With the increase of age, the average branch Angle of different grades is totally increase.
各级分枝的平均分枝角度随树龄的增大而呈现不同的波动趋势，但总体来说略有增加。
If the cross section area of the two branching ducts are different, the Included angle on the smaller branch duct against the extended center line is usually bigger.
若分枝管的截面积大小不同，则细支管与母管中心延长线间的夹角较大；
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