SECONDARY GROWTH :
- Apart from primary growth the dicot plant exhibit an increase in girth is called secondary growth.
- The tissues involved in secondary growth are two lateral meristem:
- Vascular cambium.
- Cork cambium.
Vascular cambium :
- Cells of cambium present between primary xylem and primary phloem is the intrafascicular cambium.
- The cells of medullary rays, adjoining these intrafascicular cambium become meristematic and form the interfascicular cambium.
- Intrafascicular cambium and interfascicular cambium joined to form complete cambium ring.
- The cambial ring becomes active and begins to cut off new cells both toward inner side and outer side.
- Cells produced toward pith mature into secondary xylem.
- Cells produced towards periphery mature into secondary phloem.
- Secondary xylem forms a compact mass, retaining primary xylem in the centre.
- At some places, the cambium forms a narrow band of parenchyma, which passes through secondary xylem and phloem is said to be secondary medullary rays.
Spring wood and autumn wood :
- In temperate region during spring season, cambium becomes more active and produces a large number of xylary elements having vessels with wider cavities. The wood formed in spring is called spring wood or early wood.
- In winter the cambium is less active and forms fewer xylary elements with narrow vessels thus called autumn wood or late wood.
- Spring wood is lighter in colour and lower density, where as the autumn wood is darker and has higher density.
- The two woods that appear as alternate concentric rings constitute an annual ring. Annual rings seen in a cut stem give an estimate of the age of the tree.
Heartwood and sapwood :
- In old trees the secondary xylem is dark brown due to deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils in the central or innermost layers of the stem.
- These substances make it hart, durable and resistance to insect.
- These regions comprise dead elements with high lignified walls and are called heartwood.
- The heartwood never conducts water, provide mechanical support to plant.
- The peripheral region of secondary xylem is lighter in colour and is known as sapwood.
- It involve in transport of water and minerals from root to leaf.
Cork cambium :
- Cork cambium or phellogen developed usually in the cortex region.
- Phellogen is a couple of layers in thick.
- Made of narrow, thin walled and nearly rectangular cells.
- Phellogen cut new cells to both inner and outer side.
- The outer cells differentiated into cork or phellem.
- The inner cells differentiated into secondary cortex or phelloderm.
- The cork is impervious to water due to suberin deposition.
- Phellogen, phellem and phelloderm are collectively known as periderm.
- Bark is a non-technical term that refers to all tissues exterior to the vascular cambium, there fore including secondary phloem and periderm.
- At certain region the phellogen cuts off parenchymatous cells instead of cork.
- These parenchymatous cells ruptured the epidermis and forms lens-shaped opening called lenticels.
- Lenticels permit the gaseous exchange between the outer atmosphere and internal tissues of the stem.
Secondary growth in Roots :
- In dicot root the vascular cambium is completely secondary in origin.
- It originates from the tissue below the phloem, a portion of pericycle tissue, above the protoxylem forming a complete and continuous wavy ring.
- Later it becomes circular. Ruther all events are similar with dicot stem.
