Freezing in Conifer Xylem

A scanning electron microscope equipped with a freezing stage was used to examine the effects of slow freezing on pit aspiration and bubble formation in living tree stems. The size (approximately 2.0 μm diameter) and the spherical or ellipsoidal shape of the bubbles found in the centre of frozen lumens indicated freezing rates greater than 25 μm s−1. Both unaspirated and aspirated bordered pits were found in the frozen xylem. The technique used did not reveal enough pits to determine whether unaspirated pits were more prevalent than aspirated pits. These results are compared with hypotheses and results from previous work on freezing in conifer xylem.