K. Yeboah Gyan, S. R. J. Woodell, Nectar Production, Sugar Content, Amino Acids and Potassium in Prunus spinosa L., Crataegus monogyna Jacq. and Rubus fruticosus L. at Wytham, Oxfordshire, Functional Ecology, Vol. 1, No. 3 (1987), pp. 251-259
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Gyan et al. (1987) studied this question.