$\displaystyle \small \bullet$ Apart from primary growth most dicotyledonous plants exhibit an increase in girth. This increase is called the secondary growth.
$\displaystyle \small \bullet$ Addition of new tissues by the activity of cambium increases thickness or girth of the plant body is called secondary growth.
$\displaystyle \small \bullet$ The tissues involved in secondary growth are the two lateral meristems: vascular cambium and cork cambium.
Intrastelar secondary growth
$\displaystyle \small \bullet$ The secondary growth takes place inside the stele is called intrastelar growth.
$\displaystyle \small \bullet$ The vascular bundles are conjoint, collateral and open type. The presence of cambium in the vascular bundle is called intrafascicular cambium.
$\displaystyle \small \bullet$ In between the vascular bundles presence of medullary rays, it is differentiated into meristem forms cambium called interfascicular cambium.
$\displaystyle \small \bullet$ The newly formed interfascicular cambium joins with intrafascicular cambium forms continuous ring called cambium ring.
$\displaystyle \small \bullet$ Cambium ring consist secondary meristem which have two types of cells namely ray initials and fascicular initials.
$\displaystyle \small \bullet$ The cambium ring divides both parallel and right angle to surface, it produces new cells both towards inner side and outer side.
$\displaystyle \small \bullet$ The cambium produces the cells on inner side are differentiate into secondary xylem and the cells outer side differentiates into secondary phloem.
$\displaystyle \small \bullet$ The secondary xylem consist tracheids, vessels, xylem fibres and wood parenchyma. The cells of secondary xylem are smaller than primary xylem. The secondary phloem is comparatively less than secondary xylem.
$\displaystyle \small \bullet$ It consist sieve tubes, companion cells, bast fibres and phloem parenchyma. The thin walled older phloem cells become crushed by pressure of new tissue.
$\displaystyle \small \bullet$ The vascular tissues are separated by radially arranged thin walled parenchyma cells called medullary rays. In the cambium ring there ray initial cells, they will not differentiate and remains as parenchyma.
Activity of the cambial ring
The cambial ring becomes active and begins divide into new cells, both towards the inner and the outer sides. The cells cut off towards pith, mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem. The cambium is generally more active on the inner side than on the outer. As a result, the amount of secondary xylem produced is more than secondary phloem. The primary and secondary phloem get gradually crushed due to the continued formation and accumulation of secondary xylem. The primary xylem however remains more or less intact, in or around the centre. At some places, the cambium forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These are the secondary medullary rays.
Spring wood and autumn wood
The activity of 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, cambium is very active and produces a large number of xylary elements having vessels with wider cavities. The wood formed during this season is called spring wood or early wood. In winter, the cambium is less active and forms fewer xylary elements that have narrow vessels, and this wood is called autumn wood or late wood. The spring wood is lighter in colour and has a lower density whereas the autumn wood is darker and has a higher density. The two kinds of woods that appear as alternate concentric rings, constitute an annual ring. The spring wood and autumn wood together forms one year growth called annular ring. Annual rings seen in a cut stem give an estimate of the age of the tree. The study of annular rings is a separate branch called dendrocronology.
Heart wood and Sap wood
As the tree grows the log trunk consist two zones namely heart wood and sap wood. The central dark brown coloured hard wood this region comprises dead elements with highly lignified walls and is called heartwood or duramen, it gradually loses the conduction. They become filled with oils, gums, tannins, etc. They are hard and it provides mechanical support.
The peripheral marginal light yellow coloured region consist young xylem, they are comparatively soft, and it is called sap wood or albernum. It serves the conduction of water. The soft wood is present in gymnosperms.
Extrastelar Growth
$\displaystyle \small \bullet$ The secondary growth takes place outside the stele i.e. in the cortex is known as extra stellar growth. The cells in the cortex are the collenchyma and parenchyma are differentiated become meristematic, it is known as cork cambium or phellogen. It consist single layer of cells which can undergo cell division produces new cells on both the sides. The cells produced toward outer side are thin walled dead cells forms cork or phellum. The cork cells deposited by subarin, it provides protection from injuries. The bottle cork is obtained commercially from phellum of plant Quercus.
$\displaystyle \small \bullet$ The cork cambium produces living cells towards inner side they become differentiated into parenchyma, they resembles the cortex hence they are called secondary cortex or phelloderm
$\displaystyle \small \bullet$ Periderm: The secondary growth in cortex produce three additional tissues they are phellum, phellogen and phelloderm, they are together forms periderm.
$\displaystyle \small \bullet$ Bark: All the tissues found outside the vascular cambium forms bark. It includes cork, cork cambium, secondary cortex and secondary phloem. It has old epidermis but it is broken during secondary growth. In some trees the bark peel off.
$\displaystyle \small \bullet$ Lenticels: At certain regions, the phellogen cuts off closely arranged parenchymatous cells on the outer side instead of cork cells. These parenchymatous cells soon rupture the epidermis, forming a lens shaped openings called lenticels. Lenticels permit the exchange of gases between the outer atmosphere and the internal tissue of the stem. These occur in most woody trees. The lenticels consist loose mass of cells, later they get suberised and increase the number of cells in secondary growth
Secondary Growth in Roots
$\displaystyle \small \bullet$ In the dicot root, the vascular cambium is completely secondary in origin.
$\displaystyle \small \bullet$ 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 circular.
$\displaystyle \small \bullet$ Further events are similar to those already described above for a dicotyledon stem.
$\displaystyle \small \bullet$ This leads into formation of a circular cambium which forms secondary xylem internally and secondary phloem externally.
$\displaystyle \small \bullet$ Secondary growth also occurs in stems and roots of gymnosperms. However, secondary growth does not occur in monocotyledons.
$\displaystyle \small \bullet$ Addition of new tissues by the activity of cambium increases thickness or girth of the plant body is called secondary growth.
$\displaystyle \small \bullet$ The tissues involved in secondary growth are the two lateral meristems: vascular cambium and cork cambium.
Intrastelar secondary growth
$\displaystyle \small \bullet$ The secondary growth takes place inside the stele is called intrastelar growth.
$\displaystyle \small \bullet$ The vascular bundles are conjoint, collateral and open type. The presence of cambium in the vascular bundle is called intrafascicular cambium.
$\displaystyle \small \bullet$ In between the vascular bundles presence of medullary rays, it is differentiated into meristem forms cambium called interfascicular cambium.
$\displaystyle \small \bullet$ The newly formed interfascicular cambium joins with intrafascicular cambium forms continuous ring called cambium ring.
$\displaystyle \small \bullet$ Cambium ring consist secondary meristem which have two types of cells namely ray initials and fascicular initials.
$\displaystyle \small \bullet$ The cambium ring divides both parallel and right angle to surface, it produces new cells both towards inner side and outer side.
$\displaystyle \small \bullet$ The cambium produces the cells on inner side are differentiate into secondary xylem and the cells outer side differentiates into secondary phloem.
$\displaystyle \small \bullet$ The secondary xylem consist tracheids, vessels, xylem fibres and wood parenchyma. The cells of secondary xylem are smaller than primary xylem. The secondary phloem is comparatively less than secondary xylem.
$\displaystyle \small \bullet$ It consist sieve tubes, companion cells, bast fibres and phloem parenchyma. The thin walled older phloem cells become crushed by pressure of new tissue.
$\displaystyle \small \bullet$ The vascular tissues are separated by radially arranged thin walled parenchyma cells called medullary rays. In the cambium ring there ray initial cells, they will not differentiate and remains as parenchyma.
Activity of the cambial ring
The cambial ring becomes active and begins divide into new cells, both towards the inner and the outer sides. The cells cut off towards pith, mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem. The cambium is generally more active on the inner side than on the outer. As a result, the amount of secondary xylem produced is more than secondary phloem. The primary and secondary phloem get gradually crushed due to the continued formation and accumulation of secondary xylem. The primary xylem however remains more or less intact, in or around the centre. At some places, the cambium forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These are the secondary medullary rays.
Spring wood and autumn wood
The activity of 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, cambium is very active and produces a large number of xylary elements having vessels with wider cavities. The wood formed during this season is called spring wood or early wood. In winter, the cambium is less active and forms fewer xylary elements that have narrow vessels, and this wood is called autumn wood or late wood. The spring wood is lighter in colour and has a lower density whereas the autumn wood is darker and has a higher density. The two kinds of woods that appear as alternate concentric rings, constitute an annual ring. The spring wood and autumn wood together forms one year growth called annular ring. Annual rings seen in a cut stem give an estimate of the age of the tree. The study of annular rings is a separate branch called dendrocronology.
Heart wood and Sap wood
As the tree grows the log trunk consist two zones namely heart wood and sap wood. The central dark brown coloured hard wood this region comprises dead elements with highly lignified walls and is called heartwood or duramen, it gradually loses the conduction. They become filled with oils, gums, tannins, etc. They are hard and it provides mechanical support.
The peripheral marginal light yellow coloured region consist young xylem, they are comparatively soft, and it is called sap wood or albernum. It serves the conduction of water. The soft wood is present in gymnosperms.
| Heart wood (duramen) | Sap wood (albernum) |
|---|---|
| It is found in central region | It is found in peripheral region |
| It consist older secondary xylem | It consist younger xylem |
| Generally dark brown in colour | Light yellow in colour |
| Lumen blocked by tannin and tyloses | Lumen is not blocked |
| It provides mechanical support | It conducts water and minerals |
Extrastelar Growth
$\displaystyle \small \bullet$ The secondary growth takes place outside the stele i.e. in the cortex is known as extra stellar growth. The cells in the cortex are the collenchyma and parenchyma are differentiated become meristematic, it is known as cork cambium or phellogen. It consist single layer of cells which can undergo cell division produces new cells on both the sides. The cells produced toward outer side are thin walled dead cells forms cork or phellum. The cork cells deposited by subarin, it provides protection from injuries. The bottle cork is obtained commercially from phellum of plant Quercus.
$\displaystyle \small \bullet$ The cork cambium produces living cells towards inner side they become differentiated into parenchyma, they resembles the cortex hence they are called secondary cortex or phelloderm
$\displaystyle \small \bullet$ Periderm: The secondary growth in cortex produce three additional tissues they are phellum, phellogen and phelloderm, they are together forms periderm.
$\displaystyle \small \bullet$ Bark: All the tissues found outside the vascular cambium forms bark. It includes cork, cork cambium, secondary cortex and secondary phloem. It has old epidermis but it is broken during secondary growth. In some trees the bark peel off.
$\displaystyle \small \bullet$ Lenticels: At certain regions, the phellogen cuts off closely arranged parenchymatous cells on the outer side instead of cork cells. These parenchymatous cells soon rupture the epidermis, forming a lens shaped openings called lenticels. Lenticels permit the exchange of gases between the outer atmosphere and the internal tissue of the stem. These occur in most woody trees. The lenticels consist loose mass of cells, later they get suberised and increase the number of cells in secondary growth
Secondary Growth in Roots
$\displaystyle \small \bullet$ In the dicot root, the vascular cambium is completely secondary in origin.
$\displaystyle \small \bullet$ 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 circular.
$\displaystyle \small \bullet$ Further events are similar to those already described above for a dicotyledon stem.
$\displaystyle \small \bullet$ This leads into formation of a circular cambium which forms secondary xylem internally and secondary phloem externally.
$\displaystyle \small \bullet$ Secondary growth also occurs in stems and roots of gymnosperms. However, secondary growth does not occur in monocotyledons.
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