Cotton fibers (cottonseed trichomes) are a well-established model system for investigating cellulose biosynthesis. Cotton ovules, excised from ovaries prior to fertilization, will continue to develop and produce trichomes in vitro under defined conditions when auxin and gibberellic acid are present. Our team is also using cotton fiber development in vitro as a model to investigate plant cell differentiation at the single-cell level and to identify the regulatory components of the transition from cell elongation to secondary wall biogenesis. These two developmental phases determine key properties of fiber quality and yield.