Arabidopsis is an excellent model for studying the molecular basis of floral induction and initiation in annual plants. Unlike Arabidopsis and other annuals, little is known about the molecular mechanisms that regulate flowering in woody perennial plants. The shoots of woody perennials are exposed to repeated seasonal environmental changes that are not experienced by annuals. Therefore, it is likely that there will be differences in the regulatory pathways leading to flowering between annuals and perennials. We believe that Populus is an appropriate model for studying flowering in woody perennials. The developmental and anatomical structure of the Populus shoot has been extensively studied. The Populus genome is relatively small and has been sequenced. EST libraries are available from various tissues. Because Populus is amenable to Agrobacterium-mediated transformation, it is possible to conduct functional genomic analyses. We have isolated 2 CONSTANS-like genes, which are homologs of the Arabidopsis flowering time gene CONSTANS, and are investigating their potential roles in regulating Populus flowering.