Commonly known as the scaly tree ferns, Cyatheaceae are a clade of ca. 600 plant species within a group of plants known as the monilophytes or ferns (Smith et al. 2006). All ferns are spore-bearing and share a highly distinctive vasculature system – with protoxylem confined to lobes of the xylem strand in a “necklace-like” arrangement or moniliformis in latin (Pryer et al. 2004).
Cyatheaceae: Leptosporangiate Ferns
In ‘A classification of extant ferns’ Smith et al. (2006) recognise four classes of ferns: Psilopsida; Equisetopsida; Marattiopsida; Polypodiopsida, which include 11 orders, and 37 families with ca. 9000 species. Cyatheaceae are placed within the class of ferns known as Polypodiopsida (or the Leptosporangiate Ferns) which contains the vast majority of fern diversity. In Leptosporangiate Ferns the sporangia develop from a single daughter epidermal cell and the mature sporangial walls are only one cell thick. Conversely, in the eusporangiate ferns (Psilopsida, Equisetopsida and Marattiaceae), as well as the epidermal daughter cell, a group of adjacent cells become involved in the formation of the sporangium (Sporne, 1966). With ca. 9000 extant species (eusporangiate ferns have ca. 300 species) in 267 genera, leptosporangiate ferns are, after angiosperms, the most diverse lineage of vascular plants. (Schuettpelz & Pryer, 2007).
Cyatheales: The tree fern clade
Along with the Podypodiales and Salviniales, the Cyatheales are one of the main lineages within the “Core Leptosporangiates”. It is thought that these lineages began to diverge around 220mya – 211 mya. The Polypodiales, which are an incredibly diverse group, are thought to be the sister clade to Cyatheales (Pryer et al. 2004).
Many of the species within Cyatheales have arborescent trunk-like stems, but other members of the order have a creeping and non-arborescent form. Tree ferns exhibit a range of morphological variations and sizes: from a few centimeters up to 20 meters. This is a well-supported monophyletic group – however, the tree ferns lack any obvious distinguishing feature or synapomorphy (Schuettpelz & Pryer, 2007). Furthermore, the arborescent growth form seen in tree ferns is not unique – it can also be seen in Blechnaceae (Korall et al. 2006).
There are two morphological synapomorphies which can assist in identifying a tree fern. All tree ferns have pneumathodes along the stipe and/or rachis, and the presence of radial symmetry in the shoots. However, the presence of pneumathodes may unite the clade, but it is also found in other leptopsporangiate ferns, and may represent the ancestral (plesiomorphic) condition of this group. Radial symmetry of the shoots is a common feature of most tree ferns, however, the character does not have a common origin within the clade (homoplastic), with reversals to dorsiventral shoots seen in Loxomataceae and Metaxyaceae (Korall et al. 2006).
Cyatheaceae and the ‘Core Tree Fern Clade’
Along with Metaxyaceae, Dicksoniaceae and Cibotiaceae; Cyatheaceae form a clade known as the “core tree ferns”. With the exception of Metaxyaceae (which consists of a single genus and two species with creeping rhizomes), the term ‘Core Tree Fern’ appears to unite those tree ferns which have a true ‘tree-like’ or aborescent habit. (Korall et al. 2006). It is estimated there are ca. 500 species of Cyatheaceae, which represent a large proportion of the total tree fern diversity. However, new species continue to be described, with three new species of Cyathea being discovered in the Northern Andes only very recently (Lehnert, 2009).
With the exception of eight species of Hymenophyllopsis, Cyatheaceae are made up exclusively of arborescent tree-like plants which includes the tallest living ferns, whose erect trunk-like rhizomes can reach over 20 m tall (Lehnert, 2009). Cyatheaceae usually have large leaves (to 5 m) that are 1-3 pinnate, and have vasculature made up of dictyosteles (Smith et al. 2006).
The genera Dicksonia ca. 25 spp. (Dicksoniaceae) and Cibotium ca. 11 spp. (Cibotiaceae/Dicksoniaceae) are mostly made up of arborescent taxa, and at first glance appear to be similar in form to the tree ferns of Cyatheaceae. However, they differ from Cyatheaceae by the presence of hairs instead of scales as indumentum (Korall et al. 2006). A further key morphological distinction is the abaxial sori in Cyatheaceae, whereas in Cibotium and Dicksonia the sori are marginal or submarginal. (Korall et al. 2006).
Images above: Hairs of Dicksoniaceae.
1A: Dicksonia sellowiana; 1B: Hairs at the stipe base of D. sellowiana (Scale: 5 x 5 cm); 1C: Hairs of D. sellowiana (Scale: 1 x 1 cm); 1D: Hairs of D. sellowiana (Scale: 1 x 1 cm). Images 1B, 1C and 1D attributed to Chris Reilly.
Images above: Scales of Cyatheaceae.
2A: Cyathea brownii ; 2B: C. brownii close-up; 2C: Scales of C. brownii (Scale: 8 x 8 cm); 2D: Scales of C. brownii (Scale 1.5 x 1.5 cm). Images above attributed to Chris Reilly.
Images above: Comparing the sori of Dicksoniaceae and Cyatheaceae.
3A: Abaxial scales of Sphaeropteris barbata (Scales: 1 x 1 cm); 3B: Abaxial scales of Cyathea capensis (Scales: 3 x 3 cm); 3C: Marginal scales of Dicksonia squarrosus (Scale: 2 x2 cm); 3D: Marginal scales of Dicksonia arborescens (Scale: 1 x 1 cm). All images above attributed to Chris Reilly and specimens provided by The University of Reading Herbarium.
Cyatheaceae has a pan-tropical southern temperate distribution. The extant scaly tree ferns are commonly found in the tropics, from the wet lowlands to mid elevations. However, some species are also found in high mountain areas in the tropics at elevations up to 3500m (Bystrikova, 2011). Ranil et al. (2011) describe ‘Cyatheaceae as one of the most interesting families due to striking morphology and wide geographical distribution combined with pronounced local endemism. Madagascar is an example of this pronounced local endemism, the island has 49 endemic species of scaly tree ferns (Janssen et al. 2007).
Hymenophyllopsidaceae: The filmy, scaly tree fern
Hymenophyllopsidaceae, was originally treated as a single genus family endemic to the Roraima formation (Guayana Shield) of Venezuela, Guyana and northernmost Brazil, where they are restricted to table mountains (tepuis) of Precambrian sandstone and quartzite (Christenhusz, 2009). Species of the genus Hymenophyllopsis are small plants with 1-4 pinnate fronds, mostly 10-30cm long. Their tissue is only 3-4 cells thick with chloroplasts present only in epidermal cells and lacking in stomates. The stems are usually short-creeping to sub-erect or erect. Hymenophyllopsis look superficially similar to filmy ferns (Hymenophyllaceae) because of the similar characteristics of the lamina. However, this is likely to be the result of convergent evolution rather than a close taxonomic relationship.
Uncertainty over the phylogenetic relationship of Hymenophyllopsis led the genus being categorised as “incertae sedis” – which alludes to the difficulty in defining the broader taxonomic relationships of the family (Wolf et al. 1999).
Image on right: Hymenophyllopsis dejecta specimen. Image attributed to Intermountain Herbarium via intermountainbiota.org
The plants of the Roraima Formation (Guayana Shield) are subjected so some extreme climatic conditions: strong winds, low temperatures, excessively high humidity and daily downpours of rain (Morton, 1932). Bystrikova et al. (2011) suggest the geographical distribution of scaly tree ferns in the wet tropics indicate a highly conserved climatic niche preference for warm temperatures, limited seasonal temperature and rainfall variation. Along with the geographical isolation of the tepuis, it appears that the climatic conditions in the Roraima formation have facilitated the evolution of a group of plants with morphological adaptations very different to those normally associated to the scaly tree ferns.
The affinity of Hymenophyllopsis with Cyatheaceae had been suggested but has only been confirmed in recent years through phylogenetic studies (Wolf et al. 1999). The diminutive Hymenophyllopsis at first appear to be unlikely scaly tree ferns, but the presence of scales and sporangial features indicate affinities to Cyatheaceae (Korall et al. 2006). Recent studies suggest strongly that Hymenophyllopsis nests within Cyathea as they both share marginal scales without apical setae and cyatheoid (cup-shaped) indusia (Korall et al. 2007). More recently, Hymenophyllopsis has been described as a subgenus within Cyathea (Christenhusz, 2009).
The generic limits of Cyatheaceae
Generic delimitations of Cyatheaceae have differed substantially across various studies and have been hotly debated in recent years. However, with evermore phylogenetic analyses and data botanists are beginning to piece together that main groups or genera within the family.
In 1970, Tryon recognised six genera within Cyatheaceae: Cyathea, Cnemidaria, Trichipterys, Alsophila, Nephelea and Sphaeropteris.
In a later study, Stein et al. (1997) proposed three evolutionary lineages: Alsophila, Cyathea and Sphaeropteris. This study moved Cnemidaria and Trichipteris which nested within Cyathea, and Nephelea now nested within Alsophila. Alsophila was reclassified to include ca. 235 species mostly occurring in the old world tropics and sub tropics; Sphaeropteris ca. 120 species with a similar distribution (but absent from Madagascar); Cyathea ca. 115 species mainly distributed in the New World with a few taxa in the islands of the Western pacific (Korall et al. 2007).
These proposed lineages are separated by differences in scale morphology. Sphaeropteris with conform scales, Cyathea marginate scales without an apical seta, and Alsophila with marginate scales with an apical seta (Korall et al. 2007)
Images above are simplified interpretations of petiole scales for Cyatheaceae, Fig.1 found in Korall et al. (2007).
4A: Conform scale found in Sphaeropteris showing cells of equal size and orientation; 4B: Marginate Scales without seta found in Cyathea showing cells at margin and have different orientation than cells that are centrally located; 4C: Marginate Scales with Seta found in Alsophila s.s. and Gymnosphaera.
Wolf et al. (1999) suggested that Hymophyllopsis is a sister clade to Cyathea, and nested within Cyatheaceae. In ‘A classification for extant ferns’ Smith et al. (2006) describes Cyatheaceae as having five genera – Alsophila (including Nephelea), Cyathea (including Cnemidaria, Hemitelia and Trichipteris), Gymnosphaera, Sphaeropteris and Hymenophyllopsis.
In the most recent phylogenetic study, Korall et al. (2007) suggested there are four major groups within Cyatheaceae: Sphareopteris, Cyathea, Alsophila sensu stricto and Gymnosphaera.
Again, the unique synapomorphy for Sphaeropteris is conform scales; non-setate marginate scales are unique to Cyathea; and, marginate scales with apical setae are found in Alsophila s.s. and Gymnosphaera. The diagnostic feature for Alsophila is the presence of 16 spores per sporangium, compared to 64 spores for the other in groups.
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