The vegetative apex grows in an indeterminate, iterative mode. When flowers are formed it changes to determinate, sequential growth. The essential change in growth at the transition to flowering seems to be the decrease in primordium size at initiation, relative to the size of the apex. In the formation of the flower itself there may be a further requirement, that primordia undergo a decrease in absolute size at initiation. Where measured, this decrease is parallelled by a decrease in the size of the stem frustum at initiation. While in the case of the stem frustum this decrease in size has a predictable consequence, the reduced size of internodes in the mature flower, it is not clear how the reduction in size of the primordium is related to the subsequent development of flower parts. Reduction in primordium size seems to be fundamental to the changes in primordium arrangement and divergence angle that typically occur on flower formation, but not sufficient to cause the subsequent differentiation of floral parts. This presumably occurs as a consequence of the interactions between the developing primordia and substances present in the flowering apex.
Recent models of flowering show that a realistic simulation of apical growth on the transition to flowering can be made using the equations of catastrophe theory. To account for the growth and development of a flower meristem models must also account for changes in divergence angle and the new developmental pathways followed by the floral organs.
The data on the role of growth substances in the transition to flowering at the apex are at present insufficient. What is needed particularly is information on the identity of the putative inhibitor of primordium initiation that may operate within the apex, and its role in the growth changes that occur during and after the floral transition.