Gymnosperms comprise a small group of seed plants characterized by naked seeds (gymno-naked, sperms-seeds) and absence of vessels (except Gnetopsids), endosperm formation independent of fertilization and commonly resulting in halploid endosperm (absence of double fertilization), absence of sieve tubes and companion cells. Group is represented by nearly 15 families, 80 genera and nearly 820 species, mostly consisting of evergreen trees and shrubs, distributed worlwide and forming extensive forests in North America, Europe and Asia. They represent some of the largest (Sequoiadendrod giganteum of California), tallest (Sequoia sempervirens of California and Oregon) and longest living (Pinus aristata) organisms in the world.