ANALYSIS OF FORM AND FUNCTION IN NORTH-AMERICAN COLUMNAR CACTI (TRIBEPACHYCEREEAE)

Simple and multivariate linear models were used to demonstrate the inf luence of mechanical design and climate on stem morphology and branchi ng architecture in 15 species of North American columnar cacti. The ef fect of phyletic inertia was tested by the method of independent contr asts. Stem with was found to increase significantly slower with increa sed height within taxa (cross-sectional stem area proportional to [pla nt height](0,603)), than across taxon (cross-sectional stem area propo rtional to [plant height](1,451)). Juveniles are shown to be mechanica lly overbuilt and subsequently grow into more slender adult forms dete rmined in part by structural limitations and the optimization of other stem functions. We make a structural analogy of relatively rigid colu mnar cacti to concrete columns and compare plants and models with simi lar growth forms lacking woody skeletons (barrel cacti). Taxa with woo dy support achieved a surface-to-volume ratio six times greater than t axa without woody support. Across taxon, coder winter temperatures wer e associated with larger stem girths, and greater annual precipitation was associated with Less frequent branching. The relationship between total plant surface and volume approaches isometry within taxa, but a cross taxon average individuals are scaled replicates. We hypothesize that architecture and average plant height are adjusted, in an evoluti onary sense, to maintain geometric similitude between surface and volu me along a climatic gradient.