Numerous studies show there is great genetic variability in tree fruit and nut species (24). Where quantitative genetic analyses have been made they generally reveal that most of this genetic variability is additive (18, 19, 20, 21, 22). Consequently, the simplest, least expensive breeding method (mass selection) should be as effective as any for improving tree fruit and nut cultivars. Furthermore, most tree fruit and nut species are perennial and can be readily cloned (most commonly by bud grafting). These properties provide geneticists powerful engineering tools that are unavailable in most agronomic and vegetable species. Taken together, these facts suggest that the genetic techniques so successful in improving agronomic and vegetable cultivars should be even more effective when applied to tree fruit and nut crops. However, this is not the case because large plant size and long periods of juvenility severely impede the process of cultivar improvement in tree fruit and nut species.