Diagram illustrating stages of leaf initiation in selected model plant species viz. (A) Arabidopsis thaliana; (B) caulescent Streptocarpus sp. (simple leaf eudicots); (C) Solanum lycopersicum (compound leaf eudicot); (D) Zea mays (simple leaf monocot); and (E) Selaginella kraussiana (microphyll). Black arrowhead indicates PIN1 polarization; white arrowhead denotes auxin maxima; blue arrow shows the direction of auxin flow; black arrow represents upregulation; blunt end indicates repression; red arrow depicts downregulation; yellow dots represent auxin; square bracket indicates leaf founder cells recruitment sites. Illustrations are adapted from Byrne et al.  for A. thaliana; Nishii et al.  for Streptocarpus sp. (caulescent); Koltai and Bird  for S. lycopersicum; Timmermans et al.  and Tsiantis et al.  for Z. mays; Harrison et al.  and Sanders and Langdale  for S. kraussiana. L1, L2 = tunica; L3 = corpus.