The only apparent source of pressure was root pressure, no doubt drawing water as the result of evaporation at the surface of the exposed xylem rays. On many stems the rays had split, clearly the result of water freezing in the stem.
The two larger plants shown on other pages. The picture on the right was taken in the afternoon, showing the new shoots already in an advanced stage.
A selection of pictures from Feb. 13 (click to enlarge):
Same stem as that on the left.