Control by Daylength
Daylength is one of the major environmental factors controlling flower initiation in many plants. A factor influencing the critical daylength is the number of favorable cycles given. It is a common observation that a brief favorable daylength regime is sufficient to bring about subsequent flower initiation even after the plant has been returned to unfavorable daylength conditions. It is a very general rule that daylength is most effectively perceived by the leaves. This was first shown by Knott in the LDP Spinacia, and rapidly confirmed and extended to a great number of species belonging to all photoperiodic response types. Production of flower buds is thus an after-effect of the favorable photoperiodic treatment, which has accordingly been referred to as "photoperiodic induction" or "photoinduction". The critical daylength, the cornerstone of classification of plants according to their photoperiodic response, is far from constant in all conditions.