Not available – first paragraph follows: When Howard B. Frost began citrus breeding at the University of California Citrus Experiment Station in 1914, knowledge of crossing relationships in the genus was limited to earlier studies in Florida by the U.S. Department of Agriculture. Frost began a wide series of crosses among edible types, but the numbers of hybrids first obtained were limited by nucellar embryony. This is the phenomenon by which somatic cells of the nucellus (tissue in the ovule but outside the embryo sac) develop into embryos. Since these nucellar embryos develop asexually, with no male cells contributing to their formation, they are usually genetically identical with the seed parent.