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Hawaiian Metrosideros distribution extends from tropical to cool-temperature climates throughout the six major islands of the Hawaiian Island chain. It forms a highly polymorphic complex that occurs in a continuous distribution over areas uith average annual rainfalls ranging from 30 to 450 inches and at elevations from sea-level to 8500 feet, and in diverse pedological and topographical habitats. All of these plants are probably derived from one or a very small number of ancestral introductions that arrived within the last 20 million years by long-distance dispersal. Seeds collected from diverse altitudinal sites on the islands of Hawaii and Maui and grown under uniform greenhouse conditions show evidence of ecotypic differentiation along altitudinal gradients. The seedlings, although from islands separated by 50 miles of ocean, show a parallelism in their altitudina