gogol

Flowers are now widely interpreted as being a stem with variously modified leaves. Actually, a better interpretation is to compare a flower to a young shoot with leaves in various stages of development. The Russian botanist Takhtajan, from the Botanical Institute in Leningrad, makes a convincing case for flowers as neotenous structures. By that he means they are stages of early development that persist into the adult. Or, conversely, that (sexual) maturity has arrived early. In the case of flowers, Takhtajan and others before him have concluded that flowers are derived from leaves. The green sepals--obvious at the base of roses, where the flower arises from its stem--are especially leaf-like. The venation (arrangement of veins) of petals is often reminiscent of leaf venation. And even such specialized structures as stamens and the pistil have leaf traces extending into them. Commonly, flowers are dioecius, since they contain both male and female reproductive parts--they produce the male gametophyte from the pollen and the female gametophyte or embryo sac from the megasporocyte. However, some flowers are monoecius, with flowers of each sex occurring on different plants.

Aposemic traits are most informative when we come to tracing the course of evolution. Neosemic traits are also useful, but more so as markers for the initiation of an innovation than for tracing lines of historical development. In the foregoing summaries of structures and life cycles, it became apparent that the Metaphyta show a number of aposemic or variable traits as well as certain neosemic ones. And among neosemes, such as vascular tissues, seed, and flowers, these also show aposemy. According to the methodology for phylogenetic analysis given earlier, once homologies have been identified by positional and compositional relationships (direct similarities that identify plesiosemes) and by serial relationship (indirect similarities that identify aposemes), we must proceed to the designation of a plesiomorph. (Neosemes show no homologies.) The plesiomorph is the actual form (fossil or living, embryonic or adult) that most closely resembles the ancestral form or forms of the group or groups in question. Let us now look for one or more metaphytan ple-siomorphs, and then, from that starting point, see what can be said about evolutionary trends, in general, and phylogenetic relations, in particular, within the Metaphyta.