Model Answer
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Coralloid roots are specialized, short-lived roots found in cycads, representing a unique symbiotic association between the plant and nitrogen-fixing cyanobacteria. These roots grow upwards towards the soil surface and develop characteristic coral-like branching patterns, hence the name. The presence of these roots allows cycads to thrive in nitrogen-poor environments. A transverse section of a coralloid root reveals a distinct anatomy adapted to accommodate the cyanobacterial symbionts and facilitate nitrogen fixation. Understanding the T.S. anatomy is crucial to comprehending the functional significance of these roots in the cycad life cycle.
T.S. of Coralloid Root of *Cycas*: A Detailed Anatomy
The transverse section of a coralloid root of *Cycas* exhibits a unique structure, markedly different from typical roots. It can be broadly divided into the following layers:
1. Epidermis and Cortex
- Epidermis: The outermost layer is a single layer of thin-walled cells. It is often broken down due to the upward growth of the root.
- Cortex: The cortex is extensively branched and is the primary site for the symbiotic cyanobacteria. It is composed of loosely arranged parenchyma cells interspersed with large cavities. These cavities are filled with blue-green cyanobacteria, primarily *Nostoc* and *Anabaena*. The cyanobacteria are responsible for nitrogen fixation.
- Mucilage Cells: Scattered within the cortex are mucilage cells, which secrete a mucilaginous substance that helps maintain moisture and provides a suitable environment for the cyanobacteria.
2. Vascular Cylinder
- Endodermis: A single layer of compactly arranged cells forming the innermost layer of the cortex. It possesses Casparian strips, though they may not be prominent.
- Pericycle: Present internal to the endodermis, it is a single layer of cells.
- Vascular Bundles: The vascular cylinder contains a ring of vascular bundles. Each bundle consists of xylem and phloem. Xylem is manarch type (protruding xylem poles).
- Pith: A small, centrally located pith is present, composed of parenchyma cells.
3. Specialized Features – Cyanobacterial Zone
- Cyanobacterial Chambers: The most distinctive feature is the presence of large, intercellular spaces within the cortex, specifically designed to house the cyanobacteria. These chambers are irregular in shape and size.
- Galls: The cortex often exhibits gall-like swellings due to the proliferation of cyanobacteria.
- Aerating Tissues: Some areas within the cortex contain loosely packed cells with large intercellular spaces, facilitating gas exchange for the cyanobacteria.
Diagrammatic Representation:
| Layer | Description |
|---|---|
| Epidermis | Single layer, often broken |
| Cortex | Loosely arranged parenchyma, cyanobacterial chambers, mucilage cells |
| Endodermis | Single layer with Casparian strips |
| Pericycle | Single layer of cells |
| Vascular Bundles | Xylem and Phloem, manarch xylem |
| Pith | Small, central parenchyma |
The symbiotic relationship is mutually beneficial. The cyanobacteria fix atmospheric nitrogen, providing the plant with a usable nitrogen source. In return, the plant provides the cyanobacteria with a protected environment, carbohydrates (produced through photosynthesis), and a constant supply of water and minerals.
Conclusion
In conclusion, the T.S. of a coralloid root of *Cycas* reveals a highly specialized anatomy adapted for a symbiotic relationship with nitrogen-fixing cyanobacteria. The extensive cortex with its intercellular spaces, mucilage cells, and aerating tissues provides an ideal environment for the cyanobacteria to thrive and contribute to the plant’s nitrogen nutrition. This unique adaptation allows cycads to colonize nutrient-poor habitats, demonstrating a remarkable example of co-evolution between plants and microorganisms. Further research into the molecular mechanisms governing this symbiosis could have implications for sustainable agriculture.
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.