Secondary Growth | Science for ACT PDF Download

Primary Growth 

Cells of apical meristems divide, differentiate and develop to form primary tissues, As a result the plant grows in length. This is called primary growth.

Secondary Growth 

•  By the activity of laterel meristems, increase in the circumference/girth of the plant organs due to the formation of secondary tissues in stelar & extra stelar regions, called secondary growth.

•  Normally secondary growth takes place in roots and stem of dicotyledons & Gymnosperms.

•   Due to lack of cambium in monocotyledons, secondary growth is absent. But exceptionally secondary growth takes place in some monocotyledons. Such as - Palm, Yucca, Dracaena, Smilax, Agave, Coconut etc.

Secondary Growth in Dicot Stem

[A] Secondary Growth in Stelar Region

Secondary growth in stelar region begins earlier than the extra stelar region.

I. Formation of Ring of Vascular Cambium
A cambium which is present inside the vascular bundle is called intrafascicular- cambium. This is a type of primary meristem.

• First of all, cells of medullary rays become meristamatic to form interfascicular cambium which is secondary lateral meristem.  Intrafascicular and interfascicular cambium are collectively known as vascular cambium. Vascular cambium is formed in the form of a complete ring which is made up of single layer of cells.

•  In diocot stem, the vascular cambium is partly primary and partly secondary in origin.

• Two types of cells are found in the ring of this vascular cambium.

(i) Fusiform initials
(ii) Ray initials

• Fusiform initials are long with pointed ends whereas ray initials are spherical or oval in shape.

• Fusiform initials are more in number ,in vascular cambium.

II. Activity of Vascular Cambium 

(a) Activity of Fusiform Initials

• Continuous periclinal divisions (parallel to longitudinal axis of a cell) or tangential division takes place in fusiform initials. Then few cells are formed towards the periphery and these cells are differentiated into secondary phloem or bast and some of the cells are formed towards the centre and these cells are differentiated into secondary xylem or wood.

• Normally more secondary xylem is formed as compared to secondary phloem due to unequal distribution of hormones. (Secondary xylem is formed 8-10 times more as compared to sec. phloem).

• By the pressure of secondary phloem, primary phloem is pushed towards the outside and gets crushed.

• By the pressure of secondary xylem, all the primary tissues- such as primary xylem, pith are pushing towards centre. The primary xylem however remains more or less intact in or around the centre. The primary and secondary phloem of the earlier get gradually crushed due to the continued formation and accumulation of secondary xylem. The materials such as tannin, resin, gums, oil, phenols and aromatic substances etc. are filled in lumen  of tracheids and vessels of secondary xylem. Due to this, central region of secondary xylem becomes dark brown. It is called Heart wood or Duramen. The peripheral region of secondary xylem which is light in colour, called Sap wood or Alburnum. The function of sap wood is conduction of water and minerals. Heart wood provides mechanical strength to stem.

Heartwood does not conduct water because:

1. Cavities of tracheids and vessels are progressively filled by waste materials.

2. The bladder like in growth of parenchyma cells which enter the lumen of vessels  (mainly) & tracheids through the pits in their wall.Such bladder like ingrowth are called as tyloses. Tyloses blocks the conduction of water.

(b) Activity of Ray Initial :-

Due to periclinal divisions ray initials cuts off parenchymatous cells. These are called Vascular rays or Sec. medullary rays which passes through the secondary xylem and secondary phloem in the radial direction. They conducts water and food in radial direction. An order of development of vascular rays is both centripetal and centrifugal manner.

(III) Formation of Annual Rings Annual rings are formed due to unequal activity of vascular cambium.

•  The activity of cambium does not remain same, it is changeable in the whole year. Activity of vascular cambium is affected by physiological and environmental factors.

•  In the spring season, vascular cambium is very active and produces a large number of secondary xylem elements having vessels with wider lumen. The wood formed during this season is called spring wood or early wood.

•  In winter, the vascular cambium is less active and forms fewer secondary xylem elements that have vessels with narrow lumen and this wood is called autumn wood or late wood.

•  The spring wood is lighter in colour and exhibits low density where as the autumn (or winter) wood is darker and has higher density.

Note :

(1) The autumn and spring wood are formed in the form of concentric rings, called growth rings.

(2) A ring of autumn wood and a ring of spring wood are collectively known as Annual ring. The number of annual rings, formed in a tree  give the idea of the age of the tree. The study of determination of age of the plant by this techniques is called Dendrochronology.

(3) The annual rings are counted from the base of the stem because basal part has maximum annual rings and upper part has less. Therefore, counting from the basal region can give the correct idea.

(4) A piece is taken from the stem up to central region from the base of stem with the help of increment borer instrument. The annual rings are counted from that piece and again inserted (fitted) into the same stem at the same place.

(5) More distinct annual rings are formed in that regions where climatic variations are sharp.

(6) More distinct annual rings are formed in temperate plants.

(7) Distinct annual rings are not formed in tropical plants. Distinct annual rings are not formed in India except Himalayan regions.

(8) Least distinct annual rings are formed in seashore regions because the climate, remains same through out the year.

(9) More clear annual rings are formed in deciduous plants as compared to evergreen plants.(In temperate region)

(10) In deserts annual rings are less distinct.

(11) Annual rings are bands of secondary xylem and xylem rays.

[B] Secondary Growth in Extra Stelar Region

•  Secondary growth takes place in extra stelar region due to the activity of cork cambium. Cork cambium is also known as Phellogen or Extra stelar cambium. The cells of the cork cambium are narrow, thin walled and nearly rectangular.

•  Cork cambium is derived from the hypodermis or from the outer layer of cortex by differentiation. Cork cambium is formed in the form of a single or few layered ring. It forms sec. tissue in extra stelar region.

•  Cork cambium also divides periclinally. It forms some cells towards the outside (epidermis) and some cells towards the inside (cortex). Those cells formed towards outside, their middle lamella become suberized. Due to this, these cells become dead. These cells are known as Cork or Phellem. Those cell formed towards the inside, are differentiated into parenchyma and may contain chloroplasts. These are called secondary cortex or Phelloderm.

Note :

1. Phellem, phellogen and phelloderm are collectively known as periderm.

2. The cork is impervious to water due to suberin deposition in the cell wall.

•  Cork is formed in high quantity and secondary cortex is in less quantity because, activity of Cork cambium is more towards outside. The highest activity of cork cambium is in winter (Autumn) season.

•  Ring of cork cambium remains living only for one year.Each year, a new cambium is formed below the previous cambium. This new cambium is derived from the secondary cortex or phelloderm.

Note:

All the tissues which occur outside the cork cambium are collectively termed as rhytidome. Rhytidome includes cork and tissues which become dead due to the pressure of cork.

Lenticels : At certain regions, the phellogen cuts off closely arranged parenchymatous cells on the outer side instead of cork cells. These thin walled, rounded, colourless loose parenchymatous cells called complementary cells. These cells are not suberized. As the complementary cell increase in number, pressure is caused against the epidermis and it ruptures, forming a lens-shaped openings called lenticels.
Note:

1. Lenticels are generally formed beneath the stomata or group of stomata of the epidermis. The may be scattered on the stems or they may be arranged in longitudinal or vertical rows. Rows of lenticels may occur opposite the medullary rays, thus facilitating free exchange of gases.

2. Lenticels are formed by the activity of phellogen.

3. In some cases, some of the complementary cells become suberized. eg. Prunus.

Function : 

1. Exchanmge of gases : Lenticels permit the exchange of gases between the outer atomosphere and the internal tissue of the stem.

2. Help in transpiration i.e., Lenticular transpiration.

3. Help in vegetative reporduction : Adventitious roots on cutting originates from the living cells of lenticels in vegetative reproduction.
Note :

•  Lenticels are found in most of the woody trees but they are absent in woody climbers.

•  Lenticels are mainly found on woody stems and they are never found on leaves. They are also present on fruits.

•  If lenticels are blocked then root dies first due to lack of food.

Bark

1. Old view :- All the tissues situated out side the cork cambium is called Bark. According to old view bark includes only dead tissues.

2. Modern view :- Bark is a non-technical term that refers to all tissues exterior to the vascular cambium, therefore including secondary phloem. According to modern view bark includes both living and dead tissues.
Bark has two layers : Outer layer of bark is called Rhytidome and inner layer of bark is secondary phloem.

Kinds of Bark

1. Ring Bark :- Continuous bark of equal thickening is called ring bark. It is formed around the stem in the form of a complete ring. In ring bark cork cambium is continuous.

Example:- Bhojpatra(Betula)- A complete distinct ring bark is formed in this plant. Its bark was used as a writing material /as a paper in ancient period. Ring bark is also formed in Eucalyptus.

2. Scaly Bark :- Dis-continuous bark of unequal thickening is called scaly bark. This bark is formed around the stem in the form of pieces orfragments. In scaly bark the ring of cork cambium is not continuous. Scaly bark is formed in Psidium guajava. Neem (Azadirachta) Mango (Mangifera indica) and Tamarindus etc plants.

Note :

•  Highly distinct scaly bark is formed in Psidium guava.

•  Scaly bark is found in most of the woody plants.

• If bark is removed in the form of ring at the base of main stem then root dies first due to lack of food.

•  If complete bark is removed then plant becomes dead due to excessive water loss.

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.

Secondary Growth In Dicot Root

• First of all, conjunctive tissue becomes meristematic during the secondary growth in a dicotyledon root and form separate curved strips of vasular cambium below phloem bundles. Then after, the cells of pericycle lying opposite to protoxylem also becomes meristematic to form additional strips of cambium. In this way a complete ring of vascular cambium is formed.

• The portion of vascular cambium formed by pericycle is less. The main portion of vascular cambium is formed by conjunctive tissue.

•  The shape of ring of vascular cambium is wavy in the beginning, but later on it becomes circular due to the pressure of secondary xylem.

The portion of vascular cambium formed by conjunctive tissue becomes meristematic first and form the secondary xyelm towards the centre. Ultimately the ring becomes circular by the pressure of secondary xylem (Pushing outwards)

• The activity of vascular cambium of root is the same as the activity of vascular cambium of stem. Secondary xylem is formed towards the inner side and secondary phloem is formed towards the outer side by vascular cambium. The portion of vascular cambium which is formed by 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 rays 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.

Note : The presence of two types of medullary rays is basic characteristic feature of roots. Only secondary medullary rays are found in stem after the secondary growth. Both of them conduct water and food in radial direction.

•  Cork cambium is developed from the pericycle in roots. Cork is formed towards the outside and secondary cortex is formed towards the inner side by the cork cambium. Lenticels are also found in roots but less in number as compared to stem. Cortex completely degenerate in roots after the secondary growth of one or two years. This falls down due to the pressure of cork, whereas in stem, it degenerates after the long duration.

Functions of Secondary Meristem

1. Healing of Wounds :- When any plant part gets injured wound is formed there. Boundary of the wound is raised outside and composed of similar type of living cells (Parechyma) called callus. Living cells of wound are responsible to form a cambium. This is called wound Cambium. It is also called inducible cambium This newly formed cambium forms cork towards the outside. This cork covers the wound entirely. Wound cambium is a secondary lateral meristem.

2. Abscission :-

• Falling of any plant organ is called as abscission.

• Abscission takes place due to formation of abscission layer at the base of plant organ and it is composed of parenchyma.

• ABA induces the formation of abscission layer.

• Middle lamella is dissolved in abscission layer during abscission and primary walls also dissolve partially or completely. Sign of leaf fall on stem is called leaf scar. It is a type of wound.The living cells of leaf scar are responsible to form cork cambium.Which produce cork. Cork covers the wound. At the site of abscission protective layer is found which is suberized.

3. Formation of Knots :- Knot is formed when branches are embedded inside the main stem.

4. Help in Grafting :- Grafting is not possible in monocots because cambium is absent there.

Old NCERT Syllabus

• In some members of plants such as Dracaena, Yucca, Agave, Aloe arboresence, Lomandra, Kingia, Sanseviera, etc. Vascular cambium is formed from the outer region of the ground tissues. Parenchyma is formed towards the  outside by the vascular cambium and secondary vascular bundles are formed toward the inner side.

• In some plants, the girth of the stem increases without cambium. Such as - Palm (Date palm), Musa, Tulipa etc. The apical meristem of these plants is of special type. This is known as primary thickening meristem. This apical meristem is responsible for the growth in both length and girth (thickness) of the plant.

•  Billiards's ball→Phytelophus (Ivory Palm) l

• Violin→ Picea (Spruce)

•  Hardet wood→ Hardwickia binata

•  Heaviest wood → Guaiacum officinale

•  Lightest wood → Ochroma pyramidale or Ochroma lagopus

•  Heaviest wood of india→Acacia sundra

•  Most durable soft wood OR strongest soft wood → Cedrus deodar

 Most durable wood → Tectona grandis

ANOMALOUS SECONDARY GROWTH IN DICOT STEM
1. Anomalus/abnormal position of vascular cambium :- Normally vascular cambium is circular,  but it is folded in stem of some plants later on these folds break and separate from each other. Each fold is responsible to form a complete vascular bundle. Many vascular bundles are formed in stem.
Example :- Thinouia, Serjania, Bauhinia.

2. Abnormal Activity of vascular Cambium :- Generally xylem and phloem is formed from the maximum part of  the vascular cambium and medullary rays are formed from the few parts of vascular cambium. But in some plants parenchyma (Medullary rays)  is formed from the maximum part of the vascular Cambium and rarely in some places xylem and phloem are formed.
Example :- Aristolochia,Vitis vinifera (Grape).

3. Sequential or successive ring of vascular cambium :- In some of the plants, a new ring of vascular cambium is formed each year. This is formed out side the previous ring.
Example :- Mirabilis, Boerhaavia, Bougainvillea etc.

4. Formation of vascular cambium from  pericycle :- Vascular cambium is formed from the pericycle in plants of Amaranthaceae and Chinopodiaceae family. A complete ring of vascular cambium is formed from the pericycle.

5. Interxylary Cork :- A cork layer that develops between the two growth rings of secondary xylem is called interxylary cork.
Example :- Artemesia tridentata.

6. Formation of cork cambium from  Epidermis Example : Malus pumila, Solanum dulcamara, Quercus suber (oak).

7.  The commercial cork is obtained from the plant Quercus suber,which is commonly found in Portugal and parts of Spain

8. Formation of cork cambium from pericycle : – Example - Clematis

9. Formation of cork cambium from phloem :- Vitis and Berberis