Iris virginica 'Alba'

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I don't think Iris virginica f. alba is a described taxon, so it's either a cultivar, 'Alba', or multiple clones improperly lumped under a single cultivar name. Anderson (1936) suggested that white forms are exceedingly rare in virginica and versicolor compared to most irises, because of their tetraploid/amphidiploid nature. The former has two and the latter three sets of genes for pigment, each of which must be mutated seperately to produce an entirely white flower.