Leaves provide the infrastructure within which solar photons and CO2 are channeled to photosynthetic mesophyll cells to support gross primary productivity, and from which absorbed energy is partitioned into latent and sensible heat loss. Both the internal environment and surface features of a leaf are the products of modification through natural selection to produce a form and function that provides advantages toward carbon and energy assimilation, and ultimately growth. For example, the structural arrangement of leaf cells affects the organized dispersion of the solar photon flux and the density and function of stomata affect the diffusive uptake of atmospheric CO2 and its relationship to H2O loss. As indicated in the quotes from two famous architects that we cite above, the concept that “form is married to function” is perpetuated across generations of multiple disciplines; it is not limited to the biological sciences. It is a fundamental philosophical relation. In fact, some of the earliest intellectual pursuits of this relation were conducted by very broad thinkers; practitioners of literature, music, and the visual arts. Johann Wolfgang von Goethe, best known for his literary works, spent many hours in discussions with Alexander von Humboldt, the “founder” of the discipline of phytogeography, discussing the concept that plant form could be used to discern plant-climate relations; discussions that no doubt influenced Humboldt’s theories on the determinants of vegetation distribution across the globe. There is no requisite sequence by which “form mandates function” or “function mandates form”; evolutionary modification occurs to both anatomy and physiology, maintaining both within a coordinated set of resource use parameters. Leaf form and function must be viewed as integrative, and the coupled nature of form and function must be viewed, in and of itself, as a target of natural selection.