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The old trees have a higher level of IAA,a plant growth regulator and growth-stimulating hormone,and a lower level of ABA,a growth-resisting hormone.However,the average cross- sectional area of trees at breast height in the ancient trees group has been increasing with aging which shows the continuous cambial cells division. Little difference was found when it comes to their ability to photosynthesize(光合作用)，grow leaves,grow seeds or resist disease compared to younger trees,showing that the ancient trees s group are actually in a healthy and mature state. Lin Jinxing of Beijing Forestry University and an author of the study said that trees usually die from outside stress like pests.They found disease-resistancea ssociated genes kept high