Sampling of xylem tissues. A transverse section from the base of the stem showing xylem tissues sampled from a Populus tree for the microarray and the RT-PCR analysis. The bark was peeled off resulting in a fracture in the cambial zone. For RT-PCR analysis, the different xylem tissues were successively scraped from the surface of the exposed stem to the border with the dead wood. For microarray analysis, the tissues were pooled into two samples: A (early fiber development) and B (fiber-cell death). The fiber-cell death sample corresponded closely to the tissues collected for construction of the fiber death cDNA library . V, dead vessel; Fs, developing fibers. Note that the development of vessels is completed within the region of cell expansion, and that the fibers develop at a much slower pace, visualized by the gradual loss of the cytoplasm of the fibers. The asterisks denote fibers close to the moment of death with barely detectable cytoplasm.