How can roots grow down through the hard ground?
The roots of small plants can open cracks in solid rocks and the roots of a tree can crack open the concrete pavement of a sidewalk. Obviously plant roots have pushing power. But until about 150 years ago, nobody had any idea of the mysterious force they use to shove through the ground. Healthy plant cells are filled with a jellified liquid called protoplasm. Each cell is a unit that carries on a variety of vital chemical activities. It is surrounded by a cell wall that keeps it in bounds, but this wall is not water tight. By a patient process called diffusion, the wall allows each cell to exchange fluids with its neighbors. The diffusion of liquids holds the key to the mysterious pushing power in a plant’s roots. The doors it unlocks between cells are special plant or animal tissues called semi permeable membranes. Membranes, of course, are skin type tissues. A non permeable membrane is a watertight tissue such as the skin that covers your body. A semi permeable membrane is partly wat
