Allelopathy is the release of chemicals by certain plants that inhibit the growth and development of competing plants. The chemicals are usually terpenes or phenols and may be found in roots, stems, leaves, fruits, or seeds. An example of this relationship among plants is the black walnut tree (Juglans nigra): A chemical compound in the leaves and green stems of the black walnut tree is leached by rainfall into the soil; it is then hydrolyzed and oxidized into another compound called juglone—a compound shown to be very toxic to many plants as well as an inhibitor of seed germination. For example, tomatoes and alfalfa will wilt when grown near black walnuts, and their seedlings will die if their roots contact the tree’s roots. Similarly, white pine (Pinus strobus) and black locust (Robinia pseudoacacia) are often killed by black walnuts growing in their vicinity.