In classical studies it was typically found that the leaves of all plants grew by means of a similar array of marginal and discrete intercalary meristems. In the second half of the twentieth century several lines of evidence, including anatomical investigations and clonal analysis studies, indicated that at least angiosperm leaves grew diffusely throughout the leaf without a conspicuous marginal meristem, while studies of a few fern lineages reaffirmed the existence of a marginal meristem. In the few taxa that have been studied, these alternative mechanisms of leaf growth are associated with alternative venation patterns: leaves with marginal growth have one or two vein orders and all vein endings only along the margin, leaves with diffuse growth have many vein orders that can include internally directed veins and vein endings dispersed throughout the leaf. The fossil record documents that laminate leaves evolved independently in at least four different lineages within a 100 million years of the first appearance of the vascular plants, but only two of these lineages are still extant, the ferns and seed plants. Surveys of venation and other morphological characteristics in fossils and extant plants suggest marginal meristematic growth evolved independently in each of these four lineages and that more angiosperm-like diffuse growth subsequently evolved a number of times independently in the ferns and seed plants. These hypotheses are now being tested with an anatomical survey of development that includes a variety of laminate organs and spans the phylogenetic and morphological diversity of living plants. Although heterogeneity exists in the different independently evolved examples of each growth type, this survey broadly confirms the correlation between development and morphology.

Key words: development, evolution, leaf, meristem, paleobotany