Changes in leaflet and leaf lobe form in developing compound and finely divided leaves

A comparative developmental study of the compound leaves Ailanthus altissima, Lupinus polyphyllus, Fraxinus americana, Rhus typhina and the highly dissected leaves of Achillea millefolium, was initiated to document the changes in form that lateral elements (leaflets/lobes) undergo at later stages in their development. Results indicate that there is a continuum in modes of development of vegetative shoots and compound or highly dissected simple leaves, where developmental similarities vary from pronounced to almost non-existent. In some situations, during early stages of development, the individual leaflet primordia appear as uniform swellings on either side of developing pinnately compound leaves. This organization is very similar to that of an entire shoot meristem on which many leaves develop. However, as the compound leaves develop, their dorsiventrality as a whole becomes more pronounced and the leaflet primordia begin to assume a different pattern of growth (A. altissima, F. americana, R. typhina). The leaflets become centrally concave, resulting in a horseshoe shape in the area facing the adaxial side of the rachis. This developmental stage is crucial because, in the pinnately compound leaves, it marks the point at which the development of a leaflet diverges from that of a simple leaf on a stem. In A. millefolium, the lateral elements (highly dissected lobes) remain flattened in the same plane retaining the same genera orientation as a simple leaf on a stem. In the palmately compound leaves of Lupinus polyphyllus such shoot features are not apparent.