The primary factors driving secondary growth in plants are the lateral meristems, specifically the vascular cambium and the cork cambium.
Secondary growth is the increase in thickness or girth of a plant, primarily in the stems and roots. Unlike primary growth, which focuses on increasing length, secondary growth adds radial dimension. This process is largely absent in herbaceous plants.
Here's a breakdown of the key factors:
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Vascular Cambium: This is the main factor in secondary growth. It is a lateral meristem that produces secondary xylem (wood) to the inside and secondary phloem (inner bark) to the outside. The activity of the vascular cambium is influenced by various factors including:
- Plant Hormones: Auxins and gibberellins play a crucial role in stimulating cell division and differentiation within the vascular cambium.
- Environmental Factors: Seasonality (temperature and day length) influences cambial activity, leading to the formation of annual growth rings in many temperate plants.
- Nutrient Availability: Adequate nutrients are essential for cell division and differentiation within the cambium.
- Water Availability: Water stress can inhibit cambial activity.
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Cork Cambium (Phellogen): This is another lateral meristem, responsible for producing the periderm, which replaces the epidermis in older stems and roots. The periderm comprises:
- Cork (Phellem): Forms the outer protective layer.
- Phelloderm: A layer of parenchyma cells formed inwards from the phellogen.
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Plant Type: Woody plants exhibit significant secondary growth, while herbaceous plants primarily undergo primary growth. This difference is due to the presence and activity of lateral meristems.
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Genetics: The genetic makeup of a plant determines its potential for secondary growth.
In summary, secondary growth is a complex process influenced by internal factors (lateral meristems, hormones, genetics) and external factors (environment, nutrients, water). The activity of the vascular cambium and cork cambium are essential for the increase in girth characteristic of woody plants.
