Patty Niehaus, age 12, of Indianapolis, Indiana, for her question:
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 water tight. Water molecules can permeate through it by diffusion, one by one. But water is a cohesive liquid, its molecules stick and cling together. When one diffuses through the wall, it brings another linked behind it, another and another. However, the water passes only in one direction. The wall of a living plant cell is a semi permeable membrane and it allows water to diffuse only from a weaker to a stronger solution.
The living protoplasm inside a cell is a strong, rich solution of dissolved chemicals. Ordinary ground water contains chemicals but the solution is much weaker. Down in the ground, the roots come in contact with tiny pockets of moisture in the crumbly soil. The root tips sprout millions of tiny, tiny hairs so that a larger surface of semi permeable membrane is spread around to absorb the moisture.
The weak solution of ground water diffuses through the surfaces of the rootlets into the cells. There it dilutes their chemical .rich protoplasm. This process is called osmosis. And osmosis builds up pressure. Water diffusing into the cells fills them and stretches their skins to the bursting point. When all the cells have as much as they can hold, they hold the root stiff and firm. Now suppose the root is growing by adding new cells. The new cells also fill up firm and strong. And there are millions of strong neighboring cells to help them. If they need more room, they can use all this strength to push their way through the hard ground. In the roots of a tree, the pushing pressure of osmosis may be ten to 50 times more than the pressure of the atmosphere. Scaled down to cell size and reduced to a snail pace, the pressure of osmosis in plant roots may equal the power of a steam engine. Patiently, cell by cell, roots can use this pushing to shove through the hard, hard ground.
Experiments show that osmosis pressure in plant roots is strongest at dawn. It dwindles to nothing as the day wears on. So some other force must be working to keep vital water supplies streaming through a plant. It is transpiration, the evaporation of water vapor through the leaves. As water molecules leave, replacements are sucked in from neighboring cells to replace them. Osmosis is a shoving process up from the roots. Respiration is a pulling process down from the leaves. Both forces work together to keep the plant cells stiff, full of moisture and giving it pushing power.