Fig. 2

Microscopic observations of different plant organs from various species in reflected-light microscopy: Leaves of B. sempervirens (a, b), leaves of G. nivalis (c, d), and buds of P. abies (e, f) at room temperature and in a frozen state. Entire leaves or shoots with terminal buds were frozen to sublethal target temperatures before sectioning was performed. Ice crystal identification was facilitated by the crossed arrangement of the polarisation filters. In leaves of B. sempervirens large ice crystals formed, which expanded the preexistent cavity (void) between the palisade tissue (pa) and spongy tissue (sp), which was formed after the first freeze event. The bright spots (red arrows) are very likely due to other birefringent tissue components, as they are visible at room temperature and in the frozen state (b). In leaves of G. nivalis ice occurred prominently in the air-filled cavities (acav) (d). In buds of P. abies ice occurred in the cavity (cav) below the crown (cr) and between the bud scales (bs) during translocated ice formation (f). White bars indicate 500 µm