Fig. 1. Isolation of the site of incipient leaf formation (I₁) from the meristem affects phyllotaxis. (A,E-I) Scanning electron micrographs (SEM); (B-D) light stereomicrographs. (A) Tomato meristem in top view. The site of incipient leaf formation (I₁), which can be predicted to be on the upper left part of the meristem (white arrowhead), was separated from the remainder of the meristem by an incision (black arrowhead) just before imaging. (B) Tomato apex just after operation as in A. (C) The same apex as is shown in B 1 day after operation. (D) The same apex as is shown in B, 3 days after operation. P₁ was removed to expose the meristem. I₂ has initiated at the expected position. Note an apparent post-meristematic increase in the divergence angle between P₁ and P₂ (blue lines), compare with (B). (E) The same apex as is shown in B, 6 days after operation. Note that I₂ (bottom) and I₃ (top) diverge by approximately 180°. (F) Control apex with normal divergence angle between I₂ and I₃. (G) Tomato apex 6 days after operation as in A. Note the extended width of the I₃ primordium. (H) Apex 6 days after operation as in A. The vertical distance between I₁ and I₂, is strongly increased compared with a control (I). P₃ and P₂ indicate the bases of pre-existing leaf primordia that were removed at the beginning of the experiment, and P₁ represents the youngest primordium; I₁, I₂ and I₃ indicate primordia formed after the operation. Black arrowheads indicate the incisions. Blue lines in B and D represent the divergence angle between P₁ and P₂. Scale bar: 100 µm.