The stomata on the bark are essentially pores or openings that were present on the surface of the stem before it fully developed into woody bark through secondary growth. While stomata are crucial for gaseous exchange in leaves and young, herbaceous stems, their function changes significantly on the bark of woody stems.
Understanding Stomata: General Definition
Stomata (singular: stoma) are microscopic pores found on the epidermis of plants. As defined, they are:
- Pores or openings present generally over the surface of leaves and stems.
- These are specialized structures guarded by guard cells that assist their opening and closing, regulating gas exchange (carbon dioxide uptake, oxygen release, and water vapor transpiration).
Stomata's Role on Woody Stems (Bark)
While stomata are initially present on young stems, their efficacy on developing bark diminishes considerably. The provided reference highlights a crucial point regarding their role on woody stems:
"In woody stems, the stomata cannot perform gaseous exchange as secondary growth occurs."
This means that as a plant stem undergoes secondary growth—a process that increases the stem's girth and forms the tough, protective outer layer known as bark—the stomata become ineffective for their primary function of gas exchange.
Why Stomata on Bark Lose Function
The transformation of a young stem into a woody stem with bark involves several changes that impede stomata's function:
- Cellular Differentiation: As secondary growth progresses, the outer epidermal layers, where stomata are located, are replaced or covered by new tissues like cork (periderm).
- Barrier Formation: The bark itself, composed of cork and other dead tissues, forms a thick, impermeable barrier that prevents efficient gaseous diffusion through the small stomatal pores.
- Lack of Control: The guard cells, which normally control the opening and closing of stomata, lose their functionality or are replaced by the dead, protective cells of the bark.
Key Takeaways for Stomata on Bark
- Stomata are present on stems before they mature into woody structures.
- Once secondary growth begins and bark forms, these stomata lose their ability to perform gaseous exchange.
- Their functional role for respiration and photosynthesis shifts to other specialized structures or is largely taken over by lenticels (though not explicitly mentioned in the provided reference, lenticels are the primary structures for gas exchange on mature bark).
Summary Table: Stomata Function
To illustrate the difference in function, consider the following:
| Feature | Stomata on Leaves/Young Stems | Stomata on Woody Stems (Bark) |
|---|---|---|
| Primary Function | Gaseous exchange | Cannot perform gaseous exchange |
| Structure | Pores guarded by active guard cells | Present, but inactivated by secondary growth |
| Efficiency of Exchange | High | Negligible/Non-existent |
| Presence | Active and functional | Residual; covered or rendered non-functional |
In essence, while stomata are indeed present on the stem that eventually forms bark, they are no longer functional in their capacity for gaseous exchange once the woody stem matures.
