Each plant must pass through a minimal 'ripeness to flower' stage. That is, even to perceive and respond to a specific photoperiodic regime, a plant must be ready. This may be age, size or stage by which a specific number of leaves must have been formed. In perennials this is called a 'Juvenile phase'. A phase shift from Juvenile to adult stage must be crossed before flowering ensures. There are speculations of the existence of a quiescent center in shoot apices but it has not been upheld. A floral apex does not produce branches but a 'thalamus' that carries floral parts. A 'Thalamus' does not bear leaves but floral parts like sepals, petals, stamens and carpels-which are modified leaf like structures. This fact was well established by the carpel theory of origin of floral parts. Indian scientists like Prof. V. Puri have more contributions in this field.