Leaves of 51 species and three varieties ofLotus(Leguminosae) were assessed quantitatively for hydrocyanic acid (HCN) reaction intensity and the data correlated with basic chromosome number and geographic distribution. The presence of HCN was determined from 0.05 grams of fresh leaf samples by the picric acid ndash; filter paper technique. A gradation of 10 different color reactions (minus;), (plusmn;), and (+1 to +8) was used to compare the HCN reaction for each species. On the basis of potassium cyanide equivalents, each gram of fresh leaf material was estimated to contain an amount of HCN which varied between the different plants and accessions from 0.5ensp;mg to 750ensp;mg. Most of the Old World species were positive for HCN, whereas the reverse was true for the North American species. This would favor the previous separation of the North American species into a separate genusHosackia. All of thenensp;=ensp;7 species have a greater concentration of HCN than thenensp;=ensp;6 species in both the Old and New World. None of thenensp;=ensp;6 species in the New World reacted positively. Therefore, there was a reduction of HCN with evolutionary development. Two colchicine-induced tetraploids (4xensp;=ensp;24, 28) gave a lower HCN reaction than their diploid counterparts. Leaves ofL. siliquosus(nensp;=ensp;7) reacted negatively to the HCN test; however, the cotyledons gave a highly positive reaction supporting the retention of the generic nameTetragonolobus(T. siliquosus(L.) Roth.) for this species. Chromosome number determinations are reported for the first time forL. holosericeusWebb and Berth. (2nensp;=ensp;14),L. mascaensisBuchard (2nensp;=ensp;28),L. nevadensisGreene (2nensp;=ensp;14),L. helleriBritton (2nensp;=ensp;14), andL. oblongifoliusvar.nevadensis(Gray) Munz (2nensp;=ensp;14).
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