ASPECTS OF THE EVOLUTION OF THE MONOCOTYLEDONS

Abstract

The available evidence, partly laid down in some recent reviews, suggests a morphological and anatomical heterogeneity of the major groups constituting the conventional Monocotyledonae (or Liliatae). A repeatedly signalised connection between certain ranalean forms and some liliate taxa is not unequivocal in that the magnolialean and nymphaealean dicots differ from all monocotyledonous orders in some essential, exclusively “dicotyledonous” features whilst, on the other hand, exhibiting some rather convincing “monocotyledonoid” traits. The only interpretation reconcilable with these more or less contradictory deductions is that – unless all features shared by monocots and dicots are explained as the result of convergent evolution – the phylogenetic history of the Liliatae goes back to a pre-angiospermous group of ancestors (or to several such groups), and that this latter taxon (or a part of it) was also progenitorial to the ranalean dicots. This interpretation renders the “derivation” of monocots from (certain types of) dicots, or vice versa, inane: the common characteristics of both groups are conceivably inherited from a common, basic group which had not yet attained the level of evolutionary advancement of truly “angiospermous” plant forms. The heterogeneity of the monocots can similarly be explained by an early phylogenetic divergence of lineages sprung from a common stock of pro-angiosperms, which lineages evolved independently for a considerable length of time. Phytochemical evidence somewhat paradoxically suggests a homogeneity of the monocots, however. In spite of the accumulation of a large amount of relevant data, the information is insufficient to permit more definite conclusions apart from the indication of some “impossible”, direct relationships between certain groups. The floral morphology of most, if not of all, liliate taxa must be based on the anthoid concept and this at least obviates all attempts to derive monocotyledonous groups directly from a magnolialean type of dicot.