Stem
The stem of a cactus plant performs photosynthesis.

Explanation:
Photosynthesis is the process by which plants convert sunlight into energy to fuel their growth. In most plants, this process occurs primarily in the leaves. However, cacti have evolved to survive in arid environments where water is scarce. As a result, their leaves have adapted to reduce water loss by developing into spines or becoming very small and waxy. Therefore, the stem of a cactus plant has taken on the role of performing photosynthesis instead.

Function of the Stem in Photosynthesis:
The stem of a cactus plant contains chlorophyll, the pigment responsible for capturing sunlight and converting it into energy. The stem is green in color due to the presence of chlorophyll. The chlorophyll molecules in the stem absorb sunlight, which is then used to produce glucose, the main source of energy for the plant. This glucose is then utilized by the cactus for growth and other metabolic processes.

Adaptations of the Stem:
The stem of a cactus has several adaptations that enable it to perform photosynthesis effectively in the absence of leaves. These adaptations include:

1. Thick and Fleshy Tissue: The stem of a cactus is typically thick and fleshy, allowing it to store water for extended periods. This adaptation helps the cactus survive in arid conditions when water is scarce.

2. Waxy Cuticle: The stem is often covered with a waxy cuticle that helps to reduce water loss through evaporation. This adaptation is crucial for preventing dehydration in the desert environment.

3. Spines: While spines may not directly contribute to photosynthesis, they serve a protective function. Spines help to deter herbivores from consuming the cactus and also provide shade, reducing the amount of direct sunlight on the stem.

Conclusion:
In summary, the stem of a cactus plant performs photosynthesis. Its adaptations, such as thick and fleshy tissue, a waxy cuticle, and spines, enable the stem to efficiently capture sunlight and convert it into energy for the cactus to survive in arid environments.