Flights of whimsy: eXtreme mesophytes

Buddleia on church in Waterford, Ireland

Why should it be that eXtreme plants can’t live in cities or towns or on university campuses like we do?

Sure, the concept of eXtreme plant, or eXtremophyte, essentially requires (or seems to require) that the environment, ecosystem or habitat in which the plant is growing is a hostile one, i.e. that it is itself extreme. For those of us who do not live in such places (which means most people, not just most university scientists), such plants are also weird; all of the adaptations discussed elsewhere on this site just don’t “compute” for someone living in the midst of the most fertile and productive farmland in the world.

Are our powers of observation so weak that we can’t find examples of eXtreme plants in our own realms? Nay! Sure we can! They are just weeds gone good. What might otherwise be considered a ruderal mesophyte – a fast growing, weedy opportunist that invades farm fields and gardens – can also manage to survive and reproduce in the least likely of places.

This gallery is photos of such plants. Most are “mesophytic chasmophytes”, meaning they are growing in cracks in otherwise pretty solid structures. While they are not desiccation tolerant, they are certainly highly drought tolerant. Some, I can’t identify, but only because I am not that good a botanist. Most, I can’t explain, really. Does “eXtreme” require that the plant be tolerant of the stresses it encounters daily? Is it sufficient for it to be able to exploit every little crack and cranny to at least partially ameliorate the stresses? Are some mesophytes actually also eXtremophytes – have you ever seen a Plantago wilt? (It turns out they do… look at the gallery).

The other major group here is “gutter plants”. These include both herbaceous and tree species in a truly complex ecosystem, high above the ground in rain gutters. The substrate is decaying leaves and woody debris with a high diversity of microbes, micro- and macro-invertebrates. The substrate retains water to the point of waterlogging for a while after a rain. Then, it becomes bone dry. Thus, the plants must be both flooding and desiccation tolerant.

Well, and there is yet another group… parking lot plants. These include many of the same found in pavement cracks, and clearly have to survive extreme heat and drought as well as being driven over several times a day. (It is no wonder that they are all prostrate, at least in this habitat).

Comments, submissions of additional photos, identification of the plants in these photos… all are more than welcome. I will add to the gallery at every possible chance.

Acknowledgment – Many thanks to Jamie Ellis (University of Illinois) for his expert assistance identifying some of the plants in these photos.

[A note about sidewalks. All the walks in these photos are poured concrete with ca. 1 cm x 1 cm joints to focus cracking due to expansion and contraction. Some, with plants, are not yet cracked as far as I can tell. None appeared to have major cracks, i.e. the roots were unlikely to have free access to air, water and nutrients below the sidewalks.]

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