Pattern and Form in Repeated Elements in the Development of Simple Leaves of Begonia dregei

Development of leaves of Begonia johnstonii and three forms of Begonia dregei was compared using morphometric methods. Lobes and teeth were considered to be homologous based on similarity of shape and structure, including venation, and the presence of equivalent hydathodes and emergences at the tips. Lobes and teeth of the same leaf form represent "repeated elements" because of their similarities in structure and their similarities in growth rates, as indicated by comparison of regression slopes. Differences in mature size of lobes and teeth resulted primarily from initial differences in size at formation. Among forms of B. dregei, there were differences in growth rates and structure of repeated elements so that shapes of mature leaves differed. Also, growth rates of measured areas in B. johnstonii differed significantly from the homologous areas of the three B dregei forms. Simple leaves may be formed by repetition of developmental programs at different size scales or times during development. This presents both a constraint on the shapes that may occur and a simple mechanism for the evolutionary diversification of leaf shape.