Most plants grow buds, flower, or lie dormant at specific times of year.

What Triggers These Seasonal Changes?

Temperature and rainfall often play a role, but the most important factor to many plants is the length of the day. At least, that’s what biologists thought until the 1940s, when the whole concept of day length sensitivity in plants was turned upside down.

Experiments showed that plants that seem to flower in response to day length actually respond to night length. In other words, what prompts irises to bloom in summer in North America isn’t longer sunny days, but shorter nights.

Here’s how biologists figured out how important nightlife can be to plants. They started with the cockle burr plant, which flowers when days are short. Midway through the day, the biologists exposed the plants to a brief period of complete darkness. What effect did this have on flowering? Absolutely none.

Next, for several nights, in the middle of the night, they exposed the plants to a brief period of light. To the biologists’ surprise, this brief interruption of the darkness inhibited flowering. Clearly it was the long period of darkness and not the short period of light that caused flowering.

Although plants are sensitive to the nighttime part of the light-and-dark cycle, biologists and gardeners often still refer to plants as day length sensitive.

For example, poinsettias are called “short-day” plants because they flower in winter. A stickler for accuracy might call poinsettias “long-night” plants, because what prompts their flowering isn’t the short days of winter, but the long nights.

　　植物的夜生活

　　大多数植物在每年的特定时间发芽，开花，或者冬眠。

　　是什么引起植物这些季节性的变化呢?

　　温度和降雨通常会有所影响，但是对许多植物来说，最重要的因素是昼长。至少在20世纪40年代之前，生物学家们是这样认为的，之后植物对昼长敏感的理论被彻底推翻。

　　实验显示，那些看起来受日长影响而开花的植物实际上是受夜长影响。换句话说，促使北美地区的鸢尾花在夏季盛开的不是更长的白天，而是更短的夜晚。

　　下面让我们来揭秘生物学家是怎样确定夜生活对植物的重要性的。他们由苍耳开始研究，这种植物在昼长较短时开花。生物学家让植物在白天一个短暂的时期内陷入完全的黑暗。这对植物开花有什么影响吗?答案是：完全没有影响。

　　接下来，他们连续几晚让植物在半夜接受短时间的光照。令生物学家吃惊的是，这对黑暗的短暂中断抑制了植物开花。很明显是由于长时间的黑暗，而不是短时间的光照导致的开花。

　　尽管植物对日夜交替中的夜间部分敏感，但是生物学家和园丁们还是通常倾向于认为植物是对昼长敏感的。

　　举个例子，一品红因为在冬天开花被叫做“短日照”植物。一个坚持细节的人为了精确可能称一品红为“长夜”植物，因为促使它们开花的不是冬天的短暂白昼，而是冬天的漫漫长夜。