may also affect the leaf form. Another very important class of regulators of leaf development are the microRNAs, whose role in this process has just begun to be documented. The coming years should see a rapid development in comparative studies on leaf development, with many EST sequences involved in the process coming online. Molecular genetics has also shed light on the relation between radial symmetry (characteristic of stems) and dorsiventral symmetry (typical for leaves). James (2009) stated that "it is now widely accepted that... radiality [characteristic of most shoots] and dorsiventrality [characteristic of leaves] are but extremes of a continuous spectrum. In fact, it is simply the timing of the KNOX gene expression!" In fact there is evidence for this continuum already at the beginning of land plant evolution. Furthermore, studies in molecular genetics confirmed that compound leaves are intermediate between simple leaves and shoots, that is, they are partially homologous with simple leaves and shoots, since "it is now generally accepted that compound leaves express both leaf and shoot properties”. This conclusion was reached by several authors on purely morphological grounds. Evolution of flowers Flower-like structures first appear in the fossil records some ~130 mya, in the Cretaceous era. The flowering plants have long been assumed to have evolved from within the gymnosperms; according to the traditional morphological view, they are closely allied to the gnetales. However, recent molecular evidence is at odds to this hypothesis, and further suggests that gnetales are more closely related to some gymnosperm groups than angiosperms, and that gymnosperms form a distinct clade to the angiosperms,. Molecular clock analysis predicts the divergence of flowering plants (anthophytes) and gymnosperms to ~ The main function of a flower is reproduction, which, before the evolution of the flower and angiosperms, was the job of microsporophylls and megasporophylls. A flower can be considered a powerful evolutionary innovation, because its presence allowed the plant world to access new means and mechanisms for reproduction. Origins of the flower It seems that on the level of the organ, the leaf may be the ancestor of the flower, or at least some floral organs. When we mutate some crucial genes involved in flower development, we end up with a cluster of leaf-like structures. Thus, sometime in history, the developmental program leading to formation of a leaf must have been altered to generate a flower. There probably also exists an overall robust framework