It happens every year: as soon as it begins to get warmer, plants once again sprout from the ground, and trees begin to grow buds.
How do they detect that it's spring? What are the mechanisms involved?
Something surprising that not many people realize is that plants can tell time. Plants have genes that suppress flowering and growth, and these genes are time sensitive. Plants actually register how many cold days have passed, and when enough have gone by, the genes are temporarily deactivated, allowing flowering and growth. Usually, the amount of cold days is equal to how many days are in winter and fall, hence why most plants do not grow much or flower in those seasons. It's important for these plants to have mechanisms that count the number of cold days, because if a plant goes into its flowering phase too early, it usually is quite bad for the plant. It usually prevents the plant from reproducing properly, and in the case of smaller plants, it is often fatal.
Another use plants have for this time-keeping mechanism is when to drop their leaves and seeds. Those processes also require certain seasons, and if they occur in the wrong seasons, there are detrimental consequences for the plant. If seeds are dropped in the wrong season, they may not sprout in the right season, or they may also all get eaten by wildlife. If leaves are dropped during the wrong season, especially in summer or spring, the plant's whole schedule will be thrown off, and it will unable to use photosynthesis properly.
Plants also keep a 24-hour clock as well as a daily one. When plants are flowering, they function on a 24-hour cycle, opening their petals in the daytime, and closing them at night. They also function like this in regard to their leaves, especially plants that live in hotter and drier areas. There is a certain group of plants called CAM plants who close their stomata (holes in the leaves that allow for gas transport) during the day to minimize water loss, and reopen them at night. Unlike most plants, the open stomata at night collect carbon dioxide to use during photosynthesis in the day, while the stomata are closed. Most other plants keep their stomata open at all times, since they tend to live in less arid conditions.
People aren't sure exactly what causes these time-keeping mechanisms to work, but they are certainly there nonetheless. Even if people don't realize they are there, they are very important factors in keeping the plant life of the world alive.