Fig. 8. Proliferation in the sepal boundaries. (A) An alcA::HistoneH4GFP (alcA::H4GFP) construct was introduced into a STM::ALCR alcA::GUS line (1) or into a LFY::ALCR alcA::GUS line (2) in order to obtain inducible H4GFP expression in the boundary domain or in the entire meristem, respectively. As STM-driven expression of the H4GFP extended towards the centre of stage 2 meristems, we considered as part of the boundary only the two marked cells files closest to the primordium (between broken and unbroken blue lines in 1). (3) The mitotic index (MI) in the sepal boundary domain (black bars) and in the entire meristem (white bars) was calculated for floral meristems before (stage 2) during (stage 3) or just after (stage 4) sepal primordia initiation. The four outermost cell layers were analysed and the number of cell counted for each class is indicated below the bars. (B) The sepal boundaries of stage 2-4 flowers were subdivided into boundaries between two sepals (S-S) or between sepals and the meristem (S-M) (1). (2) The MI of these two domains was calculated. The four outermost cell layers were analysed and the number of cells counted for each class is indicated below the bars. (C) The orientation of the division axis of dividing cells relative to the axis of the boundary was measured for the cells of the outermost layer. The division axis was defined as the axis joining the two future daughter cells and is perpendicular to the axis of the metaphase plate or the new cell wall (1,2). The orientation of a S-M dividing cell was calculated relative to the boundary axis (1). The boundary axis was defined as a line tangent to the outer limit of the boundary domain (recognisable as the limit between GFP-expressing and non-expressing cells). The orientation of a S-S dividing cell was calculated relative to the two adjacent boundaries (2). The insets (1,2) show magnifications of the dividing cell. (3) The number of dividing cells was plotted against the orientation of the division axis. Orientations with a high angle value (see 1) are perpendicular to the boundary whereas low values (see 2) are parallel. m, meristem centre; s, sepal primordium.