There are three main leaf types based on their adaxial–abaxial structure: bifacial, unifacial, and equifacial. In conventional, bifacial leaves, the adaxial (upper) domain of the leaf consists of an epidermis with a relatively thick cuticle and densely packed palisade mesophyll cells, which optimize light capture. The abaxial (lower) domain of the leaf consists of an epidermis with abundant stomata and spongy mesophyll cells, which function in gas exchange and the regulation of transpiration (see textbook Figure 1.3). In bifacial leaves, the leaf vasculature is also aligned along the adaxial–abaxial axis, with xylem tissue differentiating adaxially and the phloem abaxially. This adaxial–abaxial development of xylem and phloem in the leaf vasculature is a reflection of the inner–outer orientation of xylem and phloem in the vascular trace emanating from the shoot axis. Unifacial leaves form tubes, as in the case of onion (Allium cepa) and rushes (Juncus glaucus), have no upper or lower sides, and appear to be derived entirely from abaxial tissues. In contrast, equifacial leaves may be flattened, as in the case of the mint Satureja, but have adaxial tissue (palisade parenchyma) on both upper and lower sides.