This dissertation lays the groundwork for writing pedagogies that meaningfully engage students in the "writing public" (Yancey) of the 21st century. Through the development of an "infrastructural" model of the public sphere, I argue that rhetoricians and compositionists can correct an overreliance upon two archetypes of public engagement—the deliberative "conversation" and the embodied, activist "counterpublic"—that have failed to attune us to the distinct forms of relationality that characterize our changing public sphere. Drawing upon public sphere theory, ecological models of rhetorical circulation, and Mikhail Bakhtin's concept of the chronotope, I develop a model of rhetorical investment that helps us better account for the ways that rhetors reinforce the spatiotemporal dynamics of the public sphere through their interactions with strangers. As a means of testing and revising the infrastructural model of the public sphere that I have developed, I study two time periods noted for the emergence of new forms of relationality—the Progressive Era and the digital age. My first case study considers the public rhetoric of Emma Elizabeth Walker, a doctor whose "social hygiene" lectures aimed to distribute contested knowledge about sex and bodies to a lay public of girls and young women in the early 1900s. By analyzing the mother-daughter chronotope that Walker deployed, I demonstrate that her lectures emerged from an ecological infrastructure born of conflicting "investments." My second case study examines public responses to the writing of Alexander Gray, whose viral #blacklivesmatter post was shared by nearly 50,000 strangers on the facebook platform. Analyzing Gray's engagement with the more than 3,500 comments written in response to this post, my study demonstrates that an individual rhetor can delimit the ways that public audiences imagine and enact their relations with one another, contributing to the building and maintenance of the ecological infrastructure that undergirds the public sphere.