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Michael D. Windham, Paul G. Wolf, Thomas A. Ranker, Factors Affecting Prolonged Spore Viability in Herbarium Collections of Three Species of Pellaea, American Fern Journal, Vol. 76, No. 3, The Use of Fern Herbarium Specimens in Biosystematic Research (Jul. - Sep., 1986), pp. 141-148
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Windham et al. (1986) studied this question.
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