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An article from my collection of Scientific Amercan entitled "LIfe Processes of Plants by A. W. Galston addresses your specific question: (Take a deep breath and relax).
"The process to which your question refers is called photoperiodism. How day length affects the flowering of some, but not all plants
Around 1920 Garner and Allard, working on tobacco plants at an USDA experimental station, discovered some very large plants with enormous leaves among a batch of seed they were raising.
They looked forward to collecting the seed with a view to developing a strain. However, whilst the normal tobacco plants set seed in late summer, the large form showed no sign of flowering. They therefore dug them up and transferred them to a greenhouse, where the plants eventually flowered in late December. The progeny of these plants exhibited the same characteristic of late flowering.
Garner and Allard experimented moving the plants from a darkhouse to a greenhouse on a daily basis and determined that these plants flowered with the combination of 9 hours daylight and 15 hours darkness, which is characteristic for Maryland’s latitude in late December.
At this point one should remember that latitude governs difference in day length. At the equator days and nights are of an equal length. The further one gets from the equator, the greater the difference in daylength.
Garner and Allard later determined that soya likewise flowers after exposure to a certain number of consecutive short days. Spinach and most cereals, however, responded to long days. Whilst some other plants, like tomato, are unaffected by day length and flower once they have reached a certain size or developed a particular number of leaves.
They named this response of plants to relative lengths of day and night photoperiodism, and classified plants as short-day, long-day and day-neutral
(Contd.)
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