The natural environment of Wyoming is, to a very large extent, geologically controlled. Rocks and sediments of all geological ages compose the mountain ranges, their bordering cuestas and the structural basins lying between the ranges. These substrates provide a broad range of geochemical and physical substrates for plant growth that are just now being understood. The spatial distribution of a cross-section of the state flora was statistically tested in relation to nine climatic variables, the basic vegetative cover types, and three geologically derived variables: bedrock type, surficial geology, and soil. The relationships between species distributions with these variables were tested by classification tree analysis, a method permitting the assessment of importance of the environmental variables. Bedrock type was one of the controlling variables in about 80% of the cases and was the first determinant for distribution in about 21% of the cases. Thus, geology is a dominating feature in the distribution of plants in an environment ranging from rolling plains, dissected basins, cuesta complexes and sedimentary and crystalline mountains. The relative importance of substrate across this range of environments will be analyzed and hypotheses for the possible causation of these relationships will be explored.