As early as 1940s, scientists believed that aging was caused by the exhaustion of plant nutrition in sexual reproduction. Many experiments have pointed out that removing flowers and fruits of plants can delay or prevent the senescence of leaves, but the problem is not that simple.

If you are interested, you might as well do an experiment. In the flowering season of soybean, the growing flower buds should be removed every day, and you will find that the senescence of soybean without flower buds is obviously delayed than that of plants without flower buds. Further observation also found that the senescence of many plant leaves occurred before flowering and fruiting. For example, when the male flowers of dioecious spinach form, the leaves begin to age.

With the gradual deepening of research work, it is now known that the protein content, RNA content and photosynthetic capacity of leaves decrease significantly during leaf senescence. Under the electron microscope, it can be seen that chloroplasts are destroyed when leaves are aging. These physiological and cytological changes are the basis of senescence, and the final result of leaf senescence is defoliation.

From the perspective of morphological anatomy, it is found that deciduous leaves are related to the special structure near the base of petiole-separation layer. Under the microscope, it can be observed that the parenchyma cells in the abscission layer are smaller than the surrounding cells. During leaf senescence, the activities of pectinase and cellulase in abscission layer and its adjacent cells increase, which leads to the dissolution of the whole cell and the formation of natural fracture surface. However, the vascular bundle cells in the petiole do not dissolve, so the aged and dead leaves are still attached to the branches. However, these vascular bundles are very slender. When the autumn wind blows, they can't resist it, breaking their bones and muscles, and the whole leaf staggers to the ground.