Origin and stages of monocotyly in the Monocotyledons II—Stages of monocotyly

Abstract

Summary Despite the extreme differences that exist between the various types of Monocotyledon embryo, research into their original embryogenical type is justified by their monophyletism. Besides having a single cotyledon, the unifying characteristic of all Monocotyledon embryos is their bilateral symmetry; those of the Dicotyledons conserve an axial symmetry. The relative sitings of, and morphological value of, both the cotyledon and the stem apex are currently still in dispute. Our results show that it is impossible to identify a unifying morphogenetical pattern, as there are in fact four main embryogenical types: Tamus, Cocos, Agapanthus and Cyperus. These constitute an intermediate series in between original, primitive forms close to the Dicotyledons, and evolved forms specific to the Monocotyledons. In the Cyperus type, what one classically terms a cotyledon does not correspond to a cotyledon in a Dicotyledon. There appears to be a correlation between the structural evolution and the functional evolution of the different parts of the embryo and of the seedling cotyledon.