1. Giant Sequoia. Giant Sequoia (Sequoiadendron Giganteum) is ranked as number one in the list of largest trees based on stem thickness. General Sherman is the most massive Giant Sequoia single stem tree.
Why some plants have thick stems?
The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.
In desert area there is shortage of water to store water in there stem they thick stems. When it rains, water is stored in the stem. The stems are photosynthetic, green, and fleshy. The inside of the stem is either spongy or hollow.