Danaea

Introduction

The genus Danaea is one of the few genera of ferns that are exclusively Neotropical. It can be found from Mexico, through all of Central and South America, south to Bolivia, Paraguay and Southeast Brazil. It also occurs on all the Greater Antilles, the more humid Lesser Antilles and on Cocos Island (Christenhusz 2007).

Danaea consists of approximately 50 species and occurs mainly in permanently wet forests on well drained soils. Several species seem to prefer steep slopes or creek banks in mostly undisturbed rainforests, although some species tolerate a moderate amount of disturbance. It  can be found in lowland rainforests, wet mountain forests and cloud forests, up to an elevation of 2650 m, but the genus is most diverse between 100 and 1000 m (Christenhusz 2007).

A study on growth rates in ferns has shown that the leaves of some Danaea species are very long-lived, and this makes the genus one of the slowest growing ferns with a long life expectancy (Sharpe 1993).

The generic name commemorates G. P. M. Dana (1736 - 1801) an Italian professor of botany.

Characteristics

Danaea can be recognized by its dimorphic fronds, the fertile being more contracted than the sterile. The synangia are fused and sunken into the lamina, in a row between the veins, generally covering most of the lower surface of the lamina.

The mature fronds are mostly once pinnate but simple (in D. carillensis and D. simplicifolia) and bipinnate (in D. bipinnata and sometimes in D. nodosa and D. urbanii). The terminal pinna can be present or replaced by a prolific bud, which can grow out into a new plant and can contribute substantially to the reproduction of a species. Between the pinnae there are usually swollen nodes, which in many species continues on the stipe, in the form of swollen articulations without pinnae. The venation is free, simple, paired at base or once branched.

The rhizomes are erect or creeping. The creeping ones can be dorsiventrally or radially arranged, the erect ones are radially arranged and held upright by numerous proproots.

Discussion of Phylogenetic Relationships

In several studies Danaea was found to be the basal lineage in the extant Marattiaceae. (Christenhusz et al., 2008; Murdock, 2008). Sometimes the genus was therefore placed into its own family the Danaeaceae, but morphological characters of vascular structure and a eusporangiate synangium clearly places this genus within the Marattiaceae.

The molecular phylogenetics of Danaea have only recently been studied on the basis of three plastid regions (Christenhusz et al, 2008). Based on morphological characters the genus can be divided into three groups (Christenhusz & Tuomisto, 2005), which agrees with the three groups found in the molecular studies (Christenhusz et al., 2008). The three groups are (after Christenhusz, 2007; naming of groups follows Presl, 1845):

• "Danaea": Generally tall species with mostly creeping (rarely erect) rhizomes, often lacking petiole nodes, and having leaves with many elongate pinnae; generally without apical proliferations.
• "Holodanaea": Generally intermediate to small species with radially arranged creeping, ascending or erect rhizomes, nodose petioles, and leaves with many pinnae that usually have denticulate apices; often with apical proliferations. This group includes species with leathery, bicolorous leaves and species with leaves of membranaceous translucent texture.
• "Arthrodanaea": Species with always erect radially arranged rhizomes, usually nodose petioles, and few, entire pinnae (at most sinuate at apex); terminal pinnae never replaced by proliferous buds.
  

"Danaea" and "Holodanaea" are most closely related; "Arthrodanaea" is sister to these.

References

Christenhusz, M. J. M. (2007). Evolutionary History and Taxnomy of Neotropical Marattioid Ferns: Studies of an ancient lineage of plants. Annales Universitatis Turkuensis ser. AII, tom. 216, pp. 1-134.

Christenhusz, M. J. M. and Tuomisto, H. (2005). Some notes on the taxonomy, biogeography and ecology of Danaea (Marattiaceae). Fern Gaz. 17, pp. 217-222.

Christenhusz, M. J. M., Tuomisto, H., Metzgar, J. S. and Pryer, K. M. (2008). Evolutionary relationships within the neotropical, eusporangiate fern genus Danaea (Marattiaceae). Molec. Phylog. Evol. 46, pp. 34-48.

Murdock, A. (2008). A taxonomic revision of the eusporangiate fern family Marattiaceae, with description of a new genus Ptisana. Taxon 57, 737-755.

Presl, C. B. (1845). Marattiaceae. In: Genera Filicacearum, Supplementum Tentaminis Pteridographiae, pp. 7-40. Prague.

Rolleri, C. H. (2004). Revisión del género Danaea (Marattiaceae-Pteridophyta). Darwiniana 42, pp. 217-301.

Sharpe, J. M. (1993). Plant-growth and demography of the neotropical herbaceous fern Danaea wendlandii (Marattiaceae) in a Costa-Rican rain-forest. Biotropica 25, pp. 85-94.

Information on the Internet

• Botany Photos: Marattiaceae. Plant Images by Maarten Christenhusz.