This chapter discusses the plausible developmental mechanisms underlying the evolution of novel plant body and the macroevolution of different plant lineages in the Late Silurian to Middle Devonian periods. The hormone auxin appears to regulate the organizational features comprising body plans of contemporary plants. It is hypothesized that evolutionary changes in auxin action were causally involved in the generation of different body plans during the radiation of early vascular plants. Major changes in auxin action occurred in the earliest land plants prior to the Late Silurian period. The characteristic body plans of different divisions of extant land plants are established during embryonic and early postembryonic development, which means that the regulatory mechanisms operating in embryonic development are also critical for generating these body plans. The progression through both somatic and zygotic embryogenesis appears to require the sequential activation of two different auxin biosynthetic pathways. During the initial stages of embryo growth, a tryptophan-dependent pathway for auxin biosynthesis produces high levels of free auxin, which are apparently critical for mediating the rapid cell divisions needed to generate the globular embryo. After that the embryo switches to a tryptophan-independent pathway for auxin biosynthesis, which exercises greater homeostatic control over the free auxin levels.