Desert ferns: Adaptations and possible origin from epiphytic ancestors

Cheilanthes gracillima

What are the desert ferns?

• Desert ferns, often referred to as Cheilanthoids, are all in the family Pteridium which includes 4 groups:

• Adiantoids• Cheilanthoids• Hemiontidoids• Pteroids

Cheilanthoids consist of 6 genera

• Argyrochosma - false cloak ferns• Aspidotis - lace ferns• Astrolepis - star-scaled cloak ferns• Cheilanthes - lip ferns• Notholaena - cloak ferns• Pellaea - cliff brakes• All are known to inhabit xeric enviroments• Source: Wikipedia

Cheilanthoids have developed many adaptations to allow them to survive in very dry environments

• Microphylly, or reduced frond and pinnea size

Argyrochosma dealbata Reference: R. H. Hevly

Adaptations continued…

• Partially sunken and fewer stomata, reduces water loss through transpiration

• Reduced cell size, reduce likelihood of plasmolysis and resulting plasmadesmata rupture

• Increased palisade tissue, spreads illumination to a larger volume of cells decreasing stress on chloroplasts and improves translocation of water, nutrients and photosynthates

Reference: R. H. Hevly

Adaptations continued…• Heavily cutinized epidermis, particularly in

glabrous (free of hair/scale) species• Lamina of nonglabrous species partially or

completely obscured by hair, scale and gland in various combinations

• Cheilanthes parryi (hair)Reference: R. H. Hevly

Adaptations continued…

• Impregnation of epidermal and cortex cell walls with suberin and lignin particularly in stipe and rhizome.

• Sclerification of cell walls extending to the stele as well as epidermal appendages of rhizome and stipe.

• Cortex cells filled with gum or tannin-like substances

• All of these deposits are designed to control water loss from the main plant body

Reference: R. H. Hevly

Some Cheilanthes have even been found to utilize mycorrhizal VAM relationships that ostensibly help them to better deal with water stress and nutrient uptake

(M. Palmieri, L.T.Swatzell)

And the most common adaptation to the xeric environment…

• Poikilohydry- curling of leaves under drought stress. It has been shown that desert ferns curl differently depending on their morphology. Ferns with coverings on the lower epidermis curl to protect the upper epidermis from exposure, while ferns without lower epidermal coverings curl such that the lower epidermis and stomata are protected

Reference: R. H. Hevly

Notholaena standeyi Maxon Pellaea mucronata

The gametophytes of Cheilanthoids are also adapted to xeric enviroments

• The prothallus is able to recover from desiccation severe enough to destroy all but a few cells

• The mode of reproduction is apomictic • Remember Cheilanthes yavapensis? (A.L. Grusz et al.)

The southwest is a biodiversity hot spot for Cheilanthoids

So how do Cheilanthoids fit into the phylogeny of ferns? (What about the epiphyte part of this talk?)

• As previously state Cheilanthoids are in the family Pteridaceae.

• They are polypods, not eupolypods• Pteridaceae have a false indusium, never a

true indusium, vertical interrupted annulus with mixed development of sporangium

• Pteridaceae is the only polypod (excluding eupolypods) not in the double indusium group

the divergance point (50) was estimated to be 124.92 MYA (K.M. Pryer et al.)

Divergence of Pteridaceae

• The initial divergence point (50 ) was estimated to be 124.92 MYA while the divergence of Pteridaceae proper (52) was 92.03MYA this put the divergence point in the mid to late Cretaceous period

Diversification of epiphytes

• According to research done by Eric Schuettpelz the bulk of epiphytic radiation occurred after the K/T boundary about 65 MYA

• However epiphytism is a trait that has arose independently as well as subsequently lost, several times in ferns

• So what does this mean? • Maybe nothing, One of the earliest families of

ferns to diverge, estimated in Pryer et al. at 272.89 MYA, during the Permian, was hymenophyllum, the filmy ferns. This family today has an very high percentage of epiphytes.

The fact is that desert ferns and epiphytic ferns share many characteristics

• Most of these are characteristics that help the plants prevent water loss.

• Most epiphytic ferns have also lost their indusium like Cheilanthoids.

Works cited• Comparative Desiccation Tolerance of Three Desert Pteridophytes: Response to Long-term Desiccation • James B. Harten, William G. Eickmeier • American Midland Naturalist, Vol. 118, No. 2 (Oct., 1987), pp. 337-347 • Published by: The University of Notre Dame • Stable URL: http://www.jstor.org/stable/2425790

• Adaptations of Cheilanthoid Ferns to Desert Environments • Richard H. Hevly • Journal of the Arizona Academy of Science, Vol. 2, No. 4 (Dec., 1963), pp. 164-175 • Published by: Arizona-Nevada Academy of Science • Stable URL: http://www.jstor.org/stable/40026172

• Mycorrhizal Fungi Associated with the Fern Cheilanthes lanosa • M. Palmieri, L. J. Swatzell • Northeastern Naturalist, Vol. 11, No. 1 (2004), pp. 57-66 • Published by: Humboldt Field Research Institute • Stable URL: http://www.jstor.org/stable/3858543

• A Fossil Fern Indicator of Epiphytism in a Tertiary Flora • Imogen Poole, Christopher N. Page • New Phytologist, Vol. 148, No. 1 (Oct., 2000), pp. 117-125 • Published by: Blackwell Publishing on behalf of the New Phytologist Trust • Stable URL: http://www.jstor.org/stable/2588907

• Phylogeny and Evolution of Ferns (Monilophytes) with a Focus on the Early Leptosporangiate Divergences • Kathleen M. Pryer, Eric Schuettpelz, Paul G. Wolf, Harald Schneider, Alan R. Smith, Raymond Cranfill • American Journal of Botany, Vol. 91, No. 10 (Oct., 2004), pp. 1582-1598 • Published by: Botanical Society of America • Stable URL: http://www.jstor.org/stable/4123853

Schuettpelz, Eric. "THE EVOLUTION AND DIVERSIFICATION OF EPIPHYTIC FERNS." Diss. Duke University, 2007. Dukespace. Web. <http://dukespace.lib.duke.edu/dspace/bitstream/10161/181/1/D_Schuettpelz_Eric_a_052007.pdf>.