We present case-study data of undergraduates describing the relationship between scientific disciplines. Rather than viewing biology, chemistry, and physics as existing in disconnected silos, or as overlapping only in narrow regions of common interest, these students exhibit a range of nuanced views about disciplinary relationships. Some students describe hierarchical arrangements that order the disciplines by degree of system complexity or by the scale used to examine a particular system. In other instances students want physics embedded in a context that positions its relationship to biology via analogy, or reference the way in which general physical principles like energy conservation or entropy maximization impose constraints on biological systems. We argue that these case studies illustrate the varied resources that students possess for seeking coherence across disciplines, as well as the potential barriers to interdisciplinary learning that such views might create when adopted to the exclusion of others.