Genetic Control of Leaf Morphology: A Partial View

The partial-shoot theory of the leaf was a controversial hypothesis revived by Arber and supported by her morphological and anatomical studies. This theory highlighted the parallels between leaves and shoots and contrasted with an alternative view that leaves, with their limited growth potential, are completely distinct from shoots. Pea morphological mutants with altered growth potential in their compound leaves are described. The unifoliata mutant has a limited growth potential relative to wild-type; cochleata , afila and insecatus have extended potentials. Characterization of the unifoliata mutation and gene expression patterns indicate that unifoliata is a common factor in pea compound leaf and floral shoot development, and so provides rudimentary support for the idea that some leaves have shoot-like characteristics. Tomato leaves are also considered to lend tentative support. The afila and insecatus leaf forms are described as bipinnate and weakly bipinnate, respectively. These and the tendril-less mutant are potential phenocopies of legume relatives, an idea based on Vavilov's law of homologous series of variation. Arber illustrated, but did not articulate in genetical terms, that morphological variation in structures within an individual plant can be interpreted as reiteration of design. Analogous with Vavilov's view, this can be considered a consequence of the same genetic programme in a different location.