This article discusses a botanical-agricultural reason for the prohibition against eating legumes on Passover, one presented by Rabbeinu Manoah (Provence, thirteenth century). According to Rabbeinu Manoah, the basis of the prohibition is an ancient agricultural concept whereby through changes in climate and annual rainfall, wheat kernels become legumes. It seems that technical changes in agriculture in the Middle Ages affected the ban. The practice of using a three-field rotation of grains and legumes in European fields increased the mixture of aftergrowths of legumes from the previous years in the grain harvest of the present year. This phenomenon strengthened the ancient concept among contemporary agriculturists that grains had changed into legumes, when, in fact, the legumes were merely aftergrowths.
Emanuel, The halakhic writings, 246–247. The explanation of the prohibition as a result of wheat that became mixed with legumes was not cited by the Mordechai, op. cit., nor by the Hagahot Maimoniyot in the version that has reached us. It was indeed cited by R. Yosef Karo in the Beit Yosef (“Oraḥ Ḥayyim,” siman 453) on behalf of Hagahot Maimoniyot, on behalf of the Semak, but as stated this is not in evidence in the Hagahot Maimoniyot in its current form. I wish to note that the version of Hagahot Maimoniyot used by R Joseph Karo here, and throughout his work, is the first edition, published in Constantinople in 1509. On this version, which has been reprinted as part of the Shabse Frenkel edition of Mishneh Torah, see Israel Mordechai Peles, “Discovery: Ancient Manuscript of Hagahot Maimoniyot—As Kushta Version,” Yeshurun 13 (2003): 744–787.
Thomas Gaskell et al., Flora Europaea (Cambridge: Cambridge University Press, 1968), vol. 2 (Vicia sp.); Abraham Fahn et al., The Cultivated Plants of Israel (Tel Aviv: Hakibbutz Hameuhad, 1998), 213 [Hebrew].
Felix, Mixed Sowing, 19. And compare to the Mishnah Ter. 2:6, which states that although in regard to kilayim, wheat and zunin are considered one species, donations of wheat to the temple are not given from zunin because they are inedible. I.e., the two are not always considered identical, and there are variations between the different halakhic fields.
White, Medieval Technology, 71; Shahar, The Medieval Heritage, 86–87. And not as stated by Emanuel, The halakhic writings, 246, n. 161: “as the field is planted alternately: one year with wheat, one year with legumes, and in the third year the field was left fallow.” Rather the field was divided in three.
Constance Hoffman Berman, Medieval Agriculture, the Southern French Countryside, and the Early Cistercians, A Study of Forty-Tree Monasteries (Philadelphia: American Philosophical Society, 1986), 91–92.