Model Answer
0 min readIntroduction
Phloem, the vascular tissue responsible for transporting sugars and other organic nutrients throughout the plant, typically occurs on the periphery of the stem and root. However, in certain plant groups, a unique type of phloem, termed ‘interxylary phloem’, is found embedded within the xylem tissue. This unusual arrangement represents an adaptive feature observed in several dicotyledonous families. Understanding the formation and function of interxylary phloem provides insights into the evolutionary adaptations of plants to various environmental conditions and growth patterns. This answer will detail the nature of interxylary phloem, its developmental process, and provide illustrative examples.
What is Interxylary Phloem?
Interxylary phloem refers to the phloem tissue that is completely surrounded by xylem elements. Unlike the normal phloem which is located at the periphery, this phloem is found internally, within the xylem cylinder. It is a characteristic feature of the secondary xylem in certain dicotyledonous plants. The presence of interxylary phloem is considered an anomalous secondary growth feature.
How is Interxylary Phloem Formed?
The formation of interxylary phloem is a complex process linked to the activity of the cambium. It arises from the remnants of procambial cells that become surrounded by differentiating xylem elements. The process can be explained in the following steps:
- Initial Stage: During secondary growth, the vascular cambium produces both secondary xylem and secondary phloem.
- Procambial Remnants: Some procambial cells, instead of differentiating directly into xylem or phloem, remain in an undifferentiated state.
- Xylem Encapsulation: As the secondary xylem develops rapidly, these undifferentiated cells become completely surrounded by xylem elements.
- Phloem Differentiation: Subsequently, these encapsulated cells differentiate into phloem elements, forming the interxylary phloem.
- Functional Phloem: The interxylary phloem, though internally located, remains functional and contributes to the overall transport of nutrients.
Types of Interxylary Phloem
Interxylary phloem can be classified into two main types based on its extent and arrangement:
- Complete Interxylary Phloem: In this type, the phloem strand is entirely surrounded by xylem on all sides.
- Incomplete Interxylary Phloem: Here, the phloem strand is surrounded by xylem on most sides, but may be open to the surrounding tissues on one or more sides.
Examples of Plants Exhibiting Interxylary Phloem
Interxylary phloem is observed in several plant families, including:
- Cucurbitaceae (Gourd Family): Members like Cucurbita (pumpkin, squash) exhibit well-developed interxylary phloem.
- Solanaceae (Nightshade Family): Plants such as Solanum melongena (brinjal/eggplant) show this feature.
- Convolvulaceae (Morning Glory Family): Species within this family, like Ipomoea (morning glory), often possess interxylary phloem.
- Malvaceae (Mallow Family): Some species of Malva also exhibit this characteristic.
Adaptive Significance
The adaptive significance of interxylary phloem is still debated, but several hypotheses have been proposed:
- Mechanical Support: The embedded phloem may contribute to the mechanical strength of the stem, particularly in rapidly growing plants.
- Efficient Nutrient Transport: It may facilitate more efficient nutrient transport to the developing tissues within the xylem cylinder.
- Protection: The surrounding xylem may offer protection to the phloem from mechanical damage.
Diagram showing the location of interxylary phloem within the secondary xylem. (Source: Wikimedia Commons)
Conclusion
Interxylary phloem represents a fascinating example of anomalous secondary growth in plants. Its formation, linked to the activity of the vascular cambium and the encapsulation of procambial cells by xylem, results in functional phloem embedded within the xylem cylinder. Observed in families like Cucurbitaceae and Solanaceae, its adaptive significance likely involves a combination of mechanical support, efficient nutrient transport, and protection. Further research is needed to fully elucidate the evolutionary pressures that have led to the development of this unique phloem type.
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.