Surface projection (top row) and optical cross-sectional (bottom row) images of regions marked in (G) and (H). Close to the meristematic notches, an initial aperture (yellow arrowhead) forms at the intersect of approximately four epidermal cells. Polarized growth of aperture-surrounding cells occludes the initial aperture in wild type. Polarized growth fails to take place in Mprop-4, and the aperture remains open. In wild type, the division of aperture-surrounding cells is oriented radially with respect to the closed aperture (yellow dotted line). In Mprop-4, division of aperture-surrounding cells can be misoriented by 90° (yellow dotted line). Asterisks mark epidermal cells forming the aperture boundary visible in the optical cross-sectional images. Scale bars: 20 μm
