It is a common sight to see parts of a sidewalk heaved up by a tree root and their force in this respect is tremendous. One of our common ferns has been known to raise a weight of over 500 pounds, and even to break through a concrete walk. Such is the force exerted by the roots of plants that we can truly think of roots as pushing through the earth almost regardless of obstacles, binding the soil together and not only serving the needs of the plants, but actually holding the soil on steep slopes. Where fire or ruthless lumbering has stopped this natural process the washing away of the soil and exposure of the bare rocks leaves desolation behind it.
While most roots live under the surface many grow in the air, and a few grow from stems that are injured. The tomato vine often produces roots at the joints or where it has been injured. Such roots, known as adventitious roots (Figures [4] and [5]), are fairly common in many plants, the common garden practice of making cuttings, which take root under favorable conditions, being based upon this fact.