Secondary growth in dicot stem

Introduction:

In higher plants, there are two separate growth stages and each of them play a significant role in the plant's life.
The first series of growth involving an increase in the length of roots and stems from the apical meristems is called primary growth.
The growth taking place later on, succeeding the primary growth, which is responsible for the increase of girth or width is termed as secondary growth.
This growth is initiated by the two lateral meristems also known as vascular cambium and cork cambium.
Normally secondary growth takes place in roots and stems of dicotyledons and Gymnosperms.
Due to lack of cambium in monocotyledons, secondary growth is absent. But exceptionally secondary growth has been reported in some monocotyledons. Such as - Palm, Yucca, Dracaena, Smilax, Agave, Coconut etc.

Topics covered:

Introduction to Secondary growth in dicot stem
Formation of Annular rings
Difference between springwood and autumn wood
Difference between heartwood and sapwood
Extra Stelar secondary growth in dicot stem
Secondary growth in dicot root

Introduction to Secondary growth in dicot stem

Introduction:

Secondary growth begins right after the apical meristems have completed the primary growth process with all parts developed properly.
Secondary growth along with dicots happens with gymnosperms as well.
Secondary growth in the dicot stem involves the formation and functioning of the vascular cambium and cork cambium.
Secondary growth provides the plants or trees with protection against mechanical stress and microbial activity.
Secondary growth increases the girth of the stem.
In the case of woody plants, the major contribution is made by secondary tissues.

Secondary growth in dicot stem can be categorised as-

I. Intrastelar secondary growth- It involves the formation of vascular cambium in the stelar region and its activity resulting in the formation of secondary xylem, secondary phloem and secondary medullary rays

II. Extra stelar secondary growth- It involves the formation and activity of cork cambium in the extra stelar region leading to the formation of cork and secondary cortex.

Detailed explanation:

(I) Intra stelar Secondary growth:

The growth in this region starts earlier than the extra stelar region.

A. Formation of ring of vascular cambium:

A cambium that lies inside the vascular bundle is called intrafascicular cambium.
This is a type of primary meristem.
Firstly the parenchymatous cells of medullary rays in the line with intrafascicular cambium become meristematic and develop a new cambium called as interfascicular cambium, which is secondary lateral meristem.
Intrafascicular and interfascicular cambium are joined to form vascular cambium.
The latter is formed in the form of a complete ring composed of a single layer of cells.
In dicot stem some part of vascular cambium is primary and some part is secondary.
Two types of cells are found in the ring of this vascular cambium.
(i) Fusiform initials (form secondary xylem and phloem)
(ii) Ray initials (form secondary medullary rays)
Fusiform initials are long with pointed ends, while ray initials are isodiametric cells. Activity of fusiform initials is more in vascular cambium.

B. The activity of the cambial ring

I. Activity of fusiform initials:

Periclinal divisions occur in fusiform initials, as a result few cells are formed towards the radius (periphery) differentiated into secondary phloem or bast and some of the cells are formed towards the central axis and these cells are differentiated into secondary xylem or wood.
Secondary xylem is formed 8-10 times more as compared to secondary phloem.
By the pressure of secondary phloem, primary phloem is pushed towards the outside and gets crushed.
Primary xylem however remains more or less intact, in or around the center.

II. Activity of ray Initials:

The ray initials give rise to a narrow band of parenchymatous rays in radial direction known as medullary rays.
The secondary medullary rays or vascular rays are often rows of radially arranged cells which pass through the secondary xylem and secondary phloem.
The primary and secondary rays conduct food,water and minerals from the center to the periphery of the organ in radial directions.

Formation of Annular rings

Introduction:

The two separate kinds of woods that appear as alternate concentric rings together constitute an annual ring or growth ring.
The cambium activity is not equal throughout an entire year.
The activity can be altered and changed due to various physiological and environmental factors.
Hence, we classify woods and their characteristics based on the impact of the environment.

Detailed explanation:

The temperate regions from North America, Europe and Asia have a variety of climatic conditions and they change every season.
The wood produced in each season has specific characteristics due to different cambial activities.

A. Springwood/ Early wood-

During the spring season, the cambium is in a very active state which is the reason for the higher production of xylary elements which include xylem parenchyma, xylem fibers, tracheids and vessels with wider vessel cavities.
The reason for wider cavities is the water loss due to transpiration is maximum in summer and hence it needs to be replenished with more and more water.
The wood that is formed in the spring season is termed as earlywood or springwood.
Springwood is much lighter in color and has low density.

B. Autumnwood/ Late Wood -

Now during the winter season, the activity of cambium reduces to a large extent and the wood produced during that time is called Autumn wood.
This reduction results in lesser xylary elements, which are narrow compared to summer because of much less requirement of water.
And thus, the wood formed in this season is termed as the latewood or autumn wood.
Xylem of this season has more thickening with small lumen and it is dark coloured(having higher density).

C. Annular rings-

The two types of wood the spring and autumn wood alternate in a dicot stem appearing like concentric rings which are together known as Annular rings/growth rings.
These rings alternate according to the season and they are of lighter and darker shades.
Now one dark (autumn wood) ring and one light (springwood) ring form a single annular ring or they are also known as growth rings.
The transition from spring to autumn wood in an annual ring is gradual but the transition from autumn wood to spring wood of the next year is sudden. Hence, the growth of each year is quite distinct.
This means one spring wood ring and one autumn wood ring together represent a year.
The rings keep on increasing as the tree grows and counting them can give us the idea of the age of the tree.
When a tree is cut there are bands that are counted to determine its age and this is known as Dendrochronology.
In tropical regions, the cambium is almost equally active throughout the year, thus there is no distinction like spring & autumn wood. The whole wood consists of uniform xylem elements, thus no annual rings are found. e.g. Chennai, Mumbai.
More distinct annual rings are formed in temperate deciduous plants. In deserts annual rings are less distinct.

Difference between springwood and autumn wood

The wood that is formed in the spring season due to high cambium activity is termed earlywood or springwood.
In the winter season the activity of cambium reduces to a large extent and the wood produced during that time is called autumn wood or latewood.

S. No.	Spring wood	Autumn wood
1.	This wood grows during the favorable season of growth.	This wood grows during the unfavorable season of growth.
2.	The cambium activity is higher.	The cambium activity is much lower.
3.	This wood majorly contributes to annual rings.	This wood forms narrow strips in the annular rings.
4.	This wood is formed at the beginning of the year.	This wood is formed much later at the end of the year.
5.	The xylary elements are larger and wider.	The xylary elements are narrower and smaller.
6.	Fibers are lesser.	Fibres are abundant.
7.	The wood is lighter in color.	The wood is darker in color.
8.	The density of xylem elements is lower.	The density of the xylem is higher.

Difference between heartwood and sapwood

Introduction:

After a few years of secondary growth, the central part of xylem becomes dark coloured whereas the peripheral part becomes light coloured.
The dark coloured, central xylem (wood) is called heart wood and light coloured, peripheral xylem (wood) is called sap wood.

Detailed explanation:

The heart wood is non-functional because its elements get blocked due to the deposition of various organic substances like oil, gum, resins, tannins, volatile oil etc. and are unable to do the function of conduction.
Heart wood is very hard and resistant to the harmful effects of bacteria, fungi and moisture.
Heartwood is also called duramen while sapwood is also known as alburnum.
Sapwood performs conduction of water & minerals.
The bladder-like ingrowth of parenchyma cells which enter the lumen of vessels (mainly) & tracheids through the pits in their wall. These are called tyloses.
Tyloses block the conduction of water. In gymnosperms tylosoids are formed in place of tyloses.
If the heart wood is destroyed in any stem, then there will be no effect on plants (any vital function is not affected), but if the sap wood is destroyed, then the plant will die due to blocking of conduction of water.
Heart wood provides stiffness to the stem.
The waste materials of heart wood are antiseptic in nature. Heart wood has a power of repelling insects, so it is resistant to the termites.Thus it is the best quality of wood used in manufacturing of furniture.

S. No.	Heartwood	Sapwood
1.	This wood is centrally located.	This wood is located towards the periphery.
2.	Heartwood is dark or tan coloured.	Sapwood is light in colour.
3.	This consists of all dead elements.	This consists of living elements.
4.	TThis is the non-functional region of the stem because of dead elements.	This is the functional region of the stem.
5.	Presence of tyloses, i.e. bulged ingrowth of xylem parenchyma into the tracheids.	Tyloses absent.
6.	The tracheids are filled with organic compounds like resins, oils, tannins, gums and aromatic substances.	There is no such deposition in this wood.
7.	The heartwood is of higher density and heavy.	The sapwood is of lighter density and hence lightweight.
8.	Because it is composed of mostly dead elements and found rather at the centre, microbial attack or pathogenic attack is not common in this region. And hence this wood is much more durable.	This region is of living cells and found towards the periphery and hence susceptible to pathogenic attacks. Making it less durable.

Extra Stelar secondary growth in dicot stem

Introduction:

Extrastelar secondary growth is provided by a specific lateral meristem called the cork cambium.
Due to secondary growth in vascular bundles, the girth of the stem keeps on increasing which leads to damage of the epidermis and cortex.
In order to replenish those layers, the cork cambium is necessary.

Detailed explanation:

A. Phellogen/ Cork cambium-

While the stem girth increases the cork cambium also works simultaneously in order to replenish the damaged epidermal and cortical layers.
These two layers are the major protective layer for any plant and hence they need to be in proper condition throughout a plant’s life for the plant to survive mechanical stress and pathogen activity.
Because the cork cambium lies right outside the stele it is known as extra-stelar cambium.
Cork cambium or Phellogen is multi-layered and thick, it is composed of narrow rectangular cells which are thin-walled.
Cork cambium usually originates from the outer layer of cortex because the latter becomes meristematic.
Cork cambium is formed in the form of a single layered ring.
It forms secondary tissues in the cortical region.
Phellogen layer divides and produces cells on either side. The cells that formed on the outer side get differentiated into cork or phellem. Whereas, the cells formed on the inner side differentiate into the secondary cortex or phelloderm.
The cork cells are dead due to deposition of suberin in their middle lamella and are impervious to water.
Phelloderm is made up of thin-walled parenchymatous cells which are living and exhibit cellulosic cell walls.
Phellogen, Phellem, Phelloderm are altogether known as Periderm.
All these layers contribute to the protective layers of the epidermis and cortex after they get damaged due to secondary growth of the vascular bundles.
As the cork cambium adds up cells into the phellem and phelloderm, the cells peripheral to phellogen die and fall off.

B. Bark

All the tissues that exist peripheral to vascular cambium are considered as a part of bark.
Bark is not a scientific term, it can represent various tissues for example secondary phloem, periderm, primary cortex, pericycle etc.
The dead tissue present outside the cork cambium is generally called outer bark & the tissues present between vascular and cork cambium constitutes the inner bark.
All the dead tissues formed outside the innermost cork cambium are called rhytidome.
The early formed bark, i.e., at the beginning of a season is known as soft bark.
The later formed bark, i.e., at the end of a season is known as hard bark.

C. Lenticels

At places, the phellogen cuts off closely packed parenchyma cells on the outer side instead of the cork.
These cells rupture the epidermis and form a lens-like opening on the outer surface of the stem. These openings are known as lenticels.
These structures are prominently found on woody trees.
Function: Lenticels mainly perform the function of exchange of gases between plants and the atmosphere. They are also responsible for lenticular transpiration.

Secondary growth in dicot root

Introduction:

The dicot root also exhibits secondary growth.
The increase in width of a dicot root takes place due to two different lateral meristems, i.e., vascular cambium and cork cambium.
The vascular cambium and cork cambium are formed as a result of secondary growth.
The process of secondary growth in dicot roots is accomplished in two steps, a) Formation and activity of the vascular cambium b)Formation and activity of the cork cambium.

Detailed explanation:

A. The Formation and activity of the vascular cambium

The vascular bundles are arranged in a radial manner.
The xylem and phloem are distinct from each other in separate patches.
The xylem is of exarch type.
The vascular cambium forms as a result of secondary growth hence it is not present right from the beginning.
Conjuctive tissue becomes meristematic below phloem bundles and form separate curved strips of vascular cambium during the secondary growth in a dicotyledon root.
Now the cells of the pericycle lying opposite to protoxylem also become meristematic to form additional strips of cambium.
In this way a complete ring of vascular cambium is formed due to joining of these two.
The shape of the ring of vascular cambium is wavy in the beginning, but later on it becomes circular due to the pressure of the secondary xylem.
The activity of vascular cambium of root is similar to activity of vascular cambium of stem.
Vascular cambium forms secondary xylem towards the inner side and secondary phloem towards the outer side.
The portion of vascular cambium which is formed by the pericycle is responsible for the formation of pith rays. These are made up of parenchyma. These pith rays are known as primary medullary rays (Multiseriate).
A few medullary or pith rays are also formed from remaining vascular cambium. These are called secondary medullary rays (uniseriate). Thus two types of medullary rays are found in the secondary structure of roots.

B. The formation and activity of the cork cambium

The pericycle cells divide to generate the cork cambium.
Thereafter the cork cambium gives rise to the periderm, which is responsible for replacing ruptured epidermis and cortical layers.
The cork cambium in root is a lot similar to the cork cambium in the stem. This cambium forms a dead cork towards the periphery and the living, parenchymatous secondary cortex towards the centre.
Pressure builds upon the peripheral layers of the phellogen, that is the epidermis and the cortical layer as a result they die and finally slough off.

Frequently asked questions (FAQ)

Q1. Why is secondary growth important?
Ans: Secondary growth makes the stems and inner structures get thicker and stronger, therefore, enabling them to support the weight of the tall angiosperms. Secondary growth forms wood that adds strength and mechanical support.

Q2. Describe the role of cork cambium in brief?
Ans:

The pericycle cells divide to generate the cork cambium.
Thereafter the cork cambium gives rise to the periderm, which is responsible for replacing ruptured epidermis and the cortical layers.
This cambium forms dead and suberised cork towards the periphery and the living, parenchymatous secondary cortex towards the centre.

Q3. Describe extra-Stelar cambium.
Ans:

As the cork cambium lies right outside the stele it is known as extra-stelar cambium..
Cork cambium or Phellogen is multi-layered and thick, it is composed of narrow rectangular cells which are thin-walled..

Q4. What is Bark and its types?
Ans:

Bark is not a scientific term, it can represent various tissues for example secondary phloem, periderm, primary cortex, pericycle etc.
The early formed bark, i.e. at the beginning of a season is known as soft bark.
The later formed bark, i.e. at the end of a season is known as hard bark.

Q5. How are annular rings formed?
Ans: The two separate kinds of woods that appear as alternate concentric rings are together known as annual Rings. The cambium activity is not equal throughout an entire year. The wood produced in each season has specific characteristics due to different cambial activities. One spring wood and one autumn wood produced in a year together constitutes an annual ring/growth ring.

Q6. Importance of secondary growth?
Ans:

In the case of woody plants, the major contribution is made by secondary tissues.
Secondary growth provides the plants or trees with protection against mechanical stress and microbial activity.
Secondary growth increases the girth of the stem.
It helps to replace the old conducting tissues with the new ones in order to meet the increased demand for long distance transport of sap and organic nutrients.

Q7. Describe springwood and its features?
Ans: The wood that is formed in the spring season is termed as earlywood or springwood.

During the spring season, the cambium is in a very active state which is the reason for the higher production of xylary elements which include xylem parenchyma, xylem fibres, tracheids and vessels with wider vessel cavities.
The reason for wider cavities is the water loss due to transpiration is maximum in summer and hence it needs to be replenished with more and more water.
Springwood is much lighter in colour and has low density.

Q8. What do you mean by heartwood?
Ans: Heartwood is the central region in the secondary xylem with mostly dead elements and highly lignified walls. The tracheids are filled with organic compounds like resins, oils, tannins, gums and aromatic substances. Due to all this deposition the core gets very strong in this kind of wood.

Q9. What is dendrochronology?
Ans: When a tree is cut there are bands that are counted to determine its age and this is known as Dendrochronology. The springwood ring and one autumn wood ring formed in a year together represent an annual ring/growth ring. The rings keep on increasing as the tree grows and counting them can give us the idea of the age of the tree.

Q10. What are lenticels and mention their function?
Ans: At places, the phellogen cuts off closely packed parenchyma cells on the outer side instead of the cork. These cells rupture the epidermis and form a lens-like opening on the outer surface of the stem. These openings are known as lenticels. These structures are prominently found on woody trees. Lenticels mainly perform the function of the exchange of gases between plants and the atmosphere. They are also responsible for lenticular transpiration.