A central goal of biology is to understand how and why diverse structures evolve. This highlights a long-standing question: do new forms arise mainly because natural selection directly favors them, or because they emerge indirectly as a consequence of how organisms grow and develop? Tackling these questions requires knowledge of how new forms originate and their fitness implications. This project uses ferns, an ecologically important and evolutionarily persistent group of plants, to address this fundamental problem by exploring the evolution of vascular architecture (the set of tubes that move water and nutrients). Ferns are a powerful system for this work because they have evolved some of the most diverse vascular systems on Earth. The project tests two leading, and potentially complementary, explanations for this diversity: that vascular patterning evolved as a direct adaptation to drought (the Drought-Driven Hypothesis) versus that it reflects developmental linkage with changes in overall plant growth (the Ontogenetic Hypothesis). By integrating physiology, development, and large-scale evolutionary analyses, the project will clarify the unresolved problem of how developmental mechanisms generate new anatomical variation and how selection acts on this new form. Understanding these mechanisms advances fundamental knowledge about how plant form evolves, while also improving the scientific basis for anticipating how plants may respond to drought - fundamental to agricultural p