A different type of behavior is exhibited in Campanula medium (Canterbury bell) and some other species: Short days acting in the leaf can completely substitute for vernalization at the apex (Web Figure 20.9.A). In this case vernalization and photoperiod appear to act as alternative pathways to flowering. The relationship between the two pathways is not known. One possibility is that vernalization may render the meristem competent to respond to low levels of a floral stimulus that is formed in non-inductive long-day photoperiods. In inductive short-day photoperiods perhaps sufficiently high levels of a floral stimulus is formed so that even a non-vernalized meristem with reduced competence is able to undergo the floral transition. In these scenarios, vernalization and photoperiod are interacting to change the thresholds of competence and floral stimulus levels required for flowering.

Web Figure 20.9.A   The control of flowering in the SDP Campanula medium (Canterbury bell). When grown in continuous long days (LD), the plant grows as a rosette and the stem does not elongate. Eight weeks of short days (SD) followed by long days results in both elongation and flowering. The short days can be substituted by 8 weeks of low temperature. Application of GA₃ results in stem elongation but not in flowering induction. (Photo from Wellensiek 1985.)