Agrobacterial transformation is a main method of creation of transgenic plants under laboratory conditions. It is based on regeneration of whole plants from cells transformed with vectors based on T-DNA of agrobacteria. In addition, natural plants are described that contain T-DNA in their genomes and have been vertically transferring it throughout generations over millennia. This DNA was called cellular T-DNA (cT-DNA), and plants containing it are referred to as naturally transgenic ones. Since evolution involves manifold acts of such plant transformation, the latter appears to play important roles. This review analyzes the significance and feasible functions of cT-DNA in the evolution. Roles of cT-DNA in control of plant morphogenetic reactions and in that of processes related to plant-microbe interactions are also discussed.