Secondary Growth

SECONDARY GROWTH

Secondary Growth:

• By the activity of laterel meristem (vascular cambium and cork cambium). Increase in the circumference girth/ thickness of the plant organs is called secondary growth.

• Normal secondary growth takes place in roots and stem of dicotyledons and gymnosperms.

• The tissues involved in secondary growth are two lateral Meristems: vascular cambium.and.cork.cambium.

• Secondary growth is not found in the leaves and monocots.

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

SECONDARY GROWTH IN DICOT STEM 

A. SECONDARY GROWTH IN STELAR REGION BY VASCULAR CAMBIUM

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

Formation of ring of vascular cambium- A Cambium which is present inside the vascular bundle (Means between primary xylem  meristem and primary phloem) is called intrafascicular cambium This is a type of primary lateral Meristems.

• First of all,cells of medullary  rays adjoining intrafasicular cambium become meristamatic to form interfascicular cambium which is a secondary lateral meristem.

intarafasicular and interfasicular cambium are collectively known  as vascular cambium or intra stelar Vascular cambium.

Vascular cambium is  formed in the form of a complete ring which made up of single layer of cells. 

In dicot 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.

1. Fusiform initials

2.Ray Initials

1.Fusiform initials are long with pointed ends, whereas ray initials are spherical/rounded or oval in shape. Fusiform initials are more in number in vascular cambium.

Activity of vascular cambium:

(a) Activity of fusiform initials:

Continuous perclinal divisions (parallel to longitudinal axis) takes place in fusiform then few cells are formed towards  the periphery and these cells are differenciated into secondary phloem or bast and the cells which are formed towards the  centre (towards pith) are differenciated into secondary xylem or wood.

The cambium is generally more active on the innerside than on the outer.

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

Different stages of secondary growth in Dicotyledonae stem

By the pressure of secendary xylem, all the primary tissues such as primary xylem, pith are pushed towards  the centre.

The primary xylem however remain more or less intact in around the center. The primary phloem and earlier secondary phloem (old secondary phloem) get gradually crushed due to the continued formation and accumulation of secondary xylem.

Activity of Ray Initials :- Due to periclinal division ray initials cuts off (form) parenchymatous cells. These are called vascular rays (Xylem rays & phloem ray) or secondary medullary rays which passes through the secondary xylem and secondary phloem in the radial direction.They conduct water and food in radial direction. The order of development of vascular rays are both centripetal and centrifugal.

a) 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 chargeable in the whole year

• Activity of vascular cambium is under the control of many physiological and environmental factors .
•  In temperate regions,the climatic conditions are not uniform through the year.

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

• The spring wood is lighter in colour and has a lower density whereas the autumn (or winter) wood is darker and hes a higher density.

Note:-The autumn  and  spring wood are formed in the form of  concentric rings called growth rings.

• The two kinds of wood that appear as alternate concentric rings, constitute an annual ring.

• 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 a tree/plant by counting annuals rings is called Dendrochronology.

• A piece of wood start the stem up to in that 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 as the same place.

• More distinct/clear annual rings are formed in that regions where climatic variations are sharp 

• More distinct annual rings are formed in temperate plants Because in temperate  regions, the climate conditions are not uniform  throughout the year.

• Distinct annual rings are not found in tropical plants. Distinct/clear annual rings are not formed in India except Himalayan regions (Shimla, Nainital ete).

• Least distinct annual rings are formed in seashore regions/coastal regions because the climate remains the same throughout the year.

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

•  In deserts annual rings are less distinct.

• In annual rings bands of secondary xylem and xylem rays (Ray parenchyma) are present.

HEART WOOD & SAP WOOD:

In old trees the greater part of secondary xylem is dark brown.

The organic compounds like tannins, resins, gums oils 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. These substances make it hard, durable and resistant to the attack of micro-organisms and insects. Heart wood comprises dead elements with highly lignified walls. Heart wood. provides mechanical strength to stem

• The peripheral region of secondary xylem which is light in colour, is called sap wood or alburnum.
• The function of sap wood is conduction of water and minerals.

• Heart wood does not conduct water because :

• Cavities of tracheids and vessels are progressively filled with waste materials.

The bladder/balloon like ingrowth of parenchyma cells enter in the lumen of vessels (mainly) & tracheids through the pits. Such bladder like ingrowths are called tyloses or tracheal plugs. Tyloses block the lumen of tracheary elements (vessels & tracheids).

Heart wood provides stiffness to the stem. The waste materials of heart wood are antiseptic in nature. Heart wood is resistant to the attacks of termites and insects and in rainy season it does not imbibe water. Thus it is the best quality of wood.

• Study of wood is known as Xylotomy. The wood is actually a secondary xylem.

• Position of youngest secondary phloem is just outside the vascular cambium.

• Position of oldest secondary phloem is just inside the primary phloem.

• Position of youngest layer of secondary xylem is just inside the vascular cambium.

• Position of oldest layer of secondary xylem is just outside the primary xylem.

• As the time passes amount of heart wood increases more as compared to sap wood.

Classification of wood:-

 A. On the basis of ammount of parenchyma wood is classified into two groups- 

1. Manoxylic wood- such type of wood contains more amount of living parenchyma. It is loose wood eg. Cycas.

2. Pycnoxylic wood- such wood contains less amount of living parenchyma.

Example- Pinus (conifers).

B. Classification based on vessels-

On the basis of presence and absence of vessels, wood is classified into two categories:-

1. Non- porous wood/Homoxylous wood- vessels are absent in this type of wood

 Example:- mostly gymnosperms.

2. Porous wood/ Heteroxylous wood- vessels are present in this type of wood .eg. mostly dicots (angiosperms) on the basis of arrangement of vessels porous wood is divided into two groups.

a. Ring porous wood- vessels are arranged in the rings in this type of wood.

Example:- in temperate region plants ex. Dalbergia.

b. Diffused porous wood- Asystematical distribution of vessels is found in this type of wood.

 Example:- in tropical region plants ex. Azadirachta ( neem).

Most durable wood - Tectona grandis (Teak= sagwan).

SECONDARY GROWTH IN EXTRA STELAR REGION BY CORK CAMBIUM

• Secondary  growth takes place in extra stelar region due to the activity of cork cambium.  Cork cambium is also known as phellogen or extrastelar cambium. the cells of the cork cambium are narrow, thin walled and nearly rectangular. Cork cambium develops usually in cortical region by hypodermis.

• As the stem continues to increase in girth due to activity of vascular cambium the outer cortical and epidermal layers get broken and need to be replaced to provide new protective cell layers. Hence sooner or later another meristamatic tissue called cork cambium or phellogen develops.

• Cork cambium is derived form the hypodermis (outer part of cortex) by dedifferentiation. Cork cambium is single or a couple of layers thick (mainly). It forms secondary tissues in extra stelar region.

• Cork cambium divides periclinally to form some cells towards the outside (towards epidermis) and some cells towards the inside (towards general cortex). Those cells which are formed towards outside become suberized. Due to this, these cells become dead. These dead cells are known as cork or phellem. Those cells which are formed towards the inside are differentiated into parenchyma and may contain chloroplast. These are called secondary cortex or phelloderm.

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

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

• Commercial cork is obtained from  Quercus suber (oak). Common bottle  Cork is made from this cork. 

• Due to activity of the cork cambium, pressure builds up on the remaining layers peripheral to phellogen and ultimately these layers dies and slough off.

• Ring of cork cambium remain living and active only for one year. each year, a new cork cambium is formed below the previous cambium. this new cambium derived from the secondary cortex or phelloderm.

• all the tissue which occur outside the innermost cork cambium are collectively termed as rhytidome. rhytidome include cork and tissue which become dead due to presence of cork. 

Lenticels : At certain regions, the phellogen (cork cambium) cuts off forms closely arranged parenchymatous cells on the outer side instead of cork cells. These thin walled, rounded, colourless, parenchymatous cells are called complementary cells. These cells are not suberized. As the complementary cells increase in number, pressure is exerted on the epidermis to which it ruptures, forming a lens-shaped openings called lenticels.

Complementary cells are formed by the activity of phellogen cork cambium.

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

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

•  Lenticels are not found in herbaceous dicots and monocot plants.

 Functions :

Exchange of gases  Lenticels permit the  Exchange of gases  between the outer atmosphere and the internal tissues of the stem. Main function 

Help in transpiration i.e. Lenticular transpiration.

BARK 

There are two views about the bark.

 Old view All the tissues situated outside the cork cambium are called bark. According to old view bark includes mainly dead tissues.

 Modem view Bark is non-technical term  that refers to all tissues exterior to vascular cambium, therefore including secondary phloem.  According to modern view bark includes both living and dead tissues. 

KIND OF BARK 

1.Ring Bark - Sheet bark continuous  Bark of equal thickening is called ring bark, It is formed around the stem in the form of a complete ring. When cork cambium in continuous then ring bark is formed. Example Bhojpatra (Betula utilis) A complete distinct ring bark is formed in this  plant. its bark was used as writing material/as a paper in ancient period, only cork layer was used. Ring bark is also formed in Eucalyptus.

2.Scaly Bark Discontinuous bark of unequal thickening is called scaly bark. This bark is formed around the stem in the form of pieces or fragments or patches.  When cork cambium is discontinuous then scaly bark is formed  eg. Guave (psidium guajava), Neem (Azadirachta indica), Mango (Magnifera indica), and Tamarind=Imli (Tamarindus) etc. Plants.

• Highly distinct scaly bark is fromed in Psidium guajava (Guava).

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

• If bark is removed in the form of a ring (Girdling) from the base of main stem then root dies first due to lack of food.

• Girdling is not possible is monocot stem because vascular bundles are scattered.

• If complete bark is removed then plant dies 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 phloem and periderm are included in bark.

SECONDARY GROWTH IN DICOT ROOT

In the dicot root, the vascular cambium is completely secondary in origin.

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

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

• The shape 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 forms the secondary xylem towards the centre. Ultimately the cambium becomes circular by the pressure of secondary xylem.

The activity of vascular cambium of root is the same as the activity of vascular cambium of stem. Secondary xylem is formed towards the innerside 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 or medullary 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 : Two types of medullary rays are formed in the dicot roots during secondary growth.The presence of two types of medullary rays is basic characteristic feature of roots. Only secondary medullary rays are formed in dicot stem during 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 degenerates 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.

1.Secondary growth is essential in roots to provide strength to the growing aerial parts of the plants and fulfill the requirement of water and minerals.

2.Generally clear annual rings are not seen in roots because roots are not effected by the changes of environment.

3.Secondary growth is not found in monocot roots.

In dicot roots, all cambia and pith rays (medullary rays) are secondary in origin.

THANK YOU FOR VISITING....:)