its outermost layer is dead and suberized.

Summary

Anatomically, a plant is made of different kinds of tissues. The plant tissues are broadly classified into meristematic (apical, lateral and intercalary) and permanent (simple and complex).

Assimilation of food and its storage, transportation of water, minerals and photosynthates, and mechanical support are the main functions of tissues.

There are three types of tissue systems -epidermal, ground and vascular.

The epidermal tissue systems are made of epidermal cells, stomata and the epidermal appendages.

The ground tissue system forms the main bulk of the plant. It is divided into three zones cortex, pericycle and pith.

The vascular tissue system is formed by the xylem and phloem,

On the basis of presence of cambium, location of xylem and phloem, the vascular bundles are of different types. The vascular bundles form the conducting tissue and trans locate water, minerals and food material.

Monocotyledonous and dicotyledonous plants show marked variation in their internal structures. They differ in type, number and location of vascular, bundles.

The secondary growth occurs in most 6fthe dicotyledonous roots and stems and it increases the girth (diameter) of the organs by the activity of the vascular cambium and the cork cambium.

The wood is actually a secondary xylem. There are different types of wood on the basis of their composition ,and time of production.

Secondary Growth

Primary growth refers to the lengthening of roots and stems with the help of the apical meristem. Most dicotyledonous plants expand in girth in addition to main growth. This is referred to as secondary growth. The two lateral meristems, vascular cambium and cork cambium, are engaged in secondary growth.

A. Vascular Cambium:

Vascular cambium is the meristematic layer responsible for cutting off vascular tissues – xylem and phloem. It appears in patches as a single layer between the xylem and phloem in the young stem. It eventually forms a complete ring.

i) Formation of cambial ring:

The intrafascicular cambium is the cambium cells found between the primary xylem and primary phloem in dicot stems. The interfascicular cambium is formed when the cells of medullary rays next to this intrafascicular cambium become meristematic. As a result, a continuous ring of cambium forms.

ii) Activity of the cambial ring:

The cambial ring becomes active and starts cutting off new cells from both the inside and outside. Secondary xylem develops from cells cut off towards the pith, while secondary phloem develops from cells cut off towards the periphery. The inner side of the cambium is often more active than the outer side. As a result, secondary xylem is formed in greater quantities than the secondary phloem, forming a compact mass. The persistent development and accumulation of secondary xylem gradually crushes the primary and secondary phloems. The principal xylem, on the other hand, is mostly intact at or near the centre.The cambium sometimes produces a narrow band of parenchyma that runs in radial directions through the secondary xylem and secondary phloem. These are the secondary medullary rays.

iii) Springwood and autumn wood:

Cambium activity is influenced by a variety of physiological and environmental variables. Climate conditions in temperate zones are rarely consistent throughout the year. The cambium is particularly active in the spring and produces a significant number of xylary components with larger vessels. Springwood, also known as earlywood, is formed during this season. Autumn wood or latewood is formed when the cambium is less active in the winter and produces fewer xylary components with thin channels. Springwood is lighter in color and denser, whereas fall wood is darker and denser. An annual ring is made up of two types of wood that appear as alternate concentric rings.Annual rings in a cut stem can be used to estimate the age of a tree.

Figure 13: Secondary growth in dicot stem

iv) Heartwood and sapwood:

The deposition of organic components such as tannins, resins, oils, gums, aromatic chemicals, and essential oils in the middle or innermost layers of the stem causes the majority of secondary xylem in ancient trees to be dark brown. These compounds make it tough, long-lasting, and resistant to bacteria and insects. Heartwood is a zone made up of dead components with heavily lignified walls. The stem is supported mechanically by the heartwood, which does not conduct water. Sapwood refers to the lighter-colored portion of the secondary xylem. It helps to transport water and minerals from the root to the leaf.

B.Cork Cambium:The outer cortical and epidermal layers break down as the stem grows in girth due to vascular cambium activity, and they must be replaced to create new protective cell layers. As a result, additional meristematic tissue is known as cork cambium or phellogen forms, usually in the cortical region, sooner or later. Phellogen is made up of several layers. It is made up of cells that are tiny, thin-walled, and practically rectangular. Both sides of the cell are cut off by Phellogen. Inner cells differentiate into secondary cortex or phelloderm, whereas exterior cells differentiate into cork or phellem. Due to suberin deposition in the cell wall, the cork is impenetrable to water. Secondary cortical cells are parenchymatous. The periderm is the combination of phellogen, phellem, and phelloderm.

Figure 14: Lenticles

Pressure goes up on the remaining layers peripheral to phellogen due to cork cambium activity, and these layers eventually die and peel off. Secondary phloem is included in the phrase "bark," which is a non-technical term that refers to all tissues outside of the vascular cambium. Periderm and secondary phloem are two forms of tissue that makeup bark. Early or soft bark refers to bark that forms early in the season. Late or hard bark develops toward the end of the season. Instead of cork cells, the phellogen breaks off tightly packed parenchymatous cells on the outer surface in some areas. These parenchymatous cells quickly break the epidermis, generating lenticels, which are lens-shaped holes.Lenticels allow gas exchange between the outside atmosphere and the stem's internal tissue. Most woody trees have these.

Figure 15: Bark

C. Secondary Growth in Roots:

The vascular cambium in dicot roots is entirely secondary in origin. It develops from tissue right beneath the phloem bundles, a piece of pericycle tissue, and above the protoxylem, forming a complete and continuous wavy ring that eventually becomes circular. The next events are similar to those outlined before for a dicotyledon stem. Gymnosperm stems and roots also experience secondary growth. Monocotyledons, on the other hand, do not have secondary growth. Secondary growth usually results in a thickening of the root diameter due to the addition of vascular tissue. When cells in the residual procambium and sections of the pericycle begin to make periclinal divisions, secondary growth begins. Periclinal divisions begin only in the pericycle cells opposite the xylem sites. The vascular cambium is formed by the inner layer of cells. The pericycle is the outer layer that is retained. Around the primary xylem, the vascular cambium is continuous. The periclinal division of the vascular cambium continues. If the daughter cells divide towards the core of the root, they become secondary xylem cells, and if they divide towards the outer surface of the root, they become secondary phloem cells.The periderm, which comes from the pericycle and replaces the epidermis, forms an outer protective layer on certain roots. The pericycle restores its meristematic nature and divides periclinallyonce more and thus forms the phellogen or cork cambium. To the outside of the plant, the cork cambium generates phellum cells (cork cells). At maturity, these cells are lifeless. They've been suberized, which makes the cells water-resistant. Phellum cells in cross-section are neatly organized into files. The phelloderm, a mature tissue made up of live cells, is likewise produced by the cork cambium.

 

Figure 16: Different stages of the secondary growth in a typical dicot root.