Conifers form a single whorl of multiple cotyledons (embryonic leaves), unlike monocots and dicots. Polar transport of the hormone auxin affects outgrowth of distinct cotyledons, but not the radial positioning of the whorl or the within-whorl spacing between cotyledons.
Holloway et al. present a mathematical model of growth regulator patterning accounting for the response to auxin transport disruption, the stability of the single whorl over the large variation in embryo size, the linear relation between cotyledon number and embryo size, and early cotyledon morphogenesis. The conifer mechanism may apply more generally to single whorls of multiple primordia in other plants.
