The mesophyll layer in leaves comprises two distinct cell types: palisade and spongy mesophyll cells, each with a unique structure and function. Palisade mesophyll cells, located just below the upper epidermis, are elongated and densely packed with chloroplasts. This arrangement maximises light absorption, as the surface area exposed to light is increased. The palisade layer is thus primarily responsible for capturing light energy for photosynthesis. In contrast, spongy mesophyll cells, found in the lower part of the mesophyll, have a more irregular shape and are loosely arranged. This creates large air spaces between them, facilitating the diffusion of gases (such as CO2 and O2) throughout the leaf. The spongy mesophyll's structure aids in efficient gas exchange between the leaf and the external environment through the stomata. This distinct separation of functions within the mesophyll – light absorption in the palisade and gas exchange in the spongy layer – reflects a highly efficient division of labour within the leaf, optimising photosynthesis.

In plants from arid regions, the cuticle, a waxy layer covering the leaf surface, is often more prominently developed compared to those in more humid environments. This adaptation is crucial for survival in dry conditions. The primary function of the cuticle is to minimise water loss through transpiration. In arid regions, water conservation is vital for plant survival due to limited water availability. A thicker cuticle acts as a more effective barrier to water loss, reducing the rate of transpiration. This adaptation, however, comes with a trade-off, as a thicker cuticle can also reduce the amount of light reaching the photosynthetic cells and may impede gaseous exchange. Therefore, plants in arid environments must balance the need to conserve water with the requirements for photosynthesis and gas exchange. This evolutionary adaptation of a thicker cuticle illustrates how plants modify their physical structures in response to environmental pressures to optimise their chances of survival.

Xylem and phloem, as components of the vascular bundles in leaves, have distinct yet complementary roles essential for leaf function and overall plant health. The xylem primarily transports water and dissolved minerals from the roots to the leaves, which is crucial for photosynthesis. This upward movement of water also facilitates the cooling of the leaf and maintains turgor pressure, essential for keeping the leaf structures erect and optimally positioned for light absorption. On the other hand, the phloem is responsible for transporting the sugars and other photosynthetic products from the leaves to other parts of the plant. This downward flow ensures that energy produced in the leaves supports growth and development in other tissues, including roots, stems, and fruits. The coordinated functioning of xylem and phloem in vascular bundles is thus vital for maintaining the balance of nutrient and water supply within the plant, supporting both the photosynthetic process in the leaves and the overall growth and development of the plant.

Air spaces in the spongy mesophyll layer play a critical role in facilitating gas exchange in leaves. These spaces, formed between the loosely arranged spongy mesophyll cells, create a network of channels that connect the internal leaf environment with the external atmosphere via the stomata. They allow for the efficient diffusion of gases – carbon dioxide (CO2) moves from the atmosphere into the leaf, and oxygen (O2) and water vapour move from the leaf to the atmosphere. The large surface area of the air spaces ensures that gases can rapidly diffuse to and from all cells within the leaf. This is particularly important for CO2, which is needed for photosynthesis in the chloroplasts of mesophyll cells. The efficient movement of CO2 into the leaf and O2 out of the leaf is crucial for maintaining high rates of photosynthesis while minimising water loss through transpiration.

Environmental factors play a crucial role in the opening and closing of stomata, thus regulating gas exchange and transpiration in plants. Light is a primary factor; in the presence of light, stomata generally open to allow CO2 in for photosynthesis. Conversely, in darkness, they close to conserve water. Water availability also influences stomatal action. In conditions of water stress, plants tend to close their stomata to prevent water loss, even at the cost of reducing photosynthesis. Additionally, internal factors such as the concentration of carbon dioxide within the leaf also impact stomatal movement. High internal CO2 levels typically result in stomatal closure. Furthermore, temperature can affect stomatal opening; higher temperatures may cause the stomata to open to facilitate cooling through transpiration, but extreme heat can lead to their closure to prevent excessive water loss. These responses demonstrate how plants balance the need for CO2 for photosynthesis with the need to conserve water, adapting to their environment to maintain optimal physiological conditions.