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Filmy fern physiology: a story of damp and shady places
For many years the filmy ferns have featured highly in my botanical affections. I recall, as a teenager, finding the two British species of Hymenophyllum and later, as a student, the thrill of peering into the entrance of a damp Irish cave to see the fronds of Trichomanes speciosum. And it is not just botanists who find this group interesting: the author John Buchan noted of one his manly heroes, stalking in the Scottish highlands, that he had by mid-morning seen both species of filmy fern (i.e. H. tunbrigense and H. wilsonii). However, as pointed out by Michael Proctor (University of Exeter, pp. 717-727), the three filmy fern species of north-western Europe are outliers of a group that has many more species in damp habitats in the tropics and sub-tropics. Nevertheless, this paper focuses on the two north-western European Hymenophyllum species, H. tunbrigense and H. wilsonii, comparing several physiological features in relation to their slightly different habitat requirements. In terms of water relations, both species behave more like bryophytes than vascular plants and can survive dry periods, although they both favour damp habitats. In the wild, H. wilsonii may be found in habitats that are more prone both to total wetting and to periodic drying, and this is reflected in a greater tolerance of submersion and a higher ability to recover from exposure to reduced water potential than H. tunbrigense. Photosynthetically, both are shade plants. Photosynthesis saturates at low irradiances, and photo-bleaching damage occurs at relatively low levels of irradiance. However, there are further differences that reflect habitat preference. H. tunbrigense is generally found in deeper shade than H. wilsonii and photosynthetic saturation occurs at lower irradiances in the former than the latter. Overall, this is an interesting set of observations giving real insights into the ecology of these fascinating species.
j.a.bryant{at}exeter.ac.uk
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