FIGURE 1.

Leaf polarities. (A) Schematic diagram of a longitudinal section of a SAM showing the adaxial–abaxial domains, proximal–distal domains, and medial-lateral domains. (B) Arabidopsis leaf with adaxial side (left) and abaxial side (right). (C) Schematic diagram of cross section through a typical dicot C3 plant leaf. A leaf is enclosed by the upper (adaxial) epidermis and lower (abaxial) epidermis. The upper epidermis has trichomes. The upper mesophyll cells are tightly organized and enriched in chloroplast, whereas the lower mesophyll cells are loosely organized with fewer chloroplasts. Vascular bundles exhibit polarity with adaxially localized xylem and abaxially localized phloem. Stoma are abundant on the lower epidermis and trichome may or may not be present on the lower epidermis. (D) Schematic diagram of cross section through a monocot C4 plant leaf. Bulliform cells are present on the adaxial surface, and bundle sheath cells enclosing the vascular bundle are large and have chloroplasts.