Secondary Growth in Dicot Stem - Practice Questions & MCQ

Secondary Growth in Intrastelar Region of Dicot Stem

• The vascular bundles present in the Dicot stem are conjoint, collateral and open.
• That means, they have a fascicular or intrafascicular cambium present the primary xylem and primary phloem.
• The intrafascicular cambium is primary meristem having lateral position in the plant body.
• Events During the Intrastelar Secondary Growth in Dicot Stem:

Formation of Cambium Ring

• The intrafascicular cambium gets activated to perform meristematic activity.
• The parenchymatous cells of the pith rays that are present adjacent to the intrafascicular cambium dedifferentiate to become meristematic.
• The meristem so formed through the dedifferentiation of the cells of the pith rays is called secondary meristem or interfascicular meristem.
• The interfascicular meristem joins with the strip of intrafascicular meristem to form a complete cambium ring or vascular cambium.

2. Formation of Secondary Vascular Tissues

• The vascular cambium produces secondary xylem towards inside, that is, towards the pith.
• It produces secondary phloem towards the outside.
• The continuous formation of secondary xylem crushes the primary phloem. The primary xylem remains intact at some places.
• The vascular cambium also produces radial parenchymatous cells that passes through the secondary xylem and secondary phloem. These are called secondary medullary rays.
• The secondary xylem has axial and radial (horizontal) system while radial system is absent in primary xylem.
• Primary xylem has long vessels and tracheids as compared to secondary xylem.
   

Secondary Growth in Intrastelar Region of Dicot Stem - Annual Rings

• The activity of the cambium is markedly affected by environmental fluctuations.     
• In temperate regions, these fluctuations have a great role in defining the activity of the cambium. 
• During the spring season, the activity of thee cambium increases. 
• It produces a large number of xylary elements with wider lumen.
• The wood formed during this season is called spring wood or early wood.
• During the autumn season, the activity of cambium reduces.
• It produces narrow vessels in secondary xylem. A large amount of xylem fibres are produced.
• The wood formed during the autumn season is called autumn wood or late wood.
• The early wood and late wood occur in the form of two concentric rings, forming a single annual ring.
• An annual ring is called so because it is produced in a span of a year.
• The annual rings reflect the age of trees.
• Determination of age of tree by counting the annual rings in called dendrochronology.
• The activity of cambium is also affected by disease, drought, defoliation, etc. In such conditions false rings can be observed.
• In tropical trees, there is no annual rings formation because of uniformity in climate.
• In roots, the annual rings are not visible because inside the soil the temperature remains constant throughout the year.
• The roots of Salix are an exception because the annual rings are visible in them.
   

Secondary Growth in Intrastelar Region of Dicot Stem - Heartwood and Sapwood

• When a transverse section of a woody trunk is taken, two regions are distinctly visible.
• In the centre, there is dark coloured wood called heartwood. 
• Heartwood is made up of dead and non-functional components of wood. 
• Heartwood is also called duramen.
• The lumen of vessels in the heartwood has been blocked by tyloses.
• Tyloses are balloon-like structures formed due to protrusion of xylem parenchyma cells into the lumen of vessels.
• These parenchyma cells become lignified and dead, thereby making the vessels non-functional.
• In the tracheids of gymnosperms, the tylosoids are present.
• The heartwood is durable and resistant to parasitic and microbial attack. 
• It gives mechanical support to the tree trunk.
• It appears dark due to deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils.
• In the periphery of the tree trunk, surrounding the heartwood, there is lighter colored wood made up of functional xylem elements.
• This is called sapwood. It is involved in the conduction of sap. 
• Sapwood is also called alburnum.
   

Secondary Growth in Extra Stelar Region

• The continuous secondary growth in the intrastelar region adds more and more secondary xylem and secondary phloem.
• This exerts pressure on the tissues lying towards the outer side, that is, cortex and epidermis.
• The epidermis ruptures due to the exertion of the pressure and the underlying tissues are exposed to the environment.
• At this stage, a secondary tissue called periderm develops.
• Periderm develops through the activity of cork cambium or phellogen.
• Cork cambium or phellogen is the secondary meristem that develops generally from the collenchymatous cells of hypodermis, sometimes from the cells of the epidermis (apple), pericycle (Cimatis), phloem (vitis).
• Phellogen gives rise to cork or phellem on the outer side and secondary cortex of phelloderm on the inner side.
• Phellem or cork is the dead tissue with suberised cell walls. It is impermeable to water and is used commercially. 
• Quercus suber is commercial cork.
• Secondary cortex or phelloderm is living tissue. It stores food and replaces the damaged primary cortex. 
• Phellem, phellogen and phelloderm are called periderm.
• Periderm and fragments of epidermis form the protective tissues in an old dicot stem.

Bark:

• This term is used to refer to all the tissues outside the vascular cambium.
• It includes secondary phloem, periderm and epidermis.
• Removal of bark , called the ringing experiment, causes serious injury to the plant because it removes the secondary phloem which restricts the conduction of food.
• Roots die first in the ringing experiment.

Rhytidome:

• All the tissues outside the cork cambium is called rhytidome
• It consists of cork and epidermis.