The transformation of vegetative apex into a floral apex is a multifactor and multistep phenomenon. Despite researches on flowering carried out during the last seventy and odd years what we know is far less than to what we do not know.
The effect of day length on flowering of some of the temperate plants was first recorded by Garner and Allard. Subsequent work has established a classification of plants on the basis of their day length requirement to flower.
1. Day Neutral Plants: Flower irrespective of the length of the daily light (cucumber, sunflower, maize)
2. Long-day plants: Require a light period longer than a "critical period" (spinnach lettuce herbane)
3. Short-day Plants: Flower when exposed to a light period less than a "critical period". The classical example is cocklebur Xanthium strumarium. Others are chrysanthemum, strawberry etc.