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Re: Pteris cretica

One argument for treating Pteris cretica as native to Florida is that it
is so regarded by Richard Wunderlin, this country's leading authority on
the vascular plants of Florida. However, fern list members should note
that Dr. Wunderlin once regarded Pteris cretica as native. Note the past
tense. His most recent opinion is that it is not native. One can check
Dr. Wunderlin's current opinion as to the nativity of any plant
occurring in Florida by going to the Atlas of Florida Vascular Plants
website and checking under "status." The page for Pteris cretica lists
its status as "Not Native":


James Rollins brings up some interesting questions in his most recent
post and some brief comments on his questions appear below. Although
James states that Pteris vittata is not native, he wonders how nativity
is determined.

> How does one go about determining if a species is native or not?

There is no one simple answer. But because the question cannot be easily
answered does not mean that the question of a fern's nativity is an
ineffable mystery.

First, if a plant is native to a region then it must occur in a natural
habitat also native to the region, at least historically. If I see a
plant that is found only along railroad embankments and nowhere else,
that is a good sign that it it not native since railroad embankments are
artificial (not natural) environments created by human activities. If
the plant is found in a pristine natural area essentially unaffected by
human activities and associating with other native plants, that is often
a sign that it is native. Obviously, an "old limestone quarry" is not a
natural environment unaffected by human activity. In what pristine
natural areas unaffected by human activity does one see Pteris vittata
in Georgia? In Florida, it occurs almost everywhere where there is
limestone and moderate shade including cracks in my driveway (I suppose
the cement is close enough to limestone rocks for its liking), it occurs
on limestone rocks used as landscape decoration in the middle of
downtown West Palm Beach, I have seen it on the walls of old brick
churches near downtown Tampa (again, the cracks in the mortar are close
enough to limestone for its liking), it occurs in cracks between the
sidewalk and the street in downtown Yauco, Puerto Rico, as well as
limestone rocks in natural areas, but usually near trails or roadsides
or other places where human activity has disturbed the natural
vegetation. Notice that it seems to favor human company and this is
another feature shared by non-native plants. This love of human company
is described scientifically by referring to Pteris vittata as
"anthropogenic." Florida has over 200 species of native ferns, but none
come up spontaneously in cracks in my driveway as does Pteris vittata.
These are good indications that it is not native.

The range of ecological conditions under which a plant grows is referred
to as its "ecologic amplitude." Because non-native plants have been
freed from their natural predators and parasites, which they left behind
in their original homeland, they often exhibit great vigor and an
incredibly wide ecologic amplitude. Very few native New World ferns have
the ecologic amplitude of Pteris vittata and this too argues that it is
not native. I have never seen any fern native to the United States
growing in the downtown metropolitan area of three different cities but
thus far I have seen Pteris vittata growing in three different downtowns
(West Palm Beach, Tampa, and Yauco). This extreme vigor and wide
ecologic amplitude is another clue that it is not native to the New
World. Interestingly, many weeds, such as Pteris vittata, are not at all
weedy in their natural range where natural biological controls keep them
in check.

Another way of ascertaining a plant's nativity is to examine whether or
not it has any specialized parasites or herbivores. If there are
insects, bacteria, fungi, nematodes, etc. that are specialized to live
off of and eat Pteris vittata in the Old World but none of these are
present in the New World, then this would argue very strongly that
Pteris vittata is an introduced element in the New World originally
native to the Old World.

One can also look at closely related species and see where they occur.
Again, if Pteris vittata's closest relatives occur in the Old World,
then that would bolster the conclusion that it is not native to the New

One can also examine linguistic evidence. If there are no names for
Pteris vittata in American Indian languages, but there are names for
Pteris vittata in Old World languages, that too would argue that it is
not native to the New World.

And, related to linguistic evidence, is ethnobotanical evidence. If
there is no record of American Indians utilizing Pteris vittata, but
there are records of Old World cultures utilizing Pteris vittata as
food, medicine, etc., then that too would argue for its Old World
origin. Pteris vittata is eaten as a food plant in China but this is not
the case anywhere in the New World. So the ethnobotanical evidence
indicates that it is not native to the New World.

Of course, there is also the historical record. If a plant is first
recorded in a far away region, say China, and then hundreds of years
later is recorded in Georgia, the chances that it is an introduction in
Georgia is extremely high. That is precisely what the historical record
indicates for Pteris vittata and this is reflected in one of its common
names, Chinese Brake.

However, none of these lines of evidence, taken in isolation, are
foolproof. But, taken together, these lines of evidence allow botanists
to confidently ascertain the nativity of the vast majority of fern
species on earth.

> Any one got any ideas as to how far a fern spore can travel, say in a hurricane?

Based on the fern flora of the Hawaiian islands, which are isolated by
2,000 miles from the nearest large land masses, the answer seems to be
that certain ferns have dispersed by way of spore at least 2,000 miles.
Whether such dispersal was aided by powerful tropical storms (cyclones
or hurricanes) or whether it was achieved by fern spore being carried
high in the atmosphere and then settling on Hawaii, I cannot say.

> If say, a fern spore came over with Columbus on one of his voyages and it
> survived and prospered would we know this [?]

Europe and the places Columbus landed, tropical Caribbean islands, have
such widely divergent fern floras that a European fern prospering on a
Caribbean island might as well have a label on it that says, "I'm an
early European introduction." So the answer is yes, we would know this.

> and could we consider it a native?

Such a fern would be considered "naturalized" but it could not be
considered "native" as that word is commonly understood in the
biological sciences.

Kind regards, Rufino
Rufino Osorio
USDA Zone 10a

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