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The Functional Role of Crystallofolia

Any unique feature or process exhibited by a species, biological or physical, may be evaluated in terms of its functional value to that species. In this light the adaptive role of crystallofolia might be questioned, recognizing, of course, that any answer would be highly speculative.

The German plant physiologist Julius Sachs in 1860 noted the potential significance of freezing phenomena for issues of natural selection.

... they merit the greatest attention not only from a physiological point of view, but are also of decided importance for plant geography and evolutionary history.... [p. 41]

In recent decades issues of freezing in plants, especially in northern climates with plants of commercial value, has received considerable attention. Jeannine Cavender-Bates (2005) begins her article with the comment [p. 401]:

The ability of different species to avoid or tolerate freezing stress through various mechanisms can go a long way in explaining speciesgeographic distributions .... Plant species that live in freezing climates have three options for survival:

1. die, leaving a protected seed bank behind,
2. remain active, or
3. become dormant.

While the first strategy is used by annual herbaceous species, long-lived woody species that leave their above-ground biomass in place require mechanisms that allow persistence during winter.

In my opinion, plants that exhibit crystallofolia don't fit nicely into any of these categories. Considering the two Central Texas perennial herbaceous species, it would seem:

• the leaves die, but don't fall;
• the stems become dormant;
• the roots remain active, producing new shoots for the coming spring;
• freezing under given conditions of moisture and temperature 'prunes' the stems to points relatively close to the ground, but [my yet to be verified guess is that] in the event that the frost does not effect a 'pruning action,' the stems will resume activity in the spring. Older plants generally have much thicker and taller stems. As of February 2008 I have two populations of V. verbesina: one which has stems opened ('pruned') down to the base as the result of freezing; the other, in a drier and only slightly different location, is still unaffected. These two groups should develop differently as spring progresses, with the second group producing new growth from the old stems (assuming that differences in soil moisture will not create a difference in the new growth). — Unfortunately this 'test' did not support my 'guess.' The unaffected population also started new growth from the base. It is interesting to note that in December 2009, with abundant soil moisture and a hard freeze (25° F), the 'unaffected' population remains unaffected. I am left to suspect a genetic difference.

In effect, these species seem to be 'adaptively self-pruning,' gaining a head start for spring growth relative to climatic conditions.

The distributions of the American species that exhibit crystallofolia has yet to be studied, but the USDA distribution maps for Cunila, Pluchea and Verbesina indicate a range that might permit its southermost populations to escape hard freezes in some years. The somewhat more northern populations are those for which frost formations seem to be reported most often. (I have sometimes jokingly referred to this distribution as a 'bible–belt' phenomenon.)

Cunila origanoides	V. virginica
Pluchea foetida	P. odorata

The distribution for Helianthemum canadense and H. bicknellii doesn't obviously support this view. But given the extreme southern distribution of H. corymbosum, which does not exhibit frost formations, a speculative case might be made for a split with a dominant non-frost variety in the south, the northern species continuing an earlier frost–conditioned feature (similar to Verbesina).

3 Helianthemum species known from the 19th Century
H. canadense — H. bicknellii — H. corymbosum