The Devonian fossil record of vascular plants provides the type of data that is essential to reconstructing phylogenies including, clear evidence of ancestral morphologies, different aspects of the life cycle and the origins of major organ systems. In turn, these phylogenies allow us to propose and test hypotheses aimed at identifying causal agents and selective regimes underlying the evolution of traits using falsificationist and/or statistical techniques. Phylogenetic analyses of early vascular plant groups and comparative method techniques designed for analyzing correlations among discrete variables (Maddison [1990]; Sillen-Tullberg [1993]; Harvey and Pagel [1991]) were used to test the following adaptive hypotheses: 1.) phylogenetic increase in height (independent variable) is a 'causal' factor for the evolution of stelar dissection (Bower, 1930), heterospory (Chaloner and Sheerin, 1979), and extensive rooting systems (dependent variables); 2.) phylogenetic increase in size of photosynthetic surfaces (independent variable) is a 'causal' factor for the evolution of arborescence (dependent variable); 3.) development of heterosporous life cycles (independent variable) is a 'causal' factor for the evolution of specialized reproductive structures such as sporangiophores and strobili (dependent variable). Tentative results support Chaloner and Sheerin's hypothesis that heterospory evolved as an adaptation to selective increases in plant height. No evidence was found in favor of Bower's hypothesis that suggests complex primary vascular systems evolved jointly with selective increases in plant size and were necessary to maintain adequate xylem surface area-to-volume ratios. Although stelar dissection and arborescence are significantly correlated, there is no statistical evidence that suggests one trait 'caused' the evolution of the other, dependent trait. The methodology of macroevolutionary process inference and the results of five tests of adaptational hypotheses presented here will hopefully point to better ways of conducting research of this type in studying the evolution of early vascular plant groups.

Key words: comparative methods, Devonian, macroevolution, phylogeny, vascular plants