Most fruit trees require pollination to produce fruit.  Some trees are capable of pollinating themselves (self-fertile), others require pollen from another tree (self-sterile).  As a general rule, pollenizers should be no more than 50 feet apart from one another, though trees up to 200 feet apart may cross-pollinize.  The adjacent chart outlines the general requirements for fruits in this catalogue.  See the variety descriptions for more specific information.