Phylogenetic relationship among seed plants recently obtained with molecular genetics fails to reveal the evolutionary steps of the formation of the basic morphotype of flowering plants. This task can be solved via morphological functional modeling of the first steps of the formation of flowering plants based on appropriate morphotypes in extant flora, irrespective of their genetic kinship. For instance, Gnetales adhere phylogenetically to conifers, but the genus Gnetum is appreciated for its numerous features convergent with angiosperms and can be used as a model of the morphological state of the angiosperm ancestor. A morphological analysis of the available data shows that the true angiosperm ancestor must have differed from Gnetum in some important respects, e.g., it must have had a more primitive xylem and bisexual fructifications. Aquatic intermediate stage modeled by water lilies is needed as a “bridge” that connects this ancestral condition with a full-fledged flowering plant. This intermediate stage differed from the extant water lilies in some crucial respects, e.g., it retained a cambium and possessed small flowers. The appearance of perianth with corolla is supposed to be possible at this aquatic stage only because it is water-wasteful. The scenario employs the concept of a megasporangia-bearing shoot, which is an alternative to the more common concept of a megasporangia-bearing leaf.
展开▼
