Homology: An Empirical View

Comparative plant morphology is an empirical discipline that provides evidence whereby plant structures may be homologized. Historical example amply illustrates how its pro- cedures are successfully applied. Developmental evidence is often critical for analysis and, by virtue of the modular or metameric construction of higher plants, ontogenetic and primordial develop- mental processes must be carefully distinguished. Both phenotypic and genotypic variation add confusion to the comparative process. The typological basis of comparative studies is emphasized, but in a non-idealistic sense. An agnostic approach is healthy, and the futility of certain compari- sons sometimes needs to be recognized. Case studies show how phylogeny, development, and systematics need to be integrated while added evidence placed in a biological or functional context may resolve apparent difficulties. In other instances ignorance of morphogenetic processes may prevent an immediate resolution. Variation may even repeat developmental patterns, so that con- vergence in structural plans is easily possible. Because of the extended ontogeny of modular or- ganisms evolutionary diversification is likely to involve extensive neotenous metamorphism, both