Model Answer
0 min readIntroduction
Secondary growth is the increase in girth of a plant stem or root, resulting from the activity of the vascular cambium and cork cambium. It is a characteristic feature of dicotyledonous and gymnospermic plants, allowing for increased structural support and efficient transport of water and nutrients. While typical secondary growth leads to a symmetrical increase in stem diameter, variations can occur, resulting in abnormal secondary growth. This abnormality arises due to uneven or localized activity of the vascular cambium, leading to unique anatomical features and adaptations. Understanding these differences is fundamental to plant anatomy and physiology.
Normal Secondary Growth in Dicot Stem
Normal secondary growth in dicot stems is characterized by uniform activity of the vascular cambium, leading to symmetrical production of secondary xylem (wood) and secondary phloem. The vascular cambium, a lateral meristem, arises from the procambium and interfascicular cambium.
- Vascular Cambium Activity: The vascular cambium produces secondary xylem towards the inner side and secondary phloem towards the outer side. The rate of production is generally uniform throughout the circumference of the stem.
- Ray Initial Division: Ray initials in the vascular cambium divide to form secondary rays, which facilitate radial transport of water and nutrients. These rays are evenly distributed.
- Annual Rings: In temperate regions, seasonal variations in cambial activity result in the formation of annual rings, visible as alternating bands of earlywood (lighter color, larger vessels) and latewood (darker color, smaller vessels).
- Periderm Formation: As the stem expands, the epidermis ruptures and is replaced by the periderm, formed by the cork cambium.
Example: Acer (Maple) exhibits typical normal secondary growth, forming distinct annual rings and a symmetrical stem structure.
Figure: Diagram illustrating normal secondary growth in a dicot stem.
Abnormal Secondary Growth in Dicot Stem
Abnormal secondary growth occurs when the vascular cambium exhibits uneven activity, leading to asymmetrical stem development. This can be caused by various factors, including localized wounding, mechanical stress, or genetic predisposition.
- Localized Cambial Activity: The vascular cambium becomes more active on one side of the stem than the other, resulting in an uneven distribution of secondary xylem and phloem.
- Formation of Reaction Wood: In response to gravitational stress or wind exposure, plants may produce reaction wood. In dicots, this is typically tension wood, characterized by thickened cell walls on the upper side of the leaning stem.
- Eccentric Rings: Annual rings are not concentric but are shifted towards the side of greater cambial activity.
- Interrupted Vascular Bundles: Sometimes, the vascular bundles may be disrupted or completely obliterated due to uneven growth.
Example: Dalbergia sissoo (Indian Rosewood) often exhibits abnormal secondary growth, particularly in response to leaning or uneven light exposure, resulting in eccentric rings and localized thickening.
Figure: Diagram illustrating abnormal secondary growth in a dicot stem.
Comparison of Normal and Abnormal Secondary Growth
| Feature | Normal Secondary Growth | Abnormal Secondary Growth |
|---|---|---|
| Cambial Activity | Uniform throughout the circumference | Uneven, localized activity |
| Stem Shape | Symmetrical, cylindrical | Asymmetrical, often eccentric |
| Annual Rings | Concentric, regular | Eccentric, irregular |
| Ray Initial Division | Evenly distributed | Uneven distribution |
| Reaction Wood | Absent | Present (tension wood in dicots) |
| Examples | Acer, Ficus | Dalbergia sissoo, Banyan |
Conclusion
In conclusion, secondary growth in dicot stems can manifest in both normal and abnormal forms. Normal secondary growth results in symmetrical stem development due to uniform cambial activity, while abnormal secondary growth leads to asymmetrical growth caused by localized cambial activity and the formation of reaction wood. Understanding these differences is crucial for interpreting plant anatomy and appreciating the adaptive responses of plants to their environment. Further research into the molecular mechanisms regulating cambial activity could provide insights into controlling wood formation and improving timber quality.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.