M. N. Westwood, H. O. Bjornstad, Some Fruit Characteristics of Interspecific Hybrids and Extent of Self- Sterility in Pyrus, Bulletin of the Torrey Botanical Club, Vol. 98, No. 1 (Jan. - Feb., 1971), pp. 22-24
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Westwood et al. (1971) studied this question.
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