Figure 2.
Sketch of the three-dimensional structure of a leaf, showing all structures that are considered in this review. The internal leaf tissue (mesophyll) is enclosed by the upper and lower epidermis. Usually, elongated cells (palisade parenchyma) are situated at the upper side of a leaf with roundish (or a more complex shape) cells at the lower side. Two opposite gradients of gas concentration develop in a leaf during gas exchange: CO2 concentration decreases from the stomatal pore to the palisade cells, whereas water vapor concentration decreases from the mesophyll to the leaf external air humidity. Please note that this sketch shows a typical broad leaf. There are many species-specific (and also individual) variations in leaf internal structure.