Biology in pictures Another cell in the wall

architecture of the maize leaf epidermis would make any bricklayer proud. Most of the epidermal cells arise through symmetric divisions in which new cell walls are made either parallel or perpendicular to the long axis of the mother cell. Long, parallel rows of rectangular cells are the result. Specialised cell types, however, are formed through asymmetric divisions. These include the diamond-shaped stomatal complexes. In the left-hand image, of a wild-type maize leaf epidermis, each complex consists of a pair of triangular subsidiary cells (stained pink) flanking a pair of long, thin guard cells (stained dark blue). In the discordia1 mutant shown on the right, the cell pattern of the epidermis is disrupted; symmetric divisions occur normally but asymmetric ones are misoriented, resulting in abnormally shaped subsidiary cells. The discordia1 gene is required for an actin-dependent process that guides new cell walls to appropriate locations during asymmetric cell divisions.