Fig. 3. Surgical and genetically induced leaf-polarity defects. (A) Vegetative tomato shoot apex, showing ablation of the L1 layer (black arrowheads), separating the incipient leaf (I1) from the meristem. Leaf primordia of increasing age are shown (P1-P4). (B) Longitudinal section of a tomato apex, showing an incision restricted to the outermost (L1) layer of the SAM (black arrowhead), which separates the P1 primordium from the meristem. (C) SEM of a tomato apex 1 day after the P1 has been surgically separated from the SAM. The P1 develops as a radial, abaxialized primordium (black arrowhead shows the scar from the ablation). By contrast, a primordium that developed in contact with the meristem (I1) is dorsoventrally flattened and develops leaflet pairs (white arrowheads). (D) Transverse sections of Antirrhinum leaves from wild-type (WT) and phan plants. Relative to wild type, the phan leaf is radial and composed of abaxial parenchyma surrounded by abaxial epidermis. Notice that, in the phan mutant, vascular polarity is lost and that the xylem no longer lies adaxial relative to normally abaxial phloem. Images A-C are reproduced with permission from Reinhardt et al. (Reinhardt et al., 2005) and D with permission from Waites and Hudson (Waites and Hudson, 1995).