Relationship of the cell cycle to xylem cell differentiation: A new model

Conflicting data have appeared in the literature concerning the necessity for DNA synthesis prior to xylem cell differentiation. In some systems DNA synthesis is not required before differentiation, while in other systems DNA synthesis appears to be an absolute necessity. The construction of a model for the cell cycle in which the G1 phase is subdivided into a separate ‘early’ and ‘late’ phase can resolve this apparent conflict.