ИСТИНА

Cotyledon number is an iconic character that distinguishes two major groups of angiosperms. Since dicotyledons are paraphyletic and monocotyledons are monophyletic, considerable research has been conducted to establish a precise pathway of transformation from the dicotylar condition to the monocotylar condition. In pre-molecular classifications, the order Nymphaeales was often considered to be the closest extant relative of monocots. It had been postulated that the embryos of Nymphaeales resemble those of monocots, supporting a putative sister-group relationship between two taxa. However, different authors implied contradictory homologies between various parts of the embryo in water-lilies and monocots. Circular reasoning was apparently employed to correlate embryological and phylogenetic data, at least in part. The origin of the monocotylar embryo should be discussed in the framework of embryo diversity across the land plants as a whole. A bipolar embryo with endoscopic development is an important diagnostic feature of the seed-plant lineage. The origin and ancestral adaptive advantages of this component of sporophyte body plan remain enigmatic, due to lack of data on embryos of early tracheophytes. With endoscopic development, cotyledons are the organs closest to the nutritive tissue(s) of the seed. Therefore, they commonly fulfil a haustorial function. This arrangement creates a spatial constraint in seed germination, because the cotyledons enclose the shoot apical meristem. Seed plants have adopted several ways of overcoming this fundamental constraint at germination, one of which is the monocotylar embryo. The superficially monocot-like seedlings of Hydatellaceae (Nymphaeales) are useful for discussing the importance of physical constraints at seed germination.