Shoot apex development in winter Norstar and spring Manitou and the near isogenic lines spring Norstar and winter Manitou. Day 0 is the start of vernalization/acclimation at 6°C following 2 d growth at 4°C plus 13 d at 20 to 22°C (15 day pre-treatment). First number in each plate indicates the days the plants were grown at 6°C. Second number (10) indicates that the plants were grown an additional 10 days at 20°C before sampling for dissection e.g., 7 + 10 indicates 7 d at 6°C followed by 10 d at 20°C. Arrow indicates double ridge. The apices of samples that bracket the vegetative to reproductive phase transition for each genotype are shown. A) Manitou 0+10 SAM at stage-5; B) Manitou 7+10 SAM at stage-7; C) Winter Manitou 28+10 SAM at stage 0; D) Winter Manitou 35+10 SAM at early stage-5; E) Spring Norstar 0+10 SAM at stage-2 F) Spring Norstar 7+10 SAM at stage-5; G) Winter Norstar 49+10 SAM at stage-2 H) Winter Norstar 56+10 SAM at stage-5. DR, double ridge, was used as a marker of a reproductive meristem.