Model Answer
0 min readIntroduction
Secondary growth in dicotyledonous plants typically involves the activity of a single vascular cambium that forms a continuous ring, producing secondary xylem towards the inside and secondary phloem towards the outside, leading to an increase in girth. However, certain dicot plants exhibit "anomalous secondary growth" where this typical pattern is deviated. This deviation can occur due to various reasons related to the origin, position, or activity of the cambium, or the formation of additional meristematic tissues. These anomalies often represent adaptations to specific ecological niches, such as the climbing habit of lianas, or are simply non-adaptive variations in plant anatomy.
Causes of Anomalous Secondary Growth in Dicotyledonous Plants
Anomalous secondary growth is a deviation from the typical cylindrical arrangement and activity of the vascular cambium found in most dicots. These deviations are broadly categorized based on the nature of the abnormality.1. Abnormal Activity of the Normal Cambium
In some dicots, the vascular cambium forms in its normal position but exhibits irregular or differential activity. This can lead to an uneven production of secondary xylem and phloem, or even the formation of these tissues in unusual arrangements.
- Unequal Activity: The cambium may be more active in certain regions, producing a disproportionate amount of xylem or phloem. For instance, in plants like Bauhinia rubiginosa, the cambium activity is restricted to certain segments, leading to ridged stems. In Bignonia (a woody climber), the cambium initially functions normally but later produces more secondary phloem in specific regions, which gets embedded in the secondary xylem, forming phloem wedges. This adaptation provides flexibility to the climbing stem.
- Formation of Fissured Xylem: In some cases, such as Tinospora, the cambium produces extensive secondary ray parenchyma in the interfascicular regions, leading to a fissured or lobed xylem cylinder.
- Included (Interxylary) Phloem: This occurs when the cambium, instead of producing xylem inwards, occasionally produces secondary phloem towards the inside, which subsequently gets embedded within the secondary xylem. This is observed in species like Strychnos and members of the Combretaceae family (e.g., Combretum, Entada).
2. Anomalous Position of Cambium
The cambium itself may originate or be located in positions different from the typical fascicular and interfascicular regions.
- Formation of Successive Rings of Cambium: In many plants, particularly members of families like Nyctaginaceae (e.g., Bougainvillea, Boerhaavia, Mirabilis), Amaranthaceae (e.g., Amaranthus, Achyranthes), and Chenopodiaceae (e.g., Beta vulgaris), successive rings of accessory cambia arise outside the oldest phloem in the cortical or pericyclic region. Each new cambium ring produces secondary vascular tissues (xylem and phloem) and often a large amount of conjunctive parenchyma.
- Cambium in Separate Strips: In some climbers like Serjania (Sapindaceae), the cambium may occur in separate strips from the beginning, resulting in a deeply lobed or compound stem structure.
3. Presence of Accessory Vascular Bundles (Primary Anomalies leading to Secondary Anomalies)
Though primarily a feature of primary anatomy, the presence of these bundles influences the subsequent secondary growth.
- Medullary Bundles: Vascular bundles present within the pith, often seen in families like Nyctaginaceae (e.g., Boerhaavia, Mirabilis) and Amaranthaceae. These bundles may or may not exhibit secondary growth themselves, but their presence alters the overall secondary thickening pattern.
- Cortical Bundles: Vascular bundles found embedded in the cortex, as observed in Calycanthus and Nyctanthes. These bundles maintain their independent course and contribute to the anomalous structure of the stem.
4. Environmental and Genetic Factors
Beyond the structural characteristics of cambium and vascular bundles, environmental and genetic factors can also play a role in inducing anomalous growth patterns.
- Genetic Predisposition: Specific genetic traits inherent to certain plant species lead to unique, non-standard secondary growth patterns.
- Environmental Influence: Factors like light, temperature, humidity, and physical stress (e.g., injury) can induce irregular growth as the plant adapts to its conditions.
| Type of Anomaly | Description | Example Plant |
|---|---|---|
| Abnormal Cambial Activity | Uneven production of xylem/phloem, included phloem, fissured xylem. | Bignonia (phloem wedges), Strychnos (included phloem), Tinospora (fissured xylem) |
| Anomalous Cambial Position | Successive rings of cambia, cambium in separate strips. | Bougainvillea (successive cambia), Serjania (separate cambial strips) |
| Accessory Vascular Bundles | Presence of medullary or cortical bundles. | Boerhaavia (medullary bundles), Nyctanthes (cortical bundles) |
Conclusion
Anomalous secondary growth in dicotyledonous plants, while deviating from the typical uniform cylindrical thickening, is a fascinating aspect of plant anatomy. These structural variations are primarily caused by the abnormal functioning or positioning of the vascular cambium, or the development of accessory cambial rings and atypical vascular bundles. Often, these anomalies are adaptive, providing mechanical support or flexibility, especially in climbing plants, or serving storage functions in root crops. Understanding these diverse patterns is crucial for comprehending the vast morphological plasticity within the plant kingdom and their evolutionary adaptations to various ecological niches.
Answer Length
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