Stomata that are not directly exposed to the surface are referred to as "sunken."
Such stomata are found in tiny pits that shield the evaporating water vapor from air currents, reducing leaf water loss.
Nerium:
Nerium is a xerophyte, thus, have sunken stomata.
A xerophyte is a plant species that have evolved to thrive in environments with little or no liquid water, including deserts or ice- or snow-covered areas.
How do sunken stomata help xerophytes?
Sunken stomata evolved as a response to plants flourishing in water-scarce environments.
They protect plants from losing water by preventing water vapor from escaping due to wind or transpiration.
The narrow hole size of their stomata makes it difficult for water to escape.