Support in plants and the human skeleton

Support means whatever holds a living thing in shape and lets it stand and move. A plant has no bones, so it builds support into its cell walls. The outer walls of the epidermis are thickened with cutin, which is waterproof, so the cells inside keep their water. Older stems and roots can be wrapped in a layer of cork whose walls contain suberin, also waterproof. Inside, cellulose or lignin is laid in the walls: collenchyma cells are thickened with cellulose and still stretch while the stem grows, and sclerenchyma cells are thickened with lignin and are hard and rigid. Where these cells sit also matters: strong cells near the edge of a young stem and around its transport tissue help hold it up. In the human body the support is a skeleton of 206 bones (the book’s count), joined by cartilages, joints, ligaments and tendons. The vertebral column is the axis: the skull sits on top, the rib cage is attached to it, and the arms and legs join it through the shoulder and hip girdles. So the skeleton has two parts. The axial skeleton is the skull, the vertebral column and the rib cage. The appendicular skeleton is the two girdles and the four limbs.