Secondary Growth in Dicot Stem & Root Notes | Free Biology Notes

This article we will discuss about Secondary Growth in Dicot Stem & Root Notes

• By the activity of lateral meristems, formation of secondary tissue which leads to increase in girth is called secondary growth
• Two types of lateral meristems involved in secondary growth:- vascular cambium and cork cambium
• Secondary growth takes place in root and stem of dicotyledons and gymnosperms
• Secondary growth in stellar region due to activity of the vascular cambium
• Secondary growth in extra stellar region due to activity of cork cambium

Secondary Growth in Dicot Stem

Secondary growth in stelar region by vascular cambium

Formation of cambium ring

• Cambium is present between xylem and phloem. It is known as intrafascicular cambium
• During secondary growth, some cells of medullary rays become active and form a strip of cambium in between vascular bundles called interfascicular cambium
• Both the intrafascicular and interfascicular cambium unite together to form a complete ring called the cambium ring
• Two types of cells are found in cambial ring:- fusiform initials & ray initials

Activity of cambial ring

I) Fusiform initials

• Numerous cells formed towards the centre and periphery regions
• Cells in centre mature into secondary xylem & cells in periphery mature into the secondary phloem
• Secondary xylem is formed 8-10 times more as compared to secondary phloem

II) Ray initials

• It’s give rise to parenchyma cells is called secondary medullary rays
• Development of vascular rays are both centripetal and centrifugal
• They conduct water and food in radial direction

Formation of annual rings

• Activity of cambium is affected by variations in temperature

In spring season

• Vascular cambium is very active and produces large number of secondary xylem
• Secondary xylem elements having vessels with wider cavities
• Wood formed during this season is called spring wood or early wood
• Spring wood is lighter in colour and has lower density

In autumn season

• Vascular cambium is less active and forms fewer secondary xylem
• Secondary xylem elements having vessels with narrow cavities
• Wood formed during this season is called autumn wood or late wood
• Autumn wood is darker in colour and has higher density
• Ring of autumn wood & spring wood are collectively known as annual ring
• The number of annual rings formed in a tree give the idea of age of the tree
• The study of determination of age of a tree is called dendrochronology
• Clear annual rings are not formed in India except Himalayan regions

Formation of heart wood and sap wood

• In old trees where sufficient amount of secondary growth has taken place
  the secondary wood of inner side lose the power of conduction
• Their cells get filled with tannins, resins, gums, essential oils
• These cells makes the plant part hard and darker called the heart wood or duramen
• Heart wood ceases the function of conducting tissue and simply provides mechanical support
• The outer region of secondary wood, which consists of younger living xylem cells, remains yellow in colour called the sap wood or laburnum
• It functions as the conducting tissue and also as the food storage tissue

Secondary growth in extra stellar region by cork cambium

• Cork cambium is also known as phellogen or extrastelar cambium
• Cork cambium is develops from cortex by dedifferentiation
• It’s form new cells towards outside and towards inside
• Those cells which are formed towards outside, become suberized.
• These cells become dead cells are known as cork or phellem
• Those cells which are formed inside, are differentiated into parenchyma. These are called secondary cortex or phelloderm
• Phellem, phellogen and phelloderm are collectively known as periderm
• Cork cambium is more active towards outside, cork is formed in high quantity and secondary cortex is less quantity
• At certain regions, the cork cambium cuts off and forms parenchymatous cells on outer side instead of cork cells
• These parenchymatous cells are not suberized. These cells are called complementary cells
• Complementary cells increase in number, pressure is exerted on the epidermis due to which it ruptures, forming lens shaped openings is called lenticels & Its helps exchange of gases and transpiration

Secondary growth in dicot root

• Secondary growth in dicot roots is essential to provide strength
• It is similar to secondary growth in dicot stem
• However, there is marked difference is manner of the formation of vascular cambium

Origin and activity of vascular cambium

• Vascular cambium is completely secondary in origin
• It originates from conjunctive tissue located just below the phloem and as a portion of pericycle tissue present above the protoxylem
• Its form a complete and continuous wavy ring
• This wavy ring produces secondary xylem and secondary phloem similar to the secondary growth in stems

Origin and activity of cork cambium

• Cork cambium is developed from pericycle in roots
• Cork is formed towards the outside and secondary cortex is formed towards inner side by the cork cambium
• Lenticels are also found in roots but less in number
• Clear annual rings are not seen in root because root are not affected by the changes of environment
• In dicot roots all cambia and pith rays are secondary in origin