A DEVELOPMENTAL PERSPECTIVE OF THE EVOLUTION OF THE STELE

The complexity of developmental processes constrains evolutionary chan ges. A rare change, an evolutionary innovation, may be inferred from t he combined information of fossil steles and of the controls of vascul ar differentiation. This change was the gradual appearance of a new sh oot apex, with determinate but dominant appendages and a weak, yet per sistent, promeristem. The controls of vascular differentiation could b e used to indicate additional evolutionary events. These could include the responses of the differentiating vascular tissues themselves and not only the formation of inductive signals by the meristematic center s. They could also involve the interactions between the phloem and the xylem, expressed by the development of vascular rays and perhaps by t he ribbing of steles. Since the specification of orientation is an ess ential aspect of vascular differentiation, continued work requires inf ormation on the three-dimensional cellular structure of fossil tissues . Complementary studies would be experimental comparisons of the morph ogenetic responses of the various groups of extant plants.