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The breathing pore in the stem of a plant is called a lenticel. Lenticels are small openings or pores that are found on the surface of stems and some roots, allowing for gas exchange between the internal tissues of the plant and the external environment. This process is crucial for the plant's respiration, enabling it to take in oxygen and release carbon dioxide.

Lenticels are particularly important in woody plants where the bark can become thick and impermeable, making it challenging for gases to diffuse in and out. The presence of lenticels provides a necessary pathway for this exchange.

In contrast, stomata are primarily found on leaves rather than stems and are involved in gas exchange and transpiration. Chloroplasts are the organelles where photosynthesis occurs, and xylem is the tissue responsible for water and nutrient transport within the plant. Each of these plays a unique role in plant physiology, but for the specific function of gas exchange at the stem level, lenticels are the appropriate answer.