A change from the vegetative to the reproductive phase as a result of detection of, or exposure to, a period of light or dark of a given length. The length of the period of light or dark required to initiate the change is set relative to a particular duration known as the 'critical day length'. The critical day length varies between species. [ https://www.worldcat.org/search?q=bn%3A0582015952 https://www.worldcat.org/search?q=bn%3A0697037754 https://www.worldcat.org/search?q=bn%3A0709408862 ]