The complexity of developmental processes constrains evolutionary changes. A rare change, an evolutionary innovation, may be inferred from the combined information of fossil steles and of the controls of vascular differentiation. This change was the gradual appearance of a new shoot apex, with determinate but dominant appendages and a weak, yet persistent, promeristem. The controls of vascular differentiation could be 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 centers. They could also involve the interactions between the phloem and the xylem, expressed by the development of vascular rays and perhaps by the ribbing of steles. Since the specification of orientation is an essential aspect of vascular differentiation, continued work requires information on the three-dimensional cellular structure of fossil tissues. Complementary studies would be experim...