Susan E. Meyer, Habitat Correlated Variation in Firecracker Penstemon (Penstemon eatonii Gray: Scrophulariaceae) Seed Germination Response, Bulletin of the Torrey Botanical Club, Vol. 119, No. 3 (Jul. - Sep., 1992), pp. 268-279
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Susan E. Meyer (1992) studied this question.
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