The internal structure of a dicot stem is characterized by distinct layers and tissues, including the epidermis, cortex (with hypodermis, general cortex, and endodermis), vascular bundles (containing xylem and phloem), pericycle, and pith. These components work together to provide support, transport nutrients, and facilitate growth.
Here's a breakdown of each layer:
Epidermis
- The outermost protective layer, typically a single layer of cells.
- Covered with a cuticle to prevent water loss.
- May contain stomata (pores for gas exchange) and trichomes (hair-like structures for protection or secretion).
Cortex
The region between the epidermis and the vascular bundles, differentiated into three main zones:
- Hypodermis: Several layers of collenchyma cells just below the epidermis providing mechanical support.
- General Cortex: Parenchyma cells with intercellular spaces; involved in food storage.
- Endodermis: The innermost layer of the cortex, often containing a Casparian strip (though less distinct than in roots), regulating water and nutrient movement.
Vascular Bundles
- Arranged in a ring around the pith, a defining characteristic of dicot stems.
- Conjoint, open, and collateral:
- Conjoint: Xylem and phloem are present in the same bundle.
- Open: Cambium (a meristematic layer) is present between xylem and phloem, allowing for secondary growth.
- Collateral: Phloem is located on the outer side of the xylem.
- Xylem: Transports water and minerals from roots to leaves, and provides structural support.
- Phloem: Transports sugars produced during photosynthesis from leaves to other parts of the plant.
Pericycle
- A layer of cells located between the vascular bundles and the pith.
- Involved in lateral root formation (though this is more common in roots than stems).
- Made up of parenchyma and sclerenchyma cells. Sclerenchyma cells are arranged as patches above the phloem, forming bundle caps.
Pith
- The central core of the stem.
- Composed of parenchyma cells.
- Functions primarily in food storage.
In summary, the dicot stem's complex internal structure is essential for the plant's survival, growth, and reproduction. Each layer plays a vital role in providing support, transport, and storage functions.
