I study how intricate forms in the natural world are the organic result of simple physical law. My thesis considers two examples of growth in a diffusion field: a network of streams that grows by attracting groundwater and a microbial community that grows by attracting nutrients. In both cases, we combine mathematical models with field observations and laboratory experiments to predict the shape and scale of growing features. By focusing on what is simple and fundamental, our results have provided insight into diverse phenomena: from Martian valleys to billion-year-old fossils.