Monocotyledons are distinguishable from dicotyledons by their subtype P2 sieve-element plastids containing cuneate protein crystals, a synapomorphic character uniformly present from basal groups through Lilioids to Commelinoids. The dicotyledon genera Asarum and Saruma (Aristolochiaceae-Asaroideae) are the only other taxa with cuneate crystals, but their sieve-element plastids include an additional large polygonal crystal, as is typical of many eumagnoliids. New investigations in Melanthiaceae s.l. revealed the same pattern (polygonal plus cuneate crystals) in the sieve-elementplastids of Japonolirion osense (Japonoliriaceae/Petrosaviaceae), of Harperocalfsflava, Pleea tenuifolia, and Tofieldia (all: Tofieldiaceae). In Narthecium ossifragum a large crystal, present in addition to cuneate ones, usually breaks up into several small crystals, whereas in Aletris glabra and Lophiola americana (Nartheciaceae) and in all of the 15 species studied and belonging to Melanthiaceae s.str. only cuneate crystals are found. Highresolution TEM pictures reveal a crystal substructure that is densely packed in both cuneate and polygonal forms, but in Tofieldiaceae the polygonal crystals stain less densely, probably as a result of the slightly wider spacing of their subunits. The small crystals of Narthecium are “loose”; that is, much more widely spaced. Such “loose” crystals are commonly found in sieve-element plastids of Velloziaceae, present there in addition to angular crystals, and together with cuneate crystals in a few Lilioids and many taxa of Poales (Commelinoids). Ontogenetic studies of the sieve elements of Saruma, Aristolochia, and several monocotyledons have shown that in their plastids cuneate crystals develop very early and independent from a polygonal one present in some taxa. Therefore, a conceivable particulation of polygonal into cuneate crystals is excluded. Consequently, mutations of some monocotyledons that contain a lone, large, polygonal crystal in their sieve-element plastids are explained as the result of a complex genetic block. The total result of all studies in sieve-element plastids suggests that Japonolirion and Tofieldiaceae are the most basal monocotyledons and that Aristolochiaceae are their dicotyledon sister group.