6.5 Secondary Growth in Stem:
The growth of roots and stems in width or girth by the activity of lateral meristem is called secondary growth.
The tissues involved in secondary growth are the two types as follow;
Vascular cambium and
Cork cambium.
6.5.1 Vascular Cambium: (Inside growth)
The meristematic tissue layer that is responsible for the formation of secondary xylem and secondary phloem is called vascular cambium.Savinay Kumar
6.5.1.1 Formation of cambial ring:
It is also called Vascular cambial ring.
The ring takes place between primary cambium (intrafascicular cambium) by joining with the help of secondary cambium in medullary region (interfascicular cambium) called cambial ring.
Difference between intrafascicular cambium and interfascicular cambium:
6.5.1.2 Activity of the cambial ring:
The cambial ring becomes active and forms new cells.
The new cells grow towards the pith/center and this is mature into secondary xylem.
The new cells grow towards the periphery and this is mature into secondary phloem.
The cambium is generally more active on an inner side than on an outer side. Due to this, the secondary xylem produced is more than secondary phloem.
Due to continued formation and accumulation of secondary xylem, the primary and secondary phloem gradually get crushed.
But the primary xylem more or less remains intact, in or around the center.
At some places, a narrow band of parenchyma is formed by the cambium. This band of parenchyma passes through the secondary xylem and secondary phloem in radial directions. These are called secondary medullary rays.
6.5.1.3 Spring wood and autumn wood:
The activity of cambium is under the control of many physiological and environmental factors.Savinay Kumar
In temperate regions, the climatic conditions are not uniform throughout the year.
Spring Wood:
It is also known as early wood.
Wood forms during the spring season (early summer).
The cambium is a very active and secondary xylem having vessels with wider cavities (cells are large in size).
The spring wood is lighter in color and has a lower density.
Autumn Wood:
It is also known as late wood.
Wood formed during the winter season (late summer).
It is very less active and the secondary xylem is narrow vessels (cells are small in size).
The autumn wood is darker and has a higher density.
The spring wood and autumn wood that appear as alternate concentric rings constitute an Annual Ring.
Estimate based on the number of annual rings and age of the tree.
Study of the age of the plant is known as dendrochronology.
6.5.1.4 Heart wood and Sap wood:
Heart wood:Savinay Kumar
In old trees, non functional xylem present in the center most region of the stem is called heart wood.
It is dark in color due to the presence of secondary metabolites (organic deposition) like tannins, resins, oils, gums etc.
Microbial resistant wood and gives mechanical support to the plant.
Wood is light and xylem parenchyma is dead.
Sap wood:
It is the young, functional secondary xylem surrounding the heartwood.
It is light in color.
Microbial susceptible wood and helps in the conduction of water and minerals.
Wood is heavy and xylem parenchyma is living.
6.5.2 Cork Cambium:
It is also known as Phellogen.
The cork cambium is a secondary lateral meristem and it is responsible for secondary growth that replaces an epidermis in roots and stems.
Reason:Savinay Kumar
This is formed due to vascular cambium activity inside the stem and root.
This leads to the outer cortical and epidermis layers getting broken and need to be replaced by new protective cell layers.
Structure:Savinay Kumar
Cork cambium is formed in the cortex region.
It is a couple of layers (double layer) and made up of narrow, thin walled and nearly rectangular cells.
It produces cells towards the inner side, that cells are known as secondary cortex or phelloderm.
It produces cells on their outer surface, that cells are known as phellem or cork.
Phellem (Cork), Phellogen (cork cambium) and phelloderm (secondary cortex) together are called periderm.
Functions:
Phelloderm helps in photosynthesis and stores food materials.
Phellogen is a synthesis of Phelloderm (new cells towards inner cortex) and Phellem (new cells towards outer epidermis).
Phellem is impervious to water due to suberin deposition in the cell wall.
6.5.3 Lenticels:Savinay Kumar
They are lens shaped structures appearing beneath the epidermis due to the activities of cork cambium and contain loosely arranged complementary cells.
It helps in exchange of gases and water.
6.5.4 Bark:Savinay Kumar
All the tissues lying outside the vascular cambium are called bark.
Bark generally consists of periderm, primary cortex, primary phloem and secondary phloem.
The bark formed early in the season is called early or soft bark.
The bark formed later in the season is called late or hard bark.
6.5.4.1 Functions of Bark:
It prevents the invasion of pathogens.
It offers protection to the inside tissue.
It prevents evaporation of water.
6.6 Secondary Growth in Roots:
Secondary growth occurs in many roots and usually results in the thickening of the root diameter by the addition ofvascular tissue.
Increases girth or width of the root with the help of lateral meristem. This is a part of secondary growth.
Secondary growth of roots is observed usually in most dicots and gymnosperms.
6.6.1 Vascular Cambium:
The vascular cambium is completely secondary in origin.
It originates from the tissue located just below the phloem bundles.
A portion of pericycle tissue, above the protoxylem forming a complete and continuous wavy ring, which later becomes a circular ring. (Formation of the Cambial ring.)
Note:
Activity of the cambial ring and cork cambium in roots are the same as the stem.
6.7 Difference between Secondary growth in stem and root:
Savinay Kumar
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