Meristic changes in flowering plants: How flowers play with numbers

• Merism is a highly conserved character in flowers, characteristic for major clades.
• Meristic changes affect all organs (isomerous) or part of the flower (anisomerous).
• Meristic change depends on proportionate size of outer perianth parts and SAM.
• Increase is linked with reduced width of outer perianth and a valvate aestivation.
• Perianth whorl is shifted in two whorls with increased width of organs.
• Carpel increase leads to a centrifugal disturbance of the merism of the flower.
• Merism above pentamery is more stable with even than with uneven numbers.

Merism in flowers is the result of a combination of a whorled phyllotaxis, the proportion of relative sizes of floral organs and flower meristem, and pressures from external organs. Different forms of merism have evolved in major clades and represent key-innovations in floral evolution. The major forms of merism are trimery, characteristic for monocots and several magnoliids, and pentamery in the core eudicots. Despite a strong conservatism of merism within given taxonomic groups, there are several cases of meristic change.It is demonstrated that changes of merism are not random but follow complex developmental patterns. Two kinds of meristic change are recognized, isomerous changes affecting all organ whorls of a flower, and anisomerous changes only affecting part of the flower. Causes for meristic changes are analysed from a developmental (physico-mechanical) and genetic perspective. It is demonstrated that meristic change in the flower depends on the relative size proportion of preexisting outer whorls versus the flower meristem: a reduction of the size of outer perianth organs and valvate aestivation is correlated with an increase in merism. The loss or reduction of perianth parts induces a loss of meristic stability. Meristic increase is also preferentially linked with even numbers of organs, as a whorl may easily switch to two alternating whorls by space restrictions. An anisomerous increase starts preferentially from the gynoecium, mainly affecting the number and position of the stamens.