V. virginicaa, Dec. 16, 2007, 24° F
P. odorataa, Dec. 25, 2007, 26° F
|(Click to enlarge.)|
This page attempts to establish a number of facets of the process of ice formation leading to crystallofolia (aka ‘frost flowers’) with Verbesina virginica & Pluchea odorata in Central Texas. I benefited greatly from insights learned from the detailed presentation of James Carter.
A significant increase in pressure must occur between the tough flexible epidermis and the structurally rigid secondary xylem core of the stem. I assume this is the result of subfreezing air temperatures impacting the cells of the cortex and vascular bundles. The xylem sap, essentially water, becomes supercooled. Once the pressure exceeds the ability of the epidermis to contain it, the stem surface and adjacent tissue rips apart, releasing the sap in the xylem vessels to spurt out with some force. No longer in a supercooled state, this water immediately freezes, perhaps assisted by contact with the adjacent tissue and ice already formed. The epidermal opening is generally longitudinal, although horizontal tearing may also occur.
Since most of the water in the formation has been drawn up from the roots following the drop of pressure in the stem, the flow of xylem sap should be greatest at the base of the rupture.
With the rupture of the stem, the pressure falls in the secondary xylem conduits connecting with the roots; and as the water exits and is frozen, additional water is drawn up from the roots by molecular adhesion. With nondormant plants the capillary force drawing water upward is primarily from transpiraton in the leaves, but with crystallofolia we assume that it is related to the drop in pressure upon the opening of the epidermis. The xylem cells at this stage are said to become "water-conducting pipelines." The formation of new ice would thus continue until a pressure equilibrium is reached or the roots no longer provide water. But the pipe–like xylem vessels, a major agent in this process, run vertically from the roots to the top of the plant, and do not extend to the periphery of the stem.
This shows clearly that the xylem sap is from the xylem rays (parenchyma rays), which carry water from the secondary xylem to the periphery of the stem. [See also: The Role of Parenchyma Rays & Vessel to Ray Transport.]
My speculative scenario for crystallofolia.