Patterns of leaf development in anisophyllous shoots

Abstract

Comparisons of the development of the dimorphic leaves of anisophyllous shoots can be used to understand how ontogenies might be modified during evolution to produce morphological change. In anisophyllous shoots, leaves of different sizes are borne on the dorsal and ventral sides of plagiotropic stems. Anisophylly is regarded as an adaptation for light interception in strongly shaded habitats since the small size of dorsal leaves and orientation of leaf blades minimizes self-shading. Anisophylly affects not only the patterns of leaf development, but also shoot symmetry, phyllotaxis, and bud development. Pronounced anisophylly is widely distributed throughout the land plants as a result of convergent evolution, possibly in response to similar selection pressures. In taxa where the expression of anisophylly is fixed, leaf primordium size and correlated characters, including the development of procambium, differ between dorsal and ventral sides of the shoot from the first plastochron. In contrast, patterns of dorsal and ventral leaf growth and correlated characteristics diverge late in development, often at the time leaves expand from the bud, in taxa where the expression of anisophylly is facultative. These observations indicate that changes in the timing of developmental events can account for many, but not all, of the ontogenetic alterations that result in divergent leaf size and shape on the same shoot and, by implication, accompany the evolution of new taxa. Key words: leaf development, anisophylly, heterochrony.