Field of Science

From Tree Moss to Tree Ferns

Close-up of Davallia canariensis frond showing terminal sori. Photo from here.


Epiphytes seem to be the way to go here at CoO lately: after having covered a family of epiphytic mosses last week, I'm going to move on to a family of epiphytic ferns. The Davalliaceae are found in tropical and warm-temperate parts of the Old World. A few species are terrestrial but the majority are good old tree-huggers, either climbing up a suitable tree from roots attached to the ground or living entirely free of the tyranny of soil.

Habitus of Araiostegiella perdurans. Photo from here, where it is identified as 'Araiostegia' perdurans. Members of the previously recognised genus Araiostegia were redistributed by Kato & Tsutsumi (2008) between the genera Davallodes and Araiostegiella.


As a group, Davalliaceae are characterised by their elongate sori (spore-pouches) in marginal positions on the fronds at the junction of branching veins. The sori are covered by an indusium prior to maturity. Like other epiphytic ferns, the Davalliaceae also have creeping rhizomes covered by closely appressed scales. The most recent generic revision of the family recognises five genera (Kato & Tsutsumi 2008) but this aspect of Davalliaceae has always been unsettled. Phylogenetically, the Davalliaceae seem to belong in a clade that also includes the families Polypodiaceae and Grammitidaceae (Tsutsumi & Kato 2006). As these families are also primarily epiphytic, it seems likely that this lifestyle was ancestral for this clade. This would make the polypodioid-davallioid clade the largest assemblage of epiphytes among the ferns. It is also worth noting that these families probably diverged from each other some time in the early Tertiary (Schneider et al. 2004). Something that really does not get enough appreciation is that, despite the linear presentation of plant evolution that plagues most textbooks (bryophytes being replaced by ferns, which are shoved aside by conifers, that bow down before the all-conquering flowering plants), a significant percentage of the major fern lineages around today are actually much younger than the major flowering plant lineages.

Humata pectinata. Photo from here.


One final detail that's of patriotic interest to me: fossil Davalliaceae are known from the Miocene of New Zealand (Conran et al. 2010). These days Davalliaceae hang on in New Zealand by the skin of their rhizomes, with only a single species represented in the northernmost part of the country by asexually-reproducing individuals only (many fern species are able to survive by reproducing asexually in habitats where conditions do not permit sexual reproduction). This means that, along with coconuts, cone shells and crocodiles, Davalliaceae were part of a diverse biota that inhabited New Zealand during the balmy Miocene, only to decline and disappear as conditions became cooler.

REFERENCES

Conran, J. G., U. Kaulfuss, J. M. Bannister, D. C. Mildenhall & D. E. Lee. 2010. Davallia (Polypodiales: Davalliaceae) macrofossils from Early Miocene Otago (New Zealand) with in situ spores. Review of Palaeobotany and Palynology 162 (1): 84-94.

Kato, M., & C. Tsutsumi. 2008. Generic classification of Davalliaceae. Acta Phytotaxonomica et Geobotanica 59 (1): 1-19.

Schneider, H., E. Schuettpelz, K. M. Pryer, R. Cranfill, S. Magallón & R. Lupia. 2004. Ferns diversified in the shadow of angiosperms. Nature 428: 553-557.

Tsutsumi, C., & M. Kato. 2006. Evolution of epiphytes in Davalliaceae and related ferns. Botanical Journal of the Linnean Society 151 (4): 495-510.

3 comments:

  1. Can someone tell me why the spore bearing parts of a fern are called a sorus while in other plants its a sporangium?

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  2. Ferns have both. One sorus in a fern contains a number of sporangia within it.

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  3. Brian Axsmith27 May 2011 22:59

    The situation is more complex. Many ferns have the sporangia aggregated into a sorus. Other ferns do not - the sporangia are scattered on the leaf or are born on specialized fronds or frond segments. The male sporangia (pollen sacs) of cycads are born in clusters on the underside of the male cone scales, and these clusters are also sometimes referred to as sori. So the term is not even strictly unique to the ferns.

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