Model Answer
0 min readIntroduction
Secondary growth, the increase in girth of plants, is a characteristic feature of many dicotyledonous and gymnosperous flowering plants. This thickening is primarily due to the activity of lateral meristems, namely the vascular cambium and cork cambium. In some plants, the vascular cambium undergoes repeated divisions, giving rise to successive cambia, which contribute significantly to the substantial increase in stem diameter observed in certain species. This process allows for enhanced mechanical support and efficient transport of water and nutrients.
Successive Cambia and Stem Thickening
The vascular cambium, a lateral meristem, is responsible for secondary growth. In plants exhibiting successive cambial activity, the initial vascular cambium produces new vascular tissues (secondary xylem and secondary phloem). However, instead of becoming quiescent, portions of the initial cambial cells may remain meristematic and divide to form new cambial rings, termed ‘successive cambia’.
Mechanism of Formation
- Initial Cambium: The primary vascular cambium arises from procambial cells and parenchyma cells between the primary xylem and primary phloem.
- Formation of Interxylary Cambium: As the stem ages, parenchyma cells present between the vascular bundles differentiate to form new cambial layers, called interxylary cambium. These arise from the remnants of primary cambium or from parenchyma cells.
- Formation of Interfascicular Cambium: Parenchyma cells between the vascular bundles also differentiate into cambial cells, forming interfascicular cambium.
- Successive Layers: These newly formed cambial layers, along with the original cambium, contribute to the production of secondary xylem and phloem, leading to increased stem thickness. Each successive cambium layer adds to the overall girth.
- Cork Cambium Activity: Concurrent with vascular cambium activity, cork cambium (phellogen) develops from the cortex and produces periderm, providing protective layers as the stem expands.
Impact on Stem Structure
The formation of successive cambia results in a complex arrangement of vascular tissues. The secondary xylem becomes highly variable in its structure, with alternating bands of early wood (spring wood) and late wood. The presence of multiple cambial layers also leads to the formation of complex ray systems, facilitating radial transport of water and nutrients.
Families Exhibiting Successive Cambial Activity
Several plant families demonstrate prominent successive cambial activity, leading to substantial stem thickening.
- Malvaceae (Mallow Family): Species like Hibiscus and Theobroma cacao (cocoa) exhibit pronounced successive cambial activity, resulting in thick, woody stems.
- Lauraceae (Laurel Family): Members of this family, such as Cinnamomum (cinnamon) and Persea americana (avocado), show significant stem thickening due to the formation of multiple cambial rings.
| Family | Example Species | Characteristic Feature |
|---|---|---|
| Malvaceae | Theobroma cacao | Thick stems with multiple cambial layers contributing to wood formation. |
| Lauraceae | Cinnamomum verum | Significant stem thickening, important for bark harvesting (cinnamon). |
Conclusion
Successive cambial activity is a specialized adaptation in certain flowering plants, enabling substantial stem thickening and providing enhanced structural support. This process involves the repeated formation of cambial layers, leading to a complex arrangement of vascular tissues. Families like Malvaceae and Lauraceae exemplify this phenomenon, showcasing its importance in their growth and development. Understanding this process is crucial for comprehending plant anatomy and physiology, and its implications for timber production and other economic uses.
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.