Plant organs, such as leaves, branches, trunks, and roots, are believed to have evolved in a coordinated manner over the past billion years as part of an optimization system that supports their various functions of growth, development, and reproduction. In recent decades, there has been significant interest in the functional traits of different plant organs owing to their vital role in plant’s adaptation to the environment. While different plant organs are structurally connected, they exhibit significant differences in function, necessitating their collaborative or coordinated adaptation to complex natural environments. Most studies have been conducted separately on leaves, branches, trunks, or roots, aiming to investigate the spatial variation and influencing factors of functional traits at different scales. Furthermore, scientists have attempted to elucidate the quantitative relationships between functional traits of specific plant organs (leaves or roots) and the structure and function of ecosystems. However, how multiple plant organs coordinate to adapt to the external environment on a large scale remains unclear, which limits our understanding of plant adaptation strategies and optimization of functions. To explain the variations in functional traits among various organs, we developed two coexisting and complementary hypotheses, convergent evolution hypothesis and divergent evolution hypothesis, by considering the structural connections and diverse functions of various plant organs. Functional traits, that can “flow” as elements or other chemical substances among different organs, converge in content because of the unique functions different plant organs serve. However, these morphological traits tend to be divergent among different organs to facilitate adaptation, optimize life history, and improve plant fitness. Herein, we further discuss the practical implications of the two hypotheses by examining the variations and influencing factors of multiple functional elements in leaves and roots, as well as the variations and evolutionary history of the C:N ratio of leaves, branches, trunks, and roots, on a large scale. This empirical evidence demonstrates significant differences in the contents of multiple elements between leaves and roots, although their spatial variations are similar. The C:N ratio significantly differs between leaves, branches, trunks, and roots; however, the directions of allometry are convergent. Few studies have examined the covariation of functional traits among multiple organs and species on a large scale, although it is important for the adaptation of plants to complex environmental conditions in nature. This area of research is promising and should be emphasized in the future, particularly in view of the global change scenarios.