Question

Explain secondary growth in dicot stem.

Answer 1

Secondary growth in the stelar region takes place due to the activity of the vascular cambium. Secondary growth in the dicot stem's stelar region is observed as a complex process. It takes place due to the activity of the vascular cambium, which is composed of fusiform cells and ray cells.

Complete answer:
The fusiform cells divide to give rise to the two daughter cells, one of which occurs as the cambium, the other one which can give rise to phloem element or xylem element. If they are developed outwards the phloem element is formed, and if these are developed inwards, the xylem element is formed.

The ray cells help to cut the parenchymatous cells both inwards and outwards giving rise to secondary medullary rays. As this secondary growth continues, the primary tissues occur outside the secondary vascular tissues; primary phloem, cortex and epidermis are pushed outside as they are pressed to pressure through inside by the growing secondary vascular cylinder and eventually crushed off. The primary xylem remains and can be observed inner to the secondary vascular cylinder. The secondary xylem that is developed shows two different alignments, axial and radial. They give rise to the bulk of vascular tissue in woody plants.

The activity of the cambium ring is not uniform overall this year. At the time of the spring season, the vascular cambium is active, resulting in a high amount of xylem element formation, whereas in autumn more phloem is developed. So per year each ring consists of a ring of spring wood and a circle of autumn wood.

Over the years as secondary growth further continues, the secondary xylem grows and it usually results as darker and darker. The inner part which is dark is termed as the heartwood or duramen, whereas the outer lighter part is the sapwood or alburnum.

The cells of heartwood are dead and form raisins, gum or tannin, whereas the sapwood is made up of living cells which play a role in conduction.
As the plant grows older, rings of sapwood bordering heartwood keep on changing into heartwood. This is a gradual and slow process at the time which the living cells of sapwood lose their protoplast, vessels are blocked by tyloses and there

Note: 1)Secondary xylem is commonly recognized as the wood.
2)Spring wood is lighter than autumn wood, and can be exceedingly identified.
3)Dendrochronology is the learn about of determination of the age of the tree by counting the rings of sapwood and heartwood.
4)In dicot stems, vascular bundles are open, conjoint and present in a ring like manner.