The olive tree is one of the few trees that can still produce fruits even in rocky and unproductive land. Trees are extremely long-lived and tolerant of drought, salinity and almost total neglect, and so have been reliable producers of food and oil for thousands of years. Oil was used for cooking as well as burning in lamps; several references are made to olive oil lamps in the Bible and other ancient writings. In Mediterranean or desert climates, olives are frequently used as yard trees. They have attractive, silver-green foliage, full canopies, are evergreen, and require very little water and maintenance. However, the fruits will stain sidewalks and cars (black-purple oily stains), and the pollen is highly allergenic. Substitute male-sterile cultivars have been developed for urban use. The olive is often mentioned in mythology and the Bible. Athene, the goddess for which Athens was named, is said to have won this right by placing the world's first olive tree on the Acropolis in Athens. Athena is often shown with an olive branch, a symbol of peace and plenty. The olive oil is still being used for medical purposes and religious purposes and it has been proved to be an essential ingredient of a healthy diet. As a monosaturated fatty acid, olive oil does not have the same cholesterol-raising effect of saturated fats. Olive oil is also a good source of antioxidants. The olive was spread throughout Mediterranean Europe and North Africa very early, due to its ease of propagation by seed, cuttings, or "ovules" (callus growths on trunks which produce rooted shoots).