ELIMINATION OF FLOWERING AND MOST CYTOLOGICAL CHANGES AFTER SELECTIVE LONG-DAY EXPOSURE OF THE SHOOT TIP OF SINAPIS-ALBA

Entire plants of Sinapis alba exposed to a single long day were induced to flower. However, if only the shoot tip was exposed to the long day, no flowering ensued. In the apical meristem of plants with only the shoot tip exposed to the long day, none of the ultrastructural changes normally observed in the meristem of induced plants were detected, except for a marked increase in the number of mitochondria per cell. We conclude that the great majority of ultrastructural changes normally occurring in the shoot meristem during floral transition are not direct effects of daylength on the tip but are caused by signal(s) generated in induced leaves.