Monocotyledons are distinct from dicotyledons by their subtype-P2 sieve-element plastids containing cuneate protein crystals, a synapomorphic character uniformly present from basal Acorales and Alismatales 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 typical of many paleoherbs. New investigations in the Melanthiaceae sensu lato revealed the same pattern (polygonal plus cuneate crystals) in the sieve-element plastids of Japonolirion osense (Nartheciaceae-Petrosavieae / Petrosaviaceae), Harperocallis flava, Pleea tenuifolia, three Tofieldia species (all: Nartheciaceae-Tofieldioideae / Tofieldiaceae), and of Narthecium ossifragum. However, Aletris glabra and Lophiola americana (Nartheciaceae-Narthecioideae) and all of the15 species studied and belonging to the Melanthiaceae sensu stricto contain cuneate crystals only. High resolution TEM pictures reveal a crystal substructure which in cuneate forms is always densely-packed, but in polygonal ones is either 'dense' (in Asarum, Saruma, and Japonolirion) or 'loose' (in Harperocallis, Pleea, and Tofieldia). In Narthecium 'loose' polygonal crystals often break up into many small pieces, a process that is also recorded within the Velloziaceae. Moreover, small 'loose' crystals in addition to 'dense' cuneate ones are found in a few Lilioids and many taxa of the Poales (Commelinoids). - These results suggest (1) that form-P2c sieve-element plastids evolved through the breakup of a single large polygonal into many cuneate crystals, (2) that this partition was incomplete, i. e. left over a smaller polygonal crystal which through steps of (3) loosening and (4) further breakup (5) eventually dissolved, whereas (6) form-P2cs, -P2cfs and -P2cf subsequently developed by the addition of starch and/or protein filaments. This hypothesis is tested against the distribution of the different subtype-P2 sieve-element plastids and phylogenetic trees derived from molecular data.

Key words: evolution of monocots, Japonolirion, Melanthiaceae, Nartheciaceae, sieve-element plastids, Tofieldiaceae