How does this plant know it's Christmas?
This plant is known as a Christmas cactus because it only blooms once a year, during the Christmas season. But plants can't read a calendar. How can a houseplant know the time of the year?
Have you seen the leaves of plants change colors? During what time of year does this happen? What causes it to happen? Plants can sense changes in the seasons. Leaves change color and drop each autumn in some climates (Figure below).
Leaves changing color is a response to the shortened length of the day in autumn.
Certain flowers, like poinsettias, only bloom during the winter. And, in the spring, the winter buds on the trees break open, and the leaves start to grow. How do plants detect time of year?
Although you might detect seasonal changes by the change in temperature, this is not the way in which plants know the seasons are changing. Plants determine the time of year by the length of daylight, known as the photoperiod. Because of the tilt of the Earth, during winter days, there are less hours of light than during summer days. That’s why, in the winter, it starts getting dark very early in the evening, and then stays dark while you’re getting ready for school the next morning. But in the summer it will be bright early in the morning, and the sun will not set until late that night. With a light-sensitive chemical, plants can sense the differences in day length.
For example, in the fall, when the days start to get shorter, the trees sense that there is less sunlight. The plant is stimulated, and it sends messages telling the leaves to change colors and fall. This is an example of photoperiodism, the reaction of organisms, such as plants, to the length of day or night. Photoperiodism is also the reaction of plants to the length of light and dark periods. Many flowering plants sense the length of night, a dark period, as a signal to flower. Each plant has a different photoperiod, or night length. When the plant senses the appropriate length of darkness, resulting in an appropriate length of daylight, it flowers. Flowering plants are classified as long-day plants or short-day plants. Long-day plants flower when the length of daylight exceeds the necessary photoperiod, and short-day plants flower when the day length is shorter than the necessary photoperiod. Long-day plants include carnations, clover, lettuce, wheat, and turnips. Short-day plants include cotton, rice, and sugar cane.
- photoperiodism: The reaction of organisms to the length of day or night.
- season: Subdivision of the year due to a change in weather, ecology, and hours of daylight.
- Plants can respond to the change of season by losing their leaves, flowering, or breaking dormancy.
- Plants go through seasonal changes after detecting differences in day length.
Use the resource below to answer the questions that follow.
- What happens to the chlorophyll in the leaves that causes them to change color?
- Why do the leaves of trees in the tropics not change color in the fall?
- Why do some trees drop their leaves in the winter?
- How does temperature affect the color of the leaves? Explain your answer fully.
- How do plants detect the change in seasons?
- What signals a tree to drop its leaves?
- Distinguish between long-day plants and short-day plants.
- Give two examples of long-day plants.