Pawpaws and Mastodons, Logferns and Pinworms
Lytton John Musselman
My presentation deals with ethnobotany, that is, how people use plants. So my primary concern is not the habitat as such but these two groups of plants which inhabit forested wetlands. Forested wetlands are not communities with the greatest diversity of species. In fact from a botanist's standpoint, such environments are usually pretty dull and for this reason seldom visited by botanists (in contrast to ecologists). I well remember being a graduate student in Al Radford's ecosystematics class and traveling throughout the Southeastern United States (usually defined as any part of the U S where grits are served for breakfast) baling hay and learning in one semester all the plants he had learned in his forty years in the field. But when we came to the Dismal Swamp, we drove right by because there was nothing of interest there. I maintain there are at least two groups of plants which are of interest in such forested wetlands, pawpaws and log ferns of the genus Dryopteris. They are of interest not only to the botanist but also because they are plants which have potential economic importance--pawpaw as a fruit, insecticide and medicine; and logferns because they are a source of an anthihelminthic medicine used to cure pinworms and other intestinal worms as well as holding promise as horticultural subjects. I'll start by picking up a few pawpaws.
Pawpaws are in the genus Asimina (which appropriately is a Native American name). There are nine species in the Southeastern United States. Only two are widespread, however. These are the dwarf pawpaw (Asimina parviflora) which occurs in deep sand and the common pawpaw, (Asimina triloba). Common pawpaw is found mainly in forested Wetlands and reaches its greatest ecological importance in alluvial forests of the Ohio Valley. In Southeastern Virginia and neighboring North Carolina, plants associated with pawpaw include red maple, Japanese honeysuckle, American holly, etc.,- all plants typical of forested wetlands. I note that in Region I, pawpaw is listed as a FACU+² but based on my field experience in the southeast, I suggest it may better be classified as a FACW.
Pawpaw is a shrub or small tree which flowers early in the spring and about the first of October produces the largest fruit of any native plant in North America. Not only is it large, it is delicious! And no wonder when you consider its relatives. They include the custard apple, soursop, and other tropical fruits which are usually considered to be the most luscious, sought after fruits. Fruits similar to the common pawpaw have been found in the Eocene in Mississippi and other fossils almost indistinguishable from common pawpaw are known from the Late Miocene. Pawpaw was apparently one of those plants fed upon by large, frugivorous mammals like the mastodon. Mastodons have gone the way of the Santa Fe Chief and after their demise pawpaws were in trouble. But with the advent of invasions of Native Americans, the pawpaw was widely distributed either by planting or, as is usual in many bush fallow cultures, by selective cutting. Native Americans not only used the fruit but also the bark for making rope. Th eir use of the plant as a medicine is well documented as well. Plants spread in a similar manner include mayapple, pond nuts, etc.
The potential of pawpaw as a commercial fruit has been known for some time. In 1916 the Journal of Heredity announced a program to find the largest pawpaw in an effort to develop a commercially marketable variety. Despite this, the pawpaw remains "an horticultural orphan" (Zimmerman, 1941). Some of the problems in developing a commercial variety include the fact that fruit set is low; yield is unpredictable; and the fruits are exceptionally perishable. There has not been the agronomic improvement necessary for a new crop.
Those of us familiar with pawpaws in the field are often amazed at the almost total lack of insect damage, a feature pawpaws share with logferns. The larvae of the zebra swallowtail butterfly (Eurytides marcellus Cramer) feed almost exclusively on pawpaw but few other insects cherish it. This is particularly interesting because the leaves are large and somewhat succulent, and would appear inviting to insects. But there is good reason why pawpaws are not attacked. They contain a natural insecticide based on a group of compounds called acetogenins. In limited tests at Purdue University, extracts from all parts of the plant have yielded significant amounts of the compound. This insecticide has shown efficacy against a broad group of insects.
Ethnobotanists (botanists who study how people use plants) have documented the use of pawpaw as an anti-cancer agent. Jerry McLaughlin of Purdue University has studied extracts of pawpaw more than anyone and shown that these compounds have value in treatment of specific cancers. In fact, he has a patent for the composition of asimicin (named after the pawpaw). This and other compounds have shown best selectivity against certain cancers. Plans are underway to plant groves of pawpaws for the extraction of this material.
In summary, pawpaw needs the proverbial further research. But in order to do this, we must identify and protect as broad a genetic base of the plant as possible.
Logferns (species of the genus Dryopteris) are plants found in diversity of habitats including Wetlands. For many years, we have been studying the Dismal Swamp logfern, Dryopteris celsa and its hybrids. This species is rare throughout much of its range; the largest populations and the greatest diversity of hybrids are in the Dismal Swamp.
Dryopteris celsa is listed (erroneously in my opinion) as OBL in Region I and OBL FAC+ nationally (page 30). It occurs only in the driest phases of swamp forests. In a study of the logferns we published in 1978 we note [American Fern Journal 68(2)]: Page 45: "Our field observations indicate that in this area [region] Dryopteris species always are ecotone plants. Although they are invariably found in areas contiguous with swamps, they seldom grow in inundated sites." Page 50: "It [Dismal Swamp logfern] is to be expected in well drained soil at the border between swamps and upland areas. An exception to this is its absence along any large river. This may be due to the fact that it cannot tolerate inundation."
Logferns which I would consider characteristic of forested wetlands include: Dryopteris celsa, D. cristata, D. ludoviciana, D. australis, and D. separabilis (not currently on the list, no doubt due to the fact that is considered such a rare plant and because of its hybrid nature). These ferns should all probably be classified as FACU+. Dryopteris atropalustris is a name which should not be used (Musselman and Wagner, 1979) as it represents a growth phase of D. celsa.
Logferns have been used for many years as a source of antihelminthic medicines. The active component of this medicine are the phloroglucinol compounds found within the ferns. Because of their pharmaceutical importance, logferns have been well studied for the distribution of phloroglucinols and related compounds. The highest concentrations are not in Wetland species although the Wetland species do contain appreciable amounts of phloroglucinols. Several of the hybrids deserve further study.
It is remarkable that both pawpaws and logferns have a strong resistance to insect damage. At many sites in the Dismal Swamp and adjacent forested wetlands, logferns grow under pawpaws! But logferns have another value unrelated to bugs. Logferns have potential as garden plants. Features which make them attractive for urban gardens include the resistance to insect attack, their evergreen habit, tolerance of shade, and easy growth. Further, since they are rhizomatous, it is easy to propagate even the rarest hybrids.
I have been growing logferns at my home in urban Norfolk, Virginia for more than fifteen years. They are luxuriant plants which receive very little attention. Sometimes I receive a request for propagules of some of the rare hybrids I have been growing and simply take an ax, hack off a piece, and mail it off. At present, there are few nurseries which carry logfern hybrids but I believe that will change. What is needed is to multiply the ferns and make them known to the public
This essay was written for an invited talk at "Forested Wetlands", a symposium held in Annapolis, Maryland in January 1991. It deals with two plants which occur in forested Wetlands which may be of economic value.