Anisophylly and Dorsiventral Shoot Symmetry.

Anisophylly is a special case of shoot dorsiventral symmetry in which leaf size and form differ between dorsal and ventral sides of the shoot. Anisophylly is normally coupled with leaf and stem asymmetry, modified phyllotaxis, and anisoclady. Anisophylly may be expressed throughout the shoot system (habitual anisophylly) or restricted to lateral shoots (lateral anisophylly). The combination of anisophylly, shoot dorsiventrality, and plagiotropy has arisen numerous times during evolution of the land plants, most notably in the primarily tropical families of Acanthaceae, Gesneriaceae, Melastomataceae, and Urticaceae. It is thought to represent an adaptation that maximizes light capture in dimly lit understories. In species for which anisophylly is an intrinsic expression of overall shoot symmetry, anisophylly and associated features of shoot dorsiventrality are expressed early in development. In contrast, when expression of anisophylly depends on growth conditions, anisophylly is secondarily imposed on early isophyllous leaf development. Despite more than a century of experimentation, very little is understood about the physiological control of anisophylly and dorsiventrality. Development of these features requires regulated suppression of some parts of the shoot and stimulation of others. Shifts in the temporal and spatial patterns of growth suppression and enhancement occur during both the ontogeny of an individual and during evolutionary change. Elucidating these processes for anisophyllous shoot development also should have broader significance for understanding the evolution of other morphological innovations.