Figure 1

Quantification of xylem embolism in the stem of a Curly birch sapling. a) Sap-filled cells and cell walls are shown in light grey. Sap-filled cells and cell walls are shown in light grey, air-filled embolized cells in a dark grey or black. In a wide zone around the pith, the majority of the wood fibers are embolized, even though most vessels remain water-filled. This zone is highlighted in red in c). The area of the cavitated zone divided by the area of the whole stem gives the percentage of cavitated stem, or PCS value (37.1%). b) Shows an intermediate step in the PCS calculation, along with a grayvalue histogram of the data. Another figure of importance is the number of embolized xylem vessels in the cross section. The arrows in a) point to the 10 embolized vessels in this cross-section. Scale bars are 400 μm. The white object on the upper-right edge of the sample is a Blu-Tack marker used to help in locating the same position of the sample.