The correct option is Lenticels.
Explanation
Plants require mechanisms for the exchange of gases (oxygen and carbon dioxide) with the atmosphere for respiration and photosynthesis. While this primarily occurs through stomata in leaves and young green stems, woody stems possess a layer of suberized (corky) bark that is impervious to gases. Therefore, specialized structures are required to facilitate this exchange in mature stems.
Option Analysis
- Lenticels is Correct: Lenticels are lens-shaped openings or pores found on the periderm (bark) of woody stems and mature roots. They consist of loosely arranged complementary cells that permit the diffusion of gases between the atmosphere and the internal tissues of the stem. Unlike stomata, lenticels remain permanently open.
- Stomata is Incorrect: Stomata are microscopic pores primarily located on the epidermis of leaves and young, herbaceous stems. In woody plants, as the stem matures, the epidermis is replaced by the periderm, and stomata are no longer the primary means of exchange in those regions.
- Tracheids is Incorrect: Tracheids are elongated, dead cells found in the xylem tissue. Their primary function is the conduction of water and minerals from roots to leaves and providing mechanical support. They are not involved in direct gaseous exchange with the environment.
- Plasmodesmata is Incorrect: Plasmodesmata are cytoplasmic channels that traverse the cell walls of adjacent plant cells. They facilitate cell-to-cell communication and transport (symplastic pathway) but do not facilitate gaseous exchange with the external atmosphere.
Key Takeaway: In woody plants, gaseous exchange in the stem occurs through lenticels, whereas in leaves and young stems, it occurs through stomata.