Sunken stomata are present in xerophytes.

Explanation:
Stomata are tiny openings or pores found in the epidermis (outermost layer) of leaves, stems, and other plant organs. They play a crucial role in the exchange of gases, such as carbon dioxide and oxygen, between the plant and its environment. Stomata allow carbon dioxide to enter the plant for photosynthesis while also enabling the release of oxygen and water vapor.

Xerophytes are plants that are adapted to survive in dry or arid environments with limited water availability. These plants have evolved various adaptations to reduce water loss and survive in such challenging conditions. One of these adaptations is the presence of sunken stomata.

Sunken stomata are stomata that are located below the surface of the leaf, in small pits or depressions. This positioning helps to protect the stomata from direct exposure to the external environment, reducing water loss through transpiration. The pits or depressions surrounding the stomata create a microenvironment that traps a layer of moist air, reducing the concentration gradient of water vapor and minimizing water loss.

The presence of sunken stomata in xerophytes is an effective adaptation to reduce water loss and maintain water balance in arid conditions. By protecting the stomata and creating a microenvironment that minimizes water vapor diffusion, xerophytes are able to conserve water and survive in dry environments.

In contrast, hydrophytes are plants that are adapted to live in aquatic or waterlogged environments, and epiphytes are plants that grow on other plants for support. These types of plants have different adaptations to suit their specific habitats, and sunken stomata are not typically found in their leaves.

Mesophytes, on the other hand, are plants that are adapted to grow in moderate moisture conditions, neither extremely dry nor extremely wet. These plants do not require the same level of water conservation as xerophytes and therefore do not possess sunken stomata.

In conclusion, sunken stomata are present in xerophytes as an adaptation to reduce water loss in dry environments.