Why do some trees drop their leaves and some don’t?

  The main function of leaves is to provide nutrients to trees through photosynthesis. The cells of leaves contain chlorophyll, a chemical that allows cells to absorb sunlight and use the absorbed energy to make nutrients. Trees that shed their leaves every year are called deciduous, and they shed their leaves through a complex and energy-intensive process called abscission. 
  When autumn comes, due to shortened sunshine hours and changes in temperature, leaves will stop producing chlorophyll, and the remaining nutrients in leaves will be transported to the trunk and roots for storage. Because chlorophyll is green, after the production of chlorophyll stops, other chemical pigments in the leaves will reveal, such as yellow and orange, so the leaves will change color at this time. Reds and purples appear when sugars remain in the leaves. At the same time, the cells in the leaf stalk begin to form a cork membrane, called the separation layer. After all the nutrients have left the leaves, the cells in the separation layer will digest themselves, and because the leaves have been isolated from the trunk, they will fall off when the adhesion is lost. 
  Berner Rubinstein, a professor of biology at UMass Amherst, said that all trees have different rates and methods of “detachment”. For example, dead leaves on oak trees stay on the trunk all winter because the separation layer doesn’t fall off completely. For the same tree, the younger the tree, the longer the leaves stay on the trunk. Since reduced sunlight is the factor that induces “detachment”, deciduous trees will fall more slowly if they are located next to artificial light from street lamps. Evergreen trees such as pine, spruce, cedar, and fir do not lose their leaves or needles every year. Because the needles of these species have a thick layer of protective wax, and the sap inside the cells contains substances that resist freezing. The needles of evergreen wood drop and renew continuously, not all at once, and the needles can even live for several years.
  It’s hard to explain exactly why deciduous wood leaves fall once a year, but evergreen leaves do not fall as often, but it’s a result of plants adapting to the climate. Rubinstein believes that deciduous wood sheds its leaves in winter to avoid the death of sensitive leaf tissue due to low temperature; as for evergreen wood, because it usually grows in the cold zone, it is more resistant to severe cold.

  About 6,000 years ago, the Mesopotamians invented the wheel. They like the number 60 because 60 is useful, has many factors, and is convenient for doing basic calculations or doing business. Mary Blocksma’s book “Reading the Numbers” mentions that the 60-digit number system from Mesopotamia was passed on to ancient Egypt, where it was used by the ancient Egyptians The system divides the circle into 360 degrees. A 360 degree circle worked really well for the ancient Egyptians. They love equilateral triangles, and a circle can fit exactly 6 equilateral triangles. Since a circle consists of exactly 6 triangles whose interior angles are all 60 degrees, it makes sense that the interior angle of the circle is 360 degrees. The Egyptians also invented the symbols of angles, and they also invented a calendar system with 360 days a year, which is only 5.25 days away from the modern calendar system. Later, the use of a 360-degree circle passed the test of time, which in turn affected the time scale: when people first recorded time on a circle, it was natural to divide every hour into 60 minutes and every minute into 60 seconds.

  Dudareva, an associate professor in the Department of Horticultural Landscape at Purdue University, answered the following: Aroma is a chemical signal that attracts pollinators to specific flowers for nectar, pollen, or both. These volatile organic molecules are very important for insects to select and find flowers, especially flowers that are pollinated by moths, which are mainly nocturnal. Flowers that are pollinated by bees and butterflies tend to have a sweet fragrance; those that are pollinated by beetles have a strong rotten, spicy or fruity smell.
  As of today, our understanding of how insects respond to individual chemicals is very limited, but what is certain is that they are able to distinguish between different odor combinations. In addition to being used to attract and guide insects to find food in flowers, floral scent can also be used by insects to identify different plants, or even a single flower of the same kind. For example, flowers of similar plants with different pollinators have different aromas, indicating that different pollinators have different olfactory sensitivities or preferences. At the same time, the successful completion of pollination (meaning sexual reproduction) can help the reproduction of the plant.
  Fragrance usually peaks when the flowers are just right for pollination and the pollinators are active. Bees and butterflies mostly stay on flowers that emit strong aromas during the day, while moths and bats visit flowers that smell mainly at night. Younger flowers that have not yet bloomed produce less aroma because they are not yet able to provide pollen and are therefore less attractive to pollinators than older flowers. Once the flower is fertilized, the scent also diminishes, which encourages insects to choose other flowers.

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