Model Answer
0 min readIntroduction
Secondary growth, the increase in girth of plants, is a characteristic feature of dicotyledonous and gymnospermic stems and roots. However, some monocotyledonous plants, like palms and *Dracaena*, exhibit secondary growth that deviates from the typical pattern observed in dicots. This atypical secondary growth is termed ‘anomalous secondary growth’. It arises due to the presence of unusual vascular bundles and activity of cambial cells that are not organized in the conventional manner. Understanding the anatomical basis of this phenomenon in *Dracaena* provides insights into the plasticity of plant development and adaptation.
Understanding Anomalous Secondary Growth
Anomalous secondary growth refers to the development of secondary tissues in monocotyledons, which typically lack a vascular cambium. Unlike dicots where a complete ring of cambium arises, monocots exhibit localized and scattered cambial activity. This results in a non-uniform increase in girth and often leads to complex vascular arrangements.
Anomalous Secondary Growth in *Dracaena* Stem
The stem of *Dracaena* exhibits anomalous secondary growth due to the presence of multiple, scattered vascular bundles. The process can be described in the following stages:
- Formation of Cambial Rings: Cambial activity originates from the remnants of vascular bundles. These remnants, along with interfascicular tissues, form incomplete rings of cambium around each vascular bundle.
- Activity of Cambial Cells: The cambial cells divide periclinally (parallel to the surface) to produce secondary xylem towards the inside and secondary phloem towards the outside.
- Conjunctive Tissue Role: The extensive conjunctive tissue present between the vascular bundles plays a crucial role. Parenchyma cells within this tissue become meristematic and contribute to the formation of complete cambial rings, though these rings are not continuous in the same way as in dicots.
- Formation of Vascular Bundles: New vascular bundles are formed by the activity of cambial cells, adding to the complexity of the vascular arrangement. These newly formed bundles are often embedded within the secondary tissues.
- Rhizophores: *Dracaena* also develops aerial roots called rhizophores, which contribute to the overall increase in girth and provide additional support.
Anatomical Features Contributing to Anomalous Growth
Several anatomical features contribute to the unique secondary growth pattern in *Dracaena*:
- Scattered Vascular Bundles: The primary vascular bundles are scattered throughout the ground tissue, unlike the ring arrangement in dicots.
- Presence of Interfascicular Cambium: Cambial activity occurs not only from the remnants of vascular bundles but also in the interfascicular regions, contributing to the formation of complete, albeit discontinuous, cambial rings.
- Extensive Conjunctive Tissue: The large amount of parenchyma cells in the conjunctive tissue provides a significant source of meristematic cells for secondary growth.
- Lack of True Wood: The secondary xylem produced is not as organized or dense as the wood found in dicots. It lacks distinct annual rings.
Comparison with Typical Dicot Secondary Growth
| Feature | Dicot Secondary Growth | Dracaena Anomalous Secondary Growth |
|---|---|---|
| Cambial Ring | Complete and continuous | Incomplete, scattered, and often discontinuous |
| Vascular Bundle Arrangement | Ring arrangement in primary stem | Scattered throughout the stem |
| Wood Formation | Formation of distinct wood with annual rings | Formation of less organized secondary xylem without clear annual rings |
| Conjunctive Tissue | Limited role | Significant role in cambial ring formation |
Conclusion
Anomalous secondary growth in *Dracaena* represents a fascinating adaptation allowing monocots to achieve increased girth and structural support. This process, driven by localized cambial activity and the contribution of conjunctive tissue, differs significantly from the typical dicot pattern. Studying such variations in plant development provides valuable insights into the evolutionary plasticity of plant anatomy and physiology, highlighting the diverse strategies plants employ to thrive in various environments.
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.