The paleontological and phylogenetic evidence have shown that the main features ultimately responsible for plant modularity were already present at a very early stage in the evolution of land plants, and are a property shared by the whole lineage. The ecological and evolutionary implications of plant modularity have frequently been highlighted following White's pioneering treatment of plant individuals as metapopulations of repeated modules. One of the consequences of plant modularity is the appearance of a distinctive source of phenotypic variance, that is, the within-plant or subindividual component. Another consequence of the multiplicity of modules is variation in the characteristics of the copies of the same organ produced on different modules of the same plant. A thesis is developed that the multiplicity of homologous structures arising from plant modularity gives rise to a subindividual level of phenotypic differences among organs of the same plant.