Flowering Dates of Viola sororia Willd. and V. pensylvanica Michx. at Different Latitudes

The response to daylength has been shown to be significant in the initiation of flowering in many species of vascular plants. Herbarium specimens of Viola sororia Willd. and V. pensylvanica Michx. from a wide range of latitudes were examined to ascertain correlation between dates of flowering and length of day. Two observations were made: the higher the latitude, the later flowering occurred in the spring; and the higher the latitude the greater the daylength at the time of open flowering. It is suggested that these north-south clines represent a genetic response arrived at through natural selection. Further studies arc proposed to test this hypothesis