Carol C. Baskin, Jerry M. Baskin, Edward W. Chester, Seed Germination Ecology of Two Mesic Woodland Winter Annuals, Nemophila aphylla and Phacelia ranunculacea (Hydrophyllaceae), Bulletin of the Torrey Botanical Club, Vol. 120, No. 1 (Jan. - Mar., 1993), pp. 29-37
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Baskin et al. (1993) studied this question.