Polystichum acrostichoides

Christmas fern has a tufted, clumping habit, with its fronds arising from a central growth point. It can form colonies, but frequently grows singly or in twos or threes. In winter, the fertile fronds die; the sterile fronds remain through the winter, and are often flattened to the ground by low temperatures and snow cover. The frond is supported by a dark brown- to black-colored stipe, or stem, which is typically a quarter to a third of the overall frond length.{{cite book|last1=Foster|first1=Boughten Cobb|others=Illustrated by Laura Louise|title=A field guide to ferns and their related families of Northeastern and Central North America with a section on species also found in the British Isles and Western Europe|date=1987|publisher=Houghton Mifflin|location=Boston|isbn=0-395-19431-8|edition=[New ed., pbk. ed.].|url-access=registration|url=https://archive.org/details/fieldguidetofern0000cobb_o6z6}} Coarse, light-brown-to-tan scales cover the stipe, and are typically about {{Convert|5|mm|in|adj=mid|-long}} and translucent. The coiled, developing fronds ("crosiers") are scaly, greyish and prominent in early spring.

Fronds are {{Convert|30 to 80|cm|in|adj=mid|-long}} and {{Convert|5 to 12|cm|in|adj=mid|-broad}}, dark green and rather leathery in texture; their undersides may be covered in very sparse hairs. They have 20 to 35 pairs of pinnae. Each pinna is typically {{Convert|4|cm|in|adj=mid|-long}} and has a finely serrulate or spiny edge, and is oblong to falcate in shape. The fine teeth or spines on the edge of the pinna are oriented towards its tip. Each pinna has a small, triangular, "thumblike" lobe at its base.{{cite book|last1=Rhoads|first1=Ann|last2=Block|first2=Timothy|title=The Plants of Pennsylvania|publisher=University of Pennsylvania press|location=Philadelphia Pa|isbn=978-0-8122-4003-0|edition=2|date=2007-09-05}} The light brown spores are produced on fertile pinnae, at the frond's tip, which are conspicuously smaller than the sterile pinnae further down the frond.{{cite web|title=Taxon Page|url=http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200004550|website=www.efloras.org|accessdate=26 June 2016}} These fertile pinnae can be described as "acrostichoid", given that the sporangia occupy most of the lower surface of the pinna.{{cite book|last1=Coulter|first1=John|title=The Botanical Gazette (volume LXIV)|date=1917|page=347|url=https://books.google.com/books?id=Va7zAAAAMAAJ|accessdate=26 June 2016}}

Christmas fern resembles the Pacific Coast sword fern, Polystichum munitum, although forming less expansive tufts and differing from it and from almost all other ferns in that fertile pinnae of the Christmas fern are noticeably reduced in size relative to the sterile pinnae, while being located on the same frond.

P. acrostichoides is known to hybridize with Polystichum braunii in areas where their ranges overlap.

P. acrostichoides is parasitized by the fungus Taphrina polystichi, which causes yellowish to whitish galls on the fronds.{{cite journal |last1= Mix|first1= A.J.|date= 1938|title= Some Taphrina on North American ferns|journal= Mycologia|volume= 30|pages= 563–579 |doi= 10.1080/00275514.1938.12017299}}

The moth Herpetogramma sphingealis has larvae that feed on Christmas ferns. This moth species is primarily found where Christmas ferns are abundant and was given a scientific description in 2011.{{cite journal |last1=Handfield |first1=Louis |last2=Handfield |first2=Daniel |title=A New Species of Herpetogramma (Lepidoptera, Crambidae, Spilomelinae) from Eastern North America |journal=ZooKeys |date=24 November 2011 |issue=149 |pages=5–15 |doi=10.3897/zookeys.149.2344|doi-access=free |pmid=22207790 |pmc=3234405 |bibcode=2011ZooK..149....5H }}

File:Clumped (8312542938).jpg| Polystichum acrostichoides growing in Tiadaghton State Forest, Lycoming County, along the Golden Eagle Trail.

Christmas fern is popular in cultivation as an ornamental plant for gardens, including natural gardens, as it is easy to cultivate in a range of environments and soils. Being evergreen, it is sometimes used in winter-oriented garden design.{{Cite web|url = http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=a710|title = Polystichum acrostichoides - Plant Finder|website = www.missouribotanicalgarden.org|access-date = 2016-03-26}}

This fern can conserve soil and allay erosion of steep slopes. The fronds are semi-erect until the first killing frost, after which they lie prostrate on the ground and effectively hold in place the duff layer of the forest floor, enabling the gradual decomposition of the duff into humus, which in turn builds the soil.

Image:Christmas Fern Polystichum acrostichoides Curl Closeup 2000xpx.jpg|Coiled immature fronds

Image:Christmas Fern Polystichum acrostichoides Unfurled 2000px.jpg|Newly unfurled frond

Image:Christmas Fern Polystichum acrostichoides Leaf 2000px.JPG|Mature sterile frond

File:Polystichum acrostichoides5.jpg|Sori

{{Reflist}}

• {{ITIS | id = 17675 | taxon = Polystichum acrostichoides | accessdate = September 25, 2006 }}

{{Commons category}}

• {{Wikispecies-inline}}
• [http://plants.usda.gov/java/profile?symbol=POAC4 USDA PLANTS Profile for Polystichum acrostichoides (Christmas fern)]
• [http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200004550 Flora of North America: Polystichum acrostichoides]
• [https://www.paenflowered.org/apgii/polypodiales/dryopteridaceae/polystichum/polystichum-acrostichoides Flora of PA: Polystichum acrostichoides]

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acrostichoides

Category:Garden plants of North America

Category:Ferns of the Americas

Category:Ferns of the United States