The shoot concept of the flower: Still up to date?

• Loss of apical growth induces fractionation.
• Fractionation is a self-organizing, space-dependent process.
• Meristem expansion goes along with repeated fractionation.
• Differential meristem activity contributes to the diversity of flowers.
• Flowers are sporangia bearing units rather than short shoots with floral organs.

The shoot concept of the flower suggests that flowers correspond to vegetative short-shoots except the fact that their lateral appendages are floral and not vegetative leaves. However, in view of the different properties of vegetative and flower meristems, this concept should be questioned. Differential meristem activity resulting in tubes, hypanthia and inferior ovaries, continuous meristem expansion providing space for stamen fascicles and additional structures and the process of (repeated) fractionation using a given space completely, are characteristics of flower meristems hardly explainable with the shoot concept. Linking instead flower development with recent findings in molecular biology and computational modeling widens the view to the fundamental relation between growth and form. Given that the same general principles characterize plant growth at all life stages, the loss of apical growth appears to play the major role in changing geometry, space availability and genetic regulation in flower meristems. The flower, thus, turns out to be the sporangia bearing tip of a shoot.