sentences:  The "ARP" function appears to have arisen early in vascular plant evolution, because members of the primitive group Lycophytes also have a functionally similar gene.Other players that have a conserved role in defining leaf primordia are the phytohormones auxin, gibberelin and cytokinin.The arrangement of leaves or phyllotaxy on the plant body can maximally harvest light and might be expected to be genetically robust.