The response of organisms to the length of day or night is called photoperiodism. The flowering plants are using a photoreceptor protein for sensing the change in night length. Those which need a short or a long night before flowering are called obligate photoperiodic flowering plants. They are classified in long-day or short-day plants, although the hours of darkness are important for their blooming, and not the hours of daylight.
Most of the long-day plants bloom during summer, in both southern and northern hemisphere. But the short-day plants need more hours of darkness in a day for blooming, such as Poinsettia, Thanksgiving or Christmas cactus, Kalanchoe, Aloe and Chrysanthemum. Many of these plants originated in the southern hemisphere and they bloom as indoor plants in the northern hemisphere when days become shorter, in the fall or winter season. To coax them to bloom for Christmas in the southern hemisphere, gardeners need to create artificial short day conditions by covering the plants with dark cloths or bags or by keeping them in dark rooms, from 5 p.m. to 8 a.m. every day, from late August to early December. This is how they force Poinsettias to bloom in California, at Paul Ecke's farm.
A few years ago, when I first started to grow Poinsettia, I read about this night and day cycle in the poinsettia's life and how important dark was for getting this plant to bloom. Later I realized that this wasn't just a tip for the Poinsettia growers who need time control of Poinsettias so the plants will bloom before Christmas for selling them. I just recently figured this out. At first I followed their advice, covered my plants with black plastic bags and had very good results: the Poinsettias bloomed very well, but it was a lot of trouble. However, last year I was too busy to cover them, but they bloomed anyway, just in time for Christmas, without forcing. That told me that Poinsettias don't need artificial dark in the northern emisphere where I'm living, because the nights are just long enough for making them bloom.
It is the same for the other short-day plants I have potted as houseplants, such as Aloe, Kalanchoe or Thanksgiving and Christmas cactus: they bloom without any interference when the days get shorter, starting in November. As for the Chrysanthemums I have in my garden, they are perennials that bloom in the late fall, as the days are getting shorter. I don't need to interfere in their life cycle because their circadian rhythm is normal in my country, so I can let them grow as they are and just enjoy their beauty.
Now that I know why some plants like short days, I wanted to share my discovery with you, whether you are in the northern or the southern hemisphere. Complicated words like circadian rythm, photoperiodism or photoreceptors have a greater meaning to us now, don't you think?