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Size and shape of pollinia, corpuscula, anther wings, and stigmatic chambers vary greatly amongst the species of Asclepias. Hense it is not surprising that interspecific hybridization within the genus is very infrequent. After over a decade spent in studying it, I doubt whether I have observed many more than a dozen instances of putative hybrids amongst the approximately 80 species.
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Robert E. Woodson (1947) studied this question.