(1) The tree ring on the sunny trunk is wider because it receives more sunlight. The temperature of bark on the sunny side is higher than that on the shady side, and the cambium divides inward quickly, and these cells form xylem. (2) On the sunny side, the root system is developed, and the one that absorbs nutrients is called the shady side. It needs a thicker conduit to dredge the water and inorganic salts. Over time, the trunk on the sunny side is thicker than the trunk on the shady side. (3) Because the temperature on the sunny side is higher than that on the shady side, the roots and crowns of trees on the sunny side are larger than those on the shady side, that is, the root-crown ratio is larger. There are also many hormones such as cytokinin, auxin and gibberellin. Causing the trunk to thicken.