Fig. 6. Formation of radially symmetric primordia after ablations of the L₁ layer. (A) Control tomato apex after 7 days in tissue culture. Besides P₁, which was the youngest primordium at the beginning of the experiment, four new primordia were formed in clockwise phyllotaxis. The youngest (I₄) is just becoming evident at the flank of the meristem. (B-F) Tomato apices 7 days after the removal of 75-100% of the L₁ layer. The apices had one preformed primordium (P₁) at the time of the operation. (B) I₁ has developed into a radially symmetric organ with a small adaxial domain at the distal end (between arrows). (C) P₁ has developed into a radially symmetric primordium. The trichomes on its surface are typical for the abaxial side of normal leaf primordia, indicating that the radially symmetric primordium has only abaxial identity. The white arrow indicates an axillary meristem growing from the leaf base of P₃. (D) P₁ developed almost normally in the distal part; however, at the base, the adaxial domain is lost after the formation of one pair of leaflets (white arrowheads; compare with P₁ in Fig. 5A). One additional primordium was initiated (I₁), which developed to be completely radially symmetric with only abaxial identity, based on the morphology and the distribution of the trichomes. (E) P₁ has developed similarly to the primordium shown in D. After the formation of one pair of leaflets (arrowheads), the adaxial domain terminates with a single central leaflet (cl). (F) P₁ has developed normally (compare with P₁ in A), and exhibits two pairs of leaflets (white arrowheads). Axilllary meristems of older primordia are induced to grow out (arrow). M, meristem; P₁, youngest preformed primordium; I₁, I₂, I₃ and I₄, first, second, third and fourth primordium, respectively, formed after the ablation. Scale bar: 200 µm.