Mechanisms of Laminar Growth in Morphologically Convergent Leaves and Flower Petals

Distinct leaf laminar growth dynamics have been found previously in some filicalean ferns and angiosperm seed plants: a discrete zone of marginal growth versus diffuse intercalary growth throughout the leaf without a conspicuous marginal growth zone. The fern pinnules for which marginal growth is known are also morphologically distinct from dicots: one or two orders of dichotomizing veins ending only along the margin versus many orders that include internally directed veins and endings dispersed throughout the leaf. Both of those morphological alternatives have evolved many times among extant and fossil plants, but whether similar developmental processes underlie each recurrence of a particular laminar morphology has not been investigated. Here, marginal growth is demonstrated in two independent examples of fernlike open venation patterns among living seed plants, frond leaflets of the cycad Ceratozamia and flower petals of Ranunculus. Thus, in these cases, morphological convergence is shown to reflect developmental convergence. Given the preponderance of simple, fernlike laminar morphologies among fossil and extant basal seed plants, angiosperm laminar development is likely to be as highly derived as their unusual laminar venation patterns.