Date of Completion


Embargo Period



petiole, biomechanics, botany, plant physiology, tradeoff, leaf, pelargonium

Major Advisor

Cynthia S. Jones

Associate Advisor

Pamela K. Diggle

Associate Advisor

Carol Auer

Field of Study

Ecology and Evolutionary Biology


Doctor of Philosophy

Open Access

Open Access


As the connection between lamina and stem, petioles are essential for plant survival. Petioles serve two major functions in service to the leaf blade. First, they hold the leaf blade away from the stem to reduce self-shading and optimize light interception, a mechanical function. Second, petioles serve as the bridge between the stem and lamina allowing water to move up the plant body and photo-assimilates to move down to the plant body from the leaf blade, a physiological function. While mechanics and physiology have been studied independently, the petiole could be a space-limited structure leading to tradeoffs between the two main petiole functions. My goal was to investigate the interactions between the mechanical and physiological functions of the petiole to determine whether they trade off. To investigate petiole interactions, I first conducted an allometric analysis of Pelargonium petiole anatomy in 11 greenhouse-grown species, all of which were evergreen shrubs. From the anatomical relationships observed, I determine that mechanics and physiology are unlikely to tradeoff because the relationship between the supporting tissues (fibers) and conducting tissues (xylem and phloem) is positive. Next, I investigated the mechanical system of 8 species of Pelargonium from two field sites in South Africa specifically with regard to shape and stiffness change along the petiole length. I found that the change in shape for species is highly variable, but that the patterns in shape change are different for geophytes than shrubs. I conclude that the differences in stiffness from shape between growth forms are likely due to the different habits of geophytes and shrubs. Finally, I investigated the interaction of physiology and mechanics by measuring traits representative of physiology and mechanics on the same leaves for six species of greenhouse-grown Pelargonium shrubs. Using a piecewise structural equation model for lamina traits predicting physiological and mechanical traits, I show that the physiological and mechanical functions of Pelargonium petioles do not trade off, but are linked by lamina area. I conclude that lamina area is an “organizing trait” that prevents a tradeoff between the vascular and mechanical tissues.