Speculations about the Bracken Ferns of the Southeastern USA Coast, Atlantic Seaboard of the East Coast, and Florida.
It will be helpful to the reader to understand to some extent the diversity of morphology of Polystichum acrostichoides (Christmas Fern, of Eastern North America) and its meaning. Specifically, occasionally produced amongst the genetic diversity of Polystichum acrostichoides are individuals that exhibit traits of ancestral species. Instead of simple un-lobed leaflets, these individuals have leaflets that are lobed to various degrees of development. Or, instead of having the fertile portion of the frond produced at the end of a vegetative frond as is usual for the species, some individuals have completely separate fertile fronds. And yet, they are all Polystichum acrostichoides. https://www.inaturalist.org/journal/mjpapay/45470-variation-in-polystichum-acrostichoides
Pteridium aquilinum pseudocaudatum (Tailed Bracken) of the Southeastern USA, of the Atlantic Seaboard of the East Coast, and Florida, exhibits a diversity of form that sometimes seemed inexplicable, until I realized two things. Tailed Bracken owes it s existence to an ancient series of hybridizations between Pteridium aquilinum latiusculum and Pteridium esculentum. In Central and Eastern North America the dominant Bracken Fern, in numbers and geographic distribution, is Eagle Fern, Pteridium aquilinum latiusculum. If we consider the map of its distribution as a sea, then into this sea on occasion in history a spore of Pteridum esculentum would fall, germinate, and grow to maturity. However, the new fern had only Pteridium aquilinum latiusculum as a breeding partner, and into the sea of Pteridium aquilinum latiusculum the genetics of the newly arrived Pteridium esculentum were dissipated. The climate of the Southeastern USA Coast, Coastal Plain, and Seaboard of the Atlantic were amenable to the descendants of the hybridization, whilst the colder inland regions occupied then, and now, by Pteridium aquilinum latiusculum were too cold for the newly arrived genetics. By and by, what we now know as Tailed Bracken, Pteridium aquilinum pseudocaudatum the product of the ancestral hybridization, inhabited the warmer (USDA Zone 6b and warmer) climate of the Coastal Plains, and Coasts. And just as occasionally amongst the individuals of Polystichum acrostichoides there are individuals that exhibit ancestral traits, so too amongst Pteridium aquilinum pseudocaudatum individuals appear that harken back to Pteridium esculentum, (or Pteridium aquilinum latiusculum) in the most pronounced forms having winged tips of the rachis (frond stem) of leaflets and sub-leaflets, as per Pteridium esculentum.
Confusingly, Pteridium aquilinum pseudocaudatum can approach in form the almost ethereal airiness of Pteridum caudatum, a matter that caused me (and I believe many others) much consternation for some time. That difficulty "fell away", so to speak, when I realized that the fairly airy individuals of Pteridium aquilinum pseudocaudatum were more expressive of their Pteridium esculentum ancestor, and yet remain Pteridum aquilinum pseudocaudatum.
Pteridium aquilinum latiusculum and Pteridium esculentum are both diploid, that is to say that their complete genetics is comprised of two sets of chromosomes. Since both of the Brackens are diploid, they can hybridize and produce fertile offspring. The two Brackens can intermingle when opportunity allows. Their genetics are miscible, like water mixed with water.
The situation is different with Pteridium caudatum for it is a tetraploid. Its complete genetics is comprised of four sets of chromosomes. This difference very effectively separates it from Pteridium aquilinum and Pteridium esculentum, for the hybridization of Pteridium caudatum with either of the other brackens will form a sterile offspring. Thus any Pteridium caudatum fern that germinates in the figurative sea of Pteridium aquilinum finds itself immiscible, like a drop of olive oil in water. All the same, hybrids with it probably do occur in the warmer parts of Florida, and any such hybrids will not be easy to separate from the airier forms of Pteridium aquilinum pseudocaudatum.
